Title of Invention

"METHOD AND SYSTEM FOR DETERMINING HOW CHANGES TO UNDERLYING DATA AFFECT CACHED OBJECTS"

Abstract

A determination can be made of how changes to underlying data affect the value of objects. Examples of applications include: caching dynamic Web pages; client-server applications whereby a server sending objects (which are changing all the time) to multiple clients can track which versions are sent to which clients and how obsolete the versions are; and any situation where it is necessary to maintain and uniquely identify several versions of objects, update obsolete objects, quantitatively assess how different two versions of the same object are, and/or maintain consistency among a set of objects. A directed graph, called an object dependence graph, may be used to represent the data dependencies between objects. Another aspect is constructing and maintaining objects to associate changes in remote data with cached objects. If data in a remote data source changes, database change notifications are used to "trigger" a dynamic rebuild of associated objects. Thus, obsolete objects can be dynamically replaced with fresh objects. The objects can be complex objects, such as dynamic Web pages or compound-complex objects, and the data can be underlying data in a database. The update can include either: storing a new version of the object in the cache; or deleting an object from the cache. Caches on multiple servers can also be synchronized with the data in a single common database. Updated information, whether new pages or delete orders, can be broadcast to a set of server nodes, permitting many systems to simultaneously benefit from the advantages of prefetching and providing a high degree of scaleability.

Full Text

determining How Changes to Underlying Data Affect Cached Objects Cross Reference to Related Patent Applications The present indention is related to co-pending United States of America Patent application S/N (to be determined), filed of even date herewith, entitled: A Scaleable Method for Maintaining and Making Consistent Updates to Caches, by Challenger et al. This co-pending application, which is commonly assigned with the present invention to the International Business Machines Corporation, Armonk, New York., is hereby incorporated herein by reference in its entirety. Background of the Invention Field of the Invention The present invention is related to an improve^ data processing system. Particular aspects relate to the World Wide Web, databases, and transaction processing systems. A more particular aspect is related to the caching of dynamic documents on the World Wi«e Web. Related Art The speed with which documents can be retrieved from the World Wide Web is one of the most important factors in how useful the Web is for transferring information and supporting electronic commerce. Web servers must be able to serve content to users quickly. Data delivered by Web servers can be divided into two categories: This data is obtained from files stored on a computer. Static data can be served relatively quickly. A high-performance Web server running on a computer such as a single RS/6000 590 node can typically deliver several hundred files per second. (2) dynamic data. This data is obtained by executing programs at the time requests are made. Dynamic data is often expensive to create. In many cases, dynamic data is one to two orders of magnitude more expensive to obtain than static data. For Web sites containing a high percentage of dynamic data, dynamic data performance can be a bottleneck. Examples of sites containing a high percentage of dynamic data include electronic commerce sites using IBM's net Commerce software such as the LL Bean Web site (www.llbean.com) and the IBM 1996 Olympics Web site. One method to reduce the overhead of dynamic data is to store dynamic pages in a cache after they are created by a program (see "A Distributed Web Server and its Performance Analysis on Multiple Platforms" by Y. H. Liu, P. Dantzig, C. E. Wu, J. Challenger, L. M. Ni, Proceedings of the International Conference for Distributed Computing Systems, May 1996). That way, subsequent requests which need to access these pages can access the copy in the cache. A page only has to be calculated by a program once. The overhead for recalculating the same page multiple times in response to multiple requests is reduced or eliminated. Caching cannot be applied to all dynamic Web pages. Some dynamic pages cause state changes to take place which must occur each time the pages arc requested. Such pages cannot be cached. * For pages that can be cached, a need remains for a method of updating a cache when changes to underlying data which may affect the value of one or more Web pages occur. For example, dynamic Web pages are often constructed from databases. When the databases change, it may be extremely difficult to determine which cache objects have become obsolete as a result of database changes. The present invention provides a solution to this problem. The solution is quite general and can be used for other situations where one needs to know how changes to underlying data iffect the values of objects. Another problem is how to keep a set of one or more caches updated when the source of the underlying data and the caches are geographically distinct The present invention has a solution to this problem which is relevant to proxy caching of both dynamic and static data. A third problem is how to make a set of updates to one or more caches consistently so that all updates are made at once and no request received by the system sees a later view of the system with respect to the updates than a request received at a later time. The present invention has a solution to die consistency problem which is of relevance to proxy caching of both static and dynamic data. It is also of relevance to transaction systems which do not necessarily involve caches. There are utilities known in the art for managing dependencies between files which are necessary to build computer programs. For example, a single program may be constructed from multiple source and object files. Tools have been developed to manage dependencies between source, object, and executable files. One of the best known utilities for managing such dependencies is the Unix make command (See e.g., IBM ADC Version 4 on-line manual pages). Utilities such as make require users to specify dependencies between files in a special file known as a makefile. For example, the following file dependency specification: foo: foo.h foo.c cc -o foo foo.c would be placed in a makefile to indicate that "foo" depends on "foo.h" and "foo.c". Any change to either "foo.h" or "foo.c" would cause "foo" to be recompiled using the command "cc -o foo foo.c" the next time the command "make foo" was issued. Utilities such as makefile have several limitations, including: (1) Makefile only allows dependencies to be specified between files. It is not possible to specify dependencies between a file and something which is not a file. A need exists for a method which allows dependencies to be specified between objects which can be stored in caches and graph objects (which include underlying data) which cannot be cached. (2) Using the makefile approach, all files are updated whenever they are found to be obsolete by the "make" command regardless of how obsolete die file may be. A need also exists for a method which doesn't require that obsolete objects always be updated; for example, so that an obsolete object which is only slightly out of date maybe retained in the cache. (3) The makefile approach also1 only allows one version of a file to exist in the file system at a time. A need exists for a method which allows multiple versions of the same object to exist in the same cache concurrently. (4) A need also exists for a method which provides a quantitative method for determining how obsolete an object is, something not provided by tools such as makefile. (5) A need exists for a quantitative method for determining how similar two versions of the same object are, something also not provided by tools such as makefile. (6) A need also exists for a method for retaining a consistent set of possibly obsolete objects, something not provided by tools such as makefile. (7) A need exists for a method for concisely specifying dependencies between objects known as relational objects, which may be part of relational databases, something not provided by tools such as makefile. The present invention addresses these needs. Summary In many cases, the value of objects depend on underlying data. The present invention is directed to determining how changes to underlying data affect the value of objects. As an example, consider a Web site where pages are created dynamically from databases. In this situation, the dynamic Web pages are objects and the underlying data include the databases. In some cases, a Web page may have a dependency on a simpler Web page which in turn has a dependency on a database. It is possible to have a whole hierarchy of dependencies where objects depend on other objects which in turn depend on other objects. It is also possible to have co-dependencies between a set of objects where an update to any object in the set affects the value of all other objects in the set. Another aspect of the present invention is directed to a system for specifying dependencies between objects and underlying data which allows a computer system to propagate updates to all objects in the system after underlying data change. This system includes one or more of the following features: 1. Dependencies can be specified in a hierarchical fashion wherein objects may depend on other objects which in turn depend on other objects, etc. 2. Sets of objects can have co-dependencies. For example, two objects can have dependencies between each other which indicate that the value of one of them changes whenever the other object changes. 3. A feature of ensuring that an object is updated (or invalidated) whenever a change to any underlying data on which it depends changes. 4. A quantitative feature for determining how different an obsolete version of an object is from the current version. 5. A feature for ensuring that an object is updated or invalidated whenever it is sufficiently different from the current version. This is useful when always keeping the object current would entail too much overhead. 6. A new version of an object is produced each time its value is recalculated. Multiple versions of the same object can be retained. For two copies of the same object, it can be determined: (a) If the two objects correspond to the same version and are therefore identical; (b) If the answer to (a) is no, which version is more current (i. e. was created later); and (c) If the answer to (a) is no, a quantitative indication of how different the objects are. 7. A feature for preserving consistency among a set of objects without requiring all objects to be current. This is useful when ensuring consistency by requiring all objects to be current would otherwise have too high an overhead. 8. A feature for managing relational objects whereby implicit data dependencies between the relational objects are automatically added by the object manager. Examples of applications of the present invention include the following situations: Caching dynamic Web pages. 1. Client-server applications: In many client-server applications, the server will send objects to multiple clients. The objects are changing all the time. Different versions of the same object may be sent to different clients depending upon when the object was requested. The server needs some method for keeping track of which versions are sent to which clients and how obsolete the versions are. 3. Any situation where it is necessary to maintain several versions of objects, uniquely identify them, update them if they become too obsolete, quantitatively assess how different two versions of the same object are, and/or maintain consistency among a set of objects. A prefened embodiment makes use of a directed graph called the object dependence graph (G), which represents the data dependencies between objects. An edge from an object oJ to another object 02 signifies that o2 has a dependency on 07. Any update to ol also changes the current value of 02. In an alternative embodiment, each edge can have a non negative number associated with it known as the weight which represents the importance of the data dependence. For example, high numbers can represent important dependencies, while low numbers represent insignificant dependencies. Objects can also have a value known as the threshold_weight associated with them. Whenever the sum of die weights corresponding to incoming data dependencies which are current falls below the threshold_weight, the object is considered to be highly obsolete. Such objects should be updated for applications requiring recent versions of objects. Each object preferably has an objected field and a version_number field. The objectjd field corresponds to something which an application program would use to identify the object (e.g., the URL), while the version_number field allows multiple objects with the same objectjd to be maintained and uniquely identified. More specifically, a process known as the object manager maintains the data dependence information. Whenever new objects are created or the data dependencies change, the object manager is responsible for updating the appropriate information. Whenever an object is updated, the object manager uses the information in the object dependence graph to propagate other updates to the system which are necessitated by data dependencies. Each object ol may also have a consistency list which contains a list of objects which must remain consistent with ol. Two objects ol and 02 are consistent if either (1) Both objects are current; or (2) At some time t in the past, both objects were current An update to ol may also necessitate updates to other objects on the consistency list for ol.It is possible to associate an object with one or more records belonging to relations which are analogous to relations in a relational database. Such an object is known as a relational object The present invention also has features which automatically add dependencies between relational objects. Brief Description of the Drawings These and other features and advantages will become apparent from the following detailed description and accompanying drawings, wherein: Figure la depicts an example of a system having features of the present invention; Figure Ib depicts an example of an object dependence graph having features of the present invention; Figure Ic depicts an example of a system having features of the present invention; Figure 2 depicts an example of a cache used in accordance with the present invention; Figure 3 depicts an example of an object information block (OIB) used in accordance with the present invention; Figure 4 depicts an example of API functions in accordance with the present invention; Figure 5 depicts a block diagram of a method for implementing the API functions of Figure 4; Figure 6 depicts a block diagram of an API function which adds an object to a cache; Figure 7 depicts a block diagram of an API function which looks for an object in a cache; Figure 8 depicts a block diagram of an API function which deletes an object from a cache; Figure 9 depicts a block diagram of an API function which adds a dependency from a record to an object; Figure 10 depicts a block diagram of an API function which deletes a dependency from a record to an object; Figure 11 depicts a block diagram of an API function which is invoked when a record changes; Figure 12a depicts another example of a system having features of the present invention; Figure 12b depicts another example of an object dependence graph having features of the present invention; Figure 12c depicts an example of die object manager of Figure 12a; Figure 12d is another depiction of an object dependence graph having features of the present invention; Figure 13 depicts an example of a cache used in accordance with an embodiment of the present invention; Figure 14 depicts an example of an object information block (OIB) used in accordance with the present invention; Figure 15 depicts an example of a dependency list used in accordance the present invention; Figure 16 depicts an example of a dependency information block (DIB) used in accordance with the present invention; Figure 17 depicts another example of API functions in accordance with the present invention; Figure 18 depicts a block diagram of a method for implementing the API functions in figure 17; Figure 19 depicts a block diagram of a cache API function which adds the latest version of an object to a cache; Figure 20 depicts a block diagram of an API function which attempts to copy a version of an object from one cache to another, Figure 2 1 depicts a block diagram of an API function which may be invoked when underlying data change; Figure 22 depicts a block diagram of part of a method for propagating changes through the object dependence graph in response to changes to underlying data; Figure 23 depicts a block diagram of part of a method for propagating changes through the object dependence graph in a depth-first manner in response to changes to underlying data; Figure 24 depicts a block diagram of part of a method for propagating changes to a specific graph object in response to changes to underlying data; Figure 25 depicts a block diagram of part of a method for updating or invalidating a cached version of an object in response to changes to underlying data; Figure 26 depicts a block diagram of part of a method for maintaining consistency when one or more objects are added to one or more caches in response to changes to underlying data; Figure 27 depicts a block diagram of a cache API function for creating graph nodes corresponding to single record objects (SRO's); Figure 28 depicts a block diagram of an API function for creating graph nodes corresponding to multiple record objects (MRO's); Figure 29a depicts a block diagram of an API function which may be invoked when records change; Figure 29b depicts another example of an object dependence graph and how it can be used for propagating changes to graph objects; Figure 30a depicts a block diagram example of a system having features of the present invention for scaleably maintaining and consistently updating caches; Figure 30b depicts a more detailed example of the Trigger Monitor of Figure 30a instantiated as a Master Trigger Monitor;, Figure 30c depicts an example of the Trigger Monitor instantiated as a Slave Trigger Monitor, Figure 30d depicts an example of the sendjrigger API of Figure 30b; Figure 30e depicts examples of transaction types in accordance with die present invention; Figure 31 depicts an example of die Object Disposition Block (ODB) of Figure 30b; Figure 32 depicts an example of the cache ID of Figure 31; Figure 33 depicts an example of a high-level organization and communication paths of the Trigger Monitor Driver and the Distribution Manager, Figure 34 depicts an example of the Receiving Thread logic of Figure 33; Figure 35 depicts an example of the Incoming Work Dispatcher Thread logic of Figure 33; Figure 36 depicts an example of the Cache Manager Communications Thread logic of Figure 33; Figure 37 depicts an example of the Object Generator Thread logic of Figure 33; Figure 38 depicts an example of the Distribution Manager Thread logic of Figure 33; Figure 39 depicts an example of the Outbound Transaction Thread logic of Figure 33; Figure 40 depicts examples of extensions and variations for analysis and translations of Trigger Events; Figure 41 depicts an example of logic for making a set of requests consistently to a system consisting of one or more caches; and Figure 42 depicts an example of logic for determining a last_lock_time if the set of cache managers receiving a request has multiple members. Detailed Description of a Method for Determining How Changes to Underlying Data Affect Cached Objects Glossary of terms While dictionary meanings are also implied by terms used herein, the following glossary of some terms may be useful: A cache is a storage area. It may be in memory, on disk, or partly in memory and partly on disk. The physical or virtual addresses corresponding to the cache may be fixed. Alternatively, they may vary over time. The definition of caches includes but is not limited to the following: - Caches for Web documents; such as the proxy cache in the IBM Internet Connection Server or the browser cache in the Netscape Navigator, - Database caches such as in IBM's DB2 database; - Processor caches such as those in the IBM RS/6000 line of computers; and - Storage repositories for data written in a high-level programming language, wherein for at least some data, the storage repository program docs not have explicit control of the virtual or physical addresses of where the data lire stored. A cache union is the combination of all caches in a system. An object is data which can be stored in one or more caches. A multiple version cache is a cache which is allowed to include multiple versions of the same object. A single version cache is a cache which is only allowed to include one version of the same object. A current version cache is a single version cache in which the version of any cached object must be current. Underlying data include all data in the system which may affect the value of one or more objects. Underlying data are a superset of all objects in the system. A complex object is an object with one or more dependencies on underlying data. The object manager is a program which determines how changes to underlying data affect the values of objects. A graph G=(V,E) consists of a finite, nonempty set of vertices Falso known as nodes and a set of edges E consisting of pairs of vertices. If die edges are ordered pairs of vertices (v, w), then the graph is said to be directed 'with v being the source and w the target of the edge. A multigraph is similar to a graph. The key difference is that multiple edges may exist between pairs of vertices. Multigraphs are supersets of graphs. A weighted graph or weighted multigraph is one hi which each edge may optionally have a number known as a weight associated with it The object dependence graph is a directed multigraph. Vertices of the object dependence graph are known as graph objects. Graph objects are supersets of objects and may include the following: (1) objects; (2) underlying data which an: not objects; and (3) virtual objects. These graph objects do not correspond to actual data. They are used as a convenience for propagating data dependencies. Virtual objects are not as frequently used as ( 1 ) and (2). An edge from a graph object o 7 to 02 indicates a data dependence (also called dependence or dependency) from ol to o2. This means that a change to ol might also change o2. Dependencies are transitive. Thus, if a has a data dependence on b and b has a data dependence on c, then a has a dependence on c. A graph object may also be a relational object (RO). ROs have relational specifiers al them. 2 examples of RO's arc: 1 . Single record objects (SRO's); the relational specifier represents a single record. 2. Multiple record objects (MRO's); the relational specifier represents multiple records. An RO rl contains (includes) an RO r2 if all records represented by r2 are also repress The outgoing adjacency list for a node v is a list containing all nodes w for which the e contained in E. The incoming adjacency list for a node v is a list containing all nodes w for which the e contained in E. A leaf node is a node which is not the target of any edges. A proper leaf node is a leaf node which is the source of at least one edge. A maximal node is a node which is not the source of any edges. A proper maximal node is a maximal node which is the target of at least one edge. A simple dependence graph is a directed graph in which each node is a leaf node or a node. Two objects ol and o2 are consistent if cither: (1 ) Both objects are current; or (2) At some time / in the past,, both objects were current A version number is data which allows different versions of the same object to be unii identified. One implementation would be to use integers for version numbers and to z newly created current version, the version number of the previous version plus 1 . Ho implementations are also possible and version numbers do not necessarily have to be numbers. For example, text strings could also be used to implement version numbers. The most recent version of an object is known as the current version. Referring now to the drawings, Figure la depicts an example of a client-server architecture having features of the present invention. As depicted, a client 90 communicates requests to a server 100 over a network 95. The server 100 maintains one or more caches 2. As is conventional, the server 100 uses the caches 2 to improve performance and lessen the CPU time for satisfying the client 90 requests. Although Figure 1 a shows the caches 2 associated with a single server, the caches 2 could be maintained across multiple servers as well. One skilled in the art could easily adapt the present invention for other applications which are not client-server based as well. An application program 97 running on the server 100 creates objects and then stores those objects (e.g., dynamic pages which do not cause state changes upon a request therefor) on one or more caches 2. The server 100 can ajso be a proxy server wherein the source of the underlying data in the database 99 and the cache 2 are geographically separated. In this embodiment, an object is data which can be stored in one or more caches 2. The objects can be constructed from underlying data stored on a database 99. Underlying data include all data in the system which may affect the value of one or more objects stored in the cache 2. Underlying data are a superset of all objects in the system. A complex object is an object with one or more dependencies on the underlying data. Also, let the caches 2 in the cache union all be current version caches. Recall that a current version cache is a single version cache in which the version of any cached object must be current, and that a single version cache is a cache which is only allowed to include one version of the same object. According to the present invention, a cache manager 1 (which is an example of an object manager) determines how changes to underlying data affect the values of objects. Although Figure la shows the cache manager 1 residing on a single server, it could be distributed across multiple servers as well. The cache manager 1 is preferably embodied as computer executable code tangibly embodied on a program storage device for execution on a computer such as the server 100 (or the client 90). Those skilled in the art will appreciate that the cache 2, cache manager 1, and database 99 can be similarly associated with the client 90, in accordance with the present invention. The cache manager 1 provides APIs (Fig. 4) for specifying what underlying data, e.g., database records, a cached object depends upon. The cache manager 1 keeps track of these dependencies. Whenever a process modifies state which could affect the value of a complex object in a cache, the process specifies the underlying data which it is updating. The cache manager then invalidates all cached objects which depend on the underlying data being updated. Figure Ib depicts an example of an object dependence graph (G) 121' having features of the present invention. Note that the object dependence graph (G) 121' in this embodiment is less complex than in the alternative embodiment (Fig. 12b). Here, die object dependence graph 121' is a simple dependence graph, i.e., a directed graph in which each node is a leaf node rl ... r3 or a maximal node col, co2. RecaH that a leaf node is a node which is not the target of any edges and a maximal node is a node which is not the source of any edges. Also note that every path is of length 1 and there is no need to specify weights for edges. Further, each proper maximal node (a maximal node which is the target of at least one edge) col, co2 is an object; and each proper leaf node rl ... r4 (a leaf node which is the source of at least one edge) in G represents underlying data which is not an object. The underlying data represented by each proper leaf node rl... r4 is referred to as a record (These records are not synonymous with records used in the second embodiment). The objects represented by proper maximal nodes col, co2 are complex objects. The cache manager 1 maintains the underlying data structures (see Figs, 2-3) which represent the object dependence graph(s) 121'. Application programs 97 communicate the structure of object dependence graphs to the cache manager 1 via a set of cache APIs (sec Figure 4). The application also uses APIs to notify the object manager 1 of records rl ... r4 which have changed. When the cache manager 1 is notified of changes to a record rl ... r4, it must identify which complex objects co 1, co2 have been affected and cause the identified complex objects to be deleted (or updated) from any caches 2 containing them. The cache manager 1 can determine which complex objects have changed by examining edges in G (see Figure 11). For example, suppose that the cache manager 1 is notified that rl has changed. G 121' implies that col has also changed. The cache manager 1 must make sure that col is deleted (or updated) from any caches 2 containing it. As another example, suppose that r2 has changed. G 121' implies that co 1 and co2 are also affected. Here, die cache manager must make sure that both col and co2 are deleted (or updated) from any caches 2 containing them. In other words, the basic approach is to construct complex objects at the application level so mat they are dependent on a set of records. The application must be able to specify which records rl ... r4 a complex object col, co2 depends upon. For every process which modifies state in a manner which could affect the value of a cached complex object, the application program must be able to specify which records arc affected. Complex objects of this type are said to be in normal form. Many preexisting Web applications create cacheable complex objects which are already in normal form. In order to use caching in these applications, it is only necessary to recognize the records underlying complex objects and to interface the application to the cache via the APIs provided. Other changes to the applications are not necessary. Preferably, the cache manager 1 is a long running process managing storage for one or more caches 2. However, one skilled in the art could easily adapt the present invention for a cache manager which is one of the following: 1. Multiple distinct processes, none of which overlap in time. 2. Multiple distinct processes, some of which may overlap in time. This includes multiple concurrent cache managers so designed to improve the throughput of the cache system. Figure 1 c depicts an example of a system in accordance with the present invention for caching dynamic Web pages. As depicted, consider a conventional Web site 100 where pages (page 1 ... page 5) are created dynamically from one or more databases 99 and stored urone or more caches 2. An example of a database 99 and database management system adaptable to the present invention is that sold by the IBM Corporation under the trademark DB2. Here, the dynamic Web pages (page 1 ... page 5) are objects and the underlying data (tables/records) include parts of databases 99. According to the present invention, a cache manager 1 provides API's (figure 4) which allow an application 97 program to specify the records that a cached object depends upon. The cache manager 1 keeps track of these dependencies. Whenever an application program 97 modifies a record (s) or Icams about changes to a record which could affect the value of a complex object in; cache, the application program 97 notifies Ac cache manager 1 of die rccord(s) which has been updated. The cache manager 1 then invalidates or updates all cached objects with dependencies on the record (s) which has changed.. For example, consider the HTML pages (page 1 ... page 5) depicted in Figure Ic. The HTML pages, which are complex objects, are constructed from a database 99 and stored in CacheS. Each HTML page may have dependencies on one or more records which are portions of the database denoted Table 1, Table2,..., Table6. The correspondence between the tables and pages can be maintained by hash tables and record lists 19. For example, if the cache manager 1 were notified of a change to Table! Tl, it would invalidate (or update) Pagel. Similarly, if the cache manager were notified of a change to Table2 T2, it would invalidate (or update) Pagel, Page2, and Page3. Figure 2 depicts an example of the cache 2. As depicted, each cache 2 preferably has 4 storage areas: a directory 3, maintains information about each cached object; an object storage 4 for storing the objects 6; auxiliary state information S which includes other state information (e.g., statistics maintained by the cache); and a hash table 19, which stores information about records, in the hash table entries 25. In a preferred embodiment, the hash table entries 25 comprise record IDs 12; and object lists 8, which include the list of objects, i.e., object id(s) 9, whose values depend on a record which may be part of a database 99. However, the present invention also allows other kinds of information to be stored in the hash table entries. The purpose of the hash table is to provide an efficient method for finding information about a particular table/record. Preferably hashing is keyed on the record ID 12. Hash tables arc well known in the art (sec e.g., The Design and Analysis of Computer Algorithms", Aho, Hopcroft, Ullman, Addison-Wesley, 1974). Hash tables provide an efficient data structure for the present invention. However, the present invention is compatible with a wide variety of other data structures and is not limited to using hash tables. The directory 3.includes an object information block (OEB) 10 for each object 6 stored in the cache. One of the components of the OIB 10 is a record list 11 (Figure 3) which is used to store all of the record ID's 12 identiiying records rl... r4 associated with a complex object col, co2. Here, the complex objects arc dynamic web pages (page 1... page 5) stored in the cache 2 and the records may be part of a database 99. Although the preferred embodiment uses text strings for record ID's, other methods are compatible as well. An application program communicates with the cache manager 1 via a set of API functions. Examples of APIs in accordance with the present invention are shown in Figure 4. Those skilled in the art that many additional APIs can be implemented in a straightforward manner within the spirit and scope of the present invention. As depicted, the example APIs are: cache_object (objectjd, object, cache Jd) 410: stores an object 6 identified by cache Jd in the cache 2 (Fig. 2) identified by cache Jd under a key objectjd 9; overwriting any previous object 6 having the same key. The present invention is compatible with a wide variety of types for objectjd, object, and cache Jd. In the preferred embodiment, the object 6 may be of several types, the objectjd is a byte string, and the cache Jd is a character string. Here, although multiple items with the same key are preferably not allowed to exist in the same cache concurrently. However, it would be easy for one skilled in the art to use the present invention in a situation where multiple items with the same key could exist in the same cache concurrently. lookup_object (objectjd, cache Jd) 415: look for an object 6 identified by cache Jd with a key objectjd 9 in the cache 2. If any such object 6 exists, return it to the application program. delete_object (objectjd, cache Jd) 420: look for an object 6 identified by cache Jd with a key objectjd 9 in the cache. If any such object 6 exists, delete it add_dependency (objectjd, cache Jd, record Jd) 430: look for an object 6 with a key objecud 9 in the cache 2 identified by cache Jd. If any such object 6 exists and there is no dependency between the object 6 and a record identified by a rccordjd 12 associated with the record Jd, add the dependency. deletejdepcndency (objectid, cacheid, record id) 440: look for an object 6 with a key objectjd 9 in the cache identified by cache id. If any such object 6 exists and there is a dependency between the object 6 and a record identified by record_id 12, delete the dependency. invalidate_record (cache_id, record _id) 450: delete all cache objects from the cache 2 identified by cache id which depend on the record identified by the record id. show_dcpendent_objects (cacheid, record id) 460: return a list of object_ids 9 for all objects in the cache 2 identified by the cache id which depend on the record identified by the record id. This function can be implemented by returning the object list 8 for the hash table entry 25 corresponding to the record identified by record id. A status variable can also be returned to indicate if either the cache 2 or the hash table entry 25 is not found. show_associated_records (cacheid, object id) 470: return a list of record_ids 12 for all records which the object 6, identified by object id in the cache 2 identified by cache id, depends on. This function can be implemented by returning the record list 11 (Fig. 3) for the object 6 identified by the object id in the cache 2 identified by the cache id. A status variable can also returned to indicate if either the cache or the object 6 is not found. Figure 5 depicts an example of the cache manager 1 logic. As depicted, in step 1010 the cache manager receives a command (Fig. 4) from an application program. In step 1020, the cache manager reads the command (Fig. 4) and invokes different logic 1100 ... 1600, described below, based on the command. Figure 6 depicts an example of the cache manager logic 1200 for a cache_object (objectid, object, cache id) 410 command. As depicted, in step 1200, the cache manager 1 determines if the cache id parameter specifies a valid cache 2. If not, the status variable to be returned to the application program is set appropriately, in step 1245. If the cache id specifies a valid cache 2, the cache 2 is preferably locked, to prevent multiple processes from accessing the cache concurrently. That way, consistency is preserved. Those skilled in the art will appreciate that other locking schemes could be used to provide higher levels of concurrency. The present invention is compatible with a wide variety of conventional locking schemes in addition to the example used in the preferred embodiment. In step 1205, the cache manager 1 searches for the object 6 by examining the directory 3 (Fig. 2). If a previous copy of the object 6 is located, the OB 10 for the object 6 is updated, the old version of the object 6 in object storage 4 is replaced by the new one, and the status variable is set appropriately, in step 1215. If, in step 1205, a previous copy of the object 6 is not found, a new OIB 10 for the object 6 is created, initialized, and stored in the directory 3, in step 1210. The cache manager 1 also stores the object 6 in the object storage 4 and sets the status variable appropriately. In step 1230, the cache is unlocked to allow other processes to update it In step 1240, the status variable indicating the result of the command is returned to the application program. Processing then returns to step 1010 (Fig. 5). Figure 7 depicts an example of logic for the lookup_object (objectid, cachejd) 415 command. As depicted, in step 1600, the cache manager 1 determines if the cache_id parameter specifies a valid cache 2. If not, in step 1640 a status variable is set appropriately and returned, in step 1680, to the application program. If the cache_id specifies a valid cache, in step 1610 the cache 2 is locked. step 1630; In step 1620, the cache manager 1 searches for an object 6 corresponding to the objectjd parameter by examining the directory 3 (Fig. 2). If the object 6 is not found: the cache 2 is unlocked in step 1650; the status variable is set in step 1670 and returned to the application program in step 1680. If in step 1620 the object 6 is found: the cache 2 is unlocked in stq and the object 6 is returned to the application program in step 1660. Figure 8 depicts an example of logic for the delete_object (objected, cache_id) 420 command. As depicted, in step 1100 the cache manager 1 determines if the cache 2 corresponding to the - cache id parameter is valid. If not valid, in step 1103 a status variable is set appropriately, and in step 1150 the status variable is returned to the application program. If in step 1100 the cache id specifies a cache 2 which is valid, that cache is locked in step 1105. In step 1107, the cache manager 1 searches for an object 6 corresponding to the object_id parameter by examining the directory 3 (Fig. 2). If the object 6 is not found: the cache is unlocked in step 1108; the status variable is set in step 1109; and in step 1150 the status variable is returned to die application program. If in step 1107 the object 6 is found, in step 1110 the cache manager 1 deletes the objects' associated record list 11 (Fig. 3) and updates the corresponding objects lists 8 (Fig. 2). The cache manager 1 scans each record ID 12 of the record list 11 (Fig. 3) corresponding to the object 6. Note that each record ID 12 on the record list 11 has a corresponding object list 8 (Fig. 2). Pointers to object id(s) 9 (Fig. 2) corresponding to the object 6 being deleted are removed from all such object lists 8. If this results in any object list 8 becoming empty, the corresponding hash table entry 25 is also deleted. After each element of the record list 11 is examined, it can be deleted. In step 1120, the object 6 is deleted from the object storage 4. In step 1130, the corresponding OIB 10 is deleted. Note that step 1120 can be performed concurrently with or before steps 1110 and 1130. In step 1140, the cache is unlocked and in step 1150, a status variable is returned to the application program. Figure 9 depicts an example of logic for the addjdependency (object_id, cache _id, record _id) 430 command. As depicted, in step 1300, the cache manager determines if the cache id parameter specifies a cache 2 which is valid. If not, a status variable is appropriately set, in step 1302 and returned to the application program, in step 1360. If in step 1300, it is determined that the cache id specifies a valid cache, the cache 2 is locked, in step 1305. In step 1310, the cache manager 1 searches for the object 6 corresponding to the object id by examining the directory 3 (Fig. 2). If in step 1310, the object 6 is not found: the cache 2 is unlocked, in step 1315; the status variable is set in step 1317; and an appropriate status variable is returned to the application program, in step 1360. If in step 1310, the object 6 is found: the cache manager 1 examines the record list 11 (Fig. 3) in step 1320 to see if an association (i.e. the dependency information) between The object 6 and a record identified by the record_idalready exists. Alternatively, it can be determined if the record corresponding to the record-id has a hash table entry 25 and if so, to search for the object_id 9 on the object list 8. If in step 1320, a dependency to the object exists, the cache 2 is unlocked in step 1325; the status variable is set appropriately in step 1327; and an appropriate status variable is returned to the application program, in step 1360. If in step 1320, no dependency to the object is found, in step 1330 an objectjd 9 is added to the object list 8 for the record. A new hash table entry 25 and object list 8 are created for the record if needed. In step 1340, a record_id12 is added to the record list 11 (Fig. 3) for the object 6. Note that step 1340 can be executed concurrently with or before step 1330. The cache 2 is unlocked, in step 1350 and the status variable is returned to the application program, in step 1360. Figure 10 depicts an example of logic for the delete_dcpendcncy (object_id cache-id. recordjd) 440 command. As depicted, in step 1400, The cache manager 1 determines if the cache Jd parameter specifies a cache 2 which is valid. If not, in step 1402 a status variable is set appropriately and returned to the application program, in step 1460. In step 1400, if it is determined that the cache -id specifies a valid cache, in step 1405 the cache is locked. In step 1410, the cache manager 1 searches for The object 6 corresponding to The objectjd by examining The directory 3 (Fig. 2). If in step 1410 The object 6 is not found: The cache 2 is unlocked, in step 1412; The status variable is set in step 1415 and returned to The application program, in step 1460. If in step 1410, the object 6 is found: The cache manager 1 examines the record list 11 (Fig. 3), in step 1420 to see if an association (i.e. The dependency information) between The object 6 and a record identified by The record_id already exists. Alternatively, it can be determined if The record corresponding to The record_id has a hash table entry 25 and if so, to search for objectjd 9 on The object list 8. If in step 1420, no dependency is found, in step 1422 The cache 2 is unlocked; The status variable is set appropriately in step 1425; and an appropriate status variable is returned to The application program, in step 1460. If in step 1420, a dependency to The object is found, in step 1430 the objectjd 9 is deleted from The object list 8 for The record. If this makes The object list empty, The hash table entry 25 for the record is deleted. In step 1440, The recordjd 12 is.removed from the record list 11 (Fig. 3) for the object 6. Note that step 1440 can be executed concurrently with or before step 1430. The cache is unlocked, in step 1450 and the status variable is returned to the application program in step 1460. Figure 11 depicts an example of logic for the invalidate_record (cachejd, recordjd) 450 command. As depicted, in step 1500, the cache manager 1 determines if the cachejd parameter specifies a cache 2 which is valid. If the cache is not valid, a status variable is set appropriately in step 1502, and returned, in step 1550 to the application program. If in step 1500, the cache manager 1 determines the cachejd parameter specifies a cache 2 which is valid, the cache 2 is locked, in step 1505. In step 1510, the cache manager determines if the values of any objects 6 are dependent on a record associated with therecord_id by seeing if the record has a hash table entry 25 (Fig. 2). If no hash table entry 25 is found for the record, the cache is unlocked in step 1515 and the status variable is set in step 1517. If in step 1510, a hash table entry 25 is found for the record, the cache manager 1 scans the object list 8 for the record, in step 1520. Each object 6 having an object ED 9 on the object list 8 is deleted from the cache. As each object 6 is deleted, all references to the object 6 from other object lists 8 are also deleted. Such references can be found by traversing the record list 11 (Fig. 3) for the object 6 being deleted. If the deletion of any such reference results in an empty object list, the corresponding hash table entry is deleted. After each element of the object list 8 associated with therecord_id12 (corresponding to therecord_idparameter) is examined, the element can be deleted. In step 1530, the hash table entry 25 for the record is deleted. The cache is unlocked in step 1540 and the status variable is returned to the application program, in step 1550. A straightforward extension of the invalidate_record function which could be implemented by one skilled in the art would be to update one or more objects which depend on therecord_id parameter instead of invalidating them. Step 1099 represents other commands which the cache manager might process. Those skilled in the art will appreciate that there are numerous extensions and variations within the scope and spirit of the present invention. For example, one variation is to allow the cache manager 1 to preserve and update the OD3 10 (Fig. 2) for an object 6 both before the object 6 is ever cached and after the object 6 has been removed from the cache. Using this approach, it would not be necessary to delete the record list 11 for an object 6 and remove the object 6 from all object lists 8 when the object 6 is removed from the cache. That way, dependency information could be preserved and even updated while the object 6 is not in the cache. Another variation would be to allow the cache manager 1 to maintain and update a hash table entry 25 for a record both before any objects are added to the object list 8 and after the object list 8 becomes empty. In other words before the cache manager is aware of any dependency on the record and after all dependencies on the record which the cache manager is aware of become obsolete. This would be particularly valuable if hash table entries 25 include other information in addition to record ID's 12 and object lists 8. Alternative Embodiment Figure 12a depicts another example of a system having features of the present invention. In this as well as the previous embodiment, the present invention can be used for improving the performance of server applications in a conventional client-server environment. One skilled in the an could easily adapt the present invention for other applications which are not client-server based as well. As depicted, a client-server architecture wherein a client 90 communicates with a server 100 over a network 95. A server 100 maintains one or more caches 2'. As is- conventional, the server 100 uses the caches 2' to improve performance and lessen the CPU time for satisfying client 90 requests. Although Figure 12a shows the caches 2' associated with a single server, the caches 2' could be maintained across multiple servers as well. An application running on the server 100 creates objects and then stores those objects on one or more caches 2'. The system can also be architected such that the source of the underlying data in the database 99 and the cache 2' are geographically separated. In this embodiment, an object is data which can be stored in one or more caches 2'. The objects can be constructed from underlying data stored on a database 99. Underlying data include all data in the system which may affect the value of one or more objects. Underlying data are a superset of all objects in the system. According to the present invention, the object manager 120 is preferably embodied as computer executable code ("program") tangibly embodied in a computer readable medium for execution on a computer such as the server 100 (or client 90). The object manager 120 helps determine how changes to underlying data affect the values of objects in the caches 2'. Although Figure 12a shows the object manager residing on a single server, it could be distributed across multiple servers as well. The object manager 120 is preferably a long running process managing storage for one or more caches 2'. The term cache is very generic and can include any application (e.g., a client 90 application) in addition to caches in the conventional sense. One skilled in the art could easily adapt the present invention for an object manager which is one of the following: 1. Multiple distinct processes, none of which overlap in time; and 2. Multiple distinct processes, some of which may overlap in time. This includes multiple concurrent object managers so designed to improve the throughput of the system. Figure 12b depicts an example of an object dependence graph 121 having features of the present invention. The object dependence graph 121 (abbreviated by G) represents the data dependencies between graph objects gobjl ... gobjn. Here, gobjl,...gobj7 represent different graph objects and the edges e in the graph represent data dependencies. For example, the edge from gobj 1 to gobj'5 indicates that if gobjl has changed, then gobjS has also changed. The weight w of the edge is an indication of how much a change to an object, which is the source of an edge, affects the object which is the target of the edge. For example, a change to gobjl would imply a more significant change in gobj'5 than a change in gobj2. This is because the weight w of the edge e from gobjl to gobj'5 is 5 times the weight w of the edge e from gobj2 to gobj'5. The object manager 120 is responsible for maintaining the underlying data structures which represent object dependence graphs (see Figs. 12a-c and 16). Application programs communicate the structure of object dependence graphs to the object manager via a set of APIs (see Figure 18a). The application also uses APIs to notify the object manager of underlying data which have changed. When the object manager 120 is notified of changes to underlying data, it must determine which other objects have changed and notify the caches 2' of the changes. It determines which other objects have changed by following edges in the object dependence graph (see Figure 21). For example, suppose that the object manager 120 is told that gobjl has changed. By following edges in the object dependence graph 121 from gobjl, it determines that both gobj'5 and gobj7 have also changed. As another example, suppose that the object manager is told that gobj7 has changed. Since there are no edges in the object dependence graph for which gobj7 is the source, the object manager concludes that no other objects are affected. Figure 12c depicts an example of an object manager 120 having features of the present invention. As depicted, the object manager 120 includes several storage areas: 1. The object dependence graph G 121 (see Figure 12d) which is implemented by multiple dependency information blocks (Offis) 128. Those skilled in the art will appreciate that the Dffis can be stored using a variety of data structures. Preferably, conventional hash tables are used wherein the Dffis are indexed by object_ids 160. Hash tables are described, for example, in "The Design and Analysis of Computer Algorithms", Aho, Hopcroft, Ullman, Addison-Wesley, 1974. , 2. The multiple record tree (MRT) 122 (see Figs. 27-28). 3. The single record tree (SRT) 123 (see Figs. 27-28). 4. Auxiliary state information 124 which includes but is not limited to the following: a. num_updates 125: a counter num_updates 125, maintained by the object manager for tracking the number of updates the object manager has propagated through the graph. b. consistency stack 128.5 : Used for maintaining consistency during updates. c. .relation info 129 (see Figs. 27-28). 5. program logic \2t Figure 13 depicts an example of the storage areas maintained by each cache 127. Each cache has a cache_id 135 field which identifies it. There are 3 main storage areas: 1. Directory 130: Maintains information about objects. The directory 130 includes multiple object information blocks (OEBs) 10'. Information about an object may be retained in an OIB 10' (FIG. 14) after the object leaves the cache. Those skilled in the art will appreciate that the Offis can be stored using a variety of data structures. Preferably, conventional hash tables are used wherein the Offis are indexed byobject_id's 160. 2. Object storage 132: Where objects contained in the cache arc stored. 3. Auxiliary state information 124: Includes other state information, e.g., the cache_id 135. Figure 14 depicts an example of an OB 10'. The OIB preferably includes the following: object_id 160: assume for the purposes of the following discussion that an object has an objected ol; version_num 141: allows the object manager to uniquely identify different versions of the same object; timestamp 142: a number which indicates how recently the object was calculated; actual_weight 143: the sum of the weights of all edges to ol from a graph object o2 such that the cached version of ol is consistent with the current version of o2; and dcpjist 144: a list representing dependencies to (he object ol. Figure 15 depicts an example of a dep_list 144 element As depicted, each list preferably includes: object_id 160: represents a graph object o2 which has a dependency edge to ol, i.e., o2 is the source and ol the target; weight_act 152: a number representing how consistent the most recent version of o2 is with the cached version of ol. The preferred embodiment uses values of 0 (totally inconsistent) or the weight 165 (Fig. 16) for the corresponding edge in the dependency information block (DIB) 128 (see Figure 16) (totally consistent). A straightforward extension would allow values in between these two extremes to represent degrees of inconsistency; and version_num 153: the version_num of o2 which is consistent with the cached version of ol; Figure 16 depicts an example of the dependency information block (DIB) 128 of Figure 12. As depicted, the DIB 128 preferably includes the following fields: objecUd 160: used by the application program to identify the graph object Assume for the purposes of the following discussion that a graph object has anobject_id ol; version_num 161: version number for the current version of the graph object; timestamp 162: timestamp for the current version of the graph object; storagejist 163 (for graph objects which are objects): list of cache_id's for all caches containing the object; incoming_dep 164: list of (object_id 160, weight 165) pairs for all graph objects o2 with dependency edges to ol. The weight 165 represents the importance of the dependency. For example, higher numbers can represent more important dependencies; outgoing_dep 166: list of allobject_id's for which there exists a dependency edge originating from ol; sum_weight 167: the sum of the weights of all dependency edges going into ol; threshold_weight 168 (forginaph objects which are objects): number representing when an object should be considered highly obsolete. Whenever the actual_weight 143 field in an Offi 10' (Fig. 14) falls below the thrcshold_weight 168 field for the object, the object is considered to be highly obsolete and should be invalidated or updated from the cache; consistency_list 169 (for graph objects which are objects): a list of object_id's 160 corresponding to other objects which must be kept consistent with the current object. Preferably, consistency is only enforced among objects; within the same cache. A straightforward extension would be to enforce consistency of objects across multiple caches. Another straightforward extension would be one which required all objects on the list 169 to be in/out of the cache whenever the object_id is in/out of the cache; latest_object 1601 (for graph objects which are objects): a pointer to the latest version of the object, null if the object manager is unaware of such a copy. This field allows an object to be updated in multiple caches without recalculating its value each time; relational_string 1602: null if the graph object is not a relational object Otherwise, this is of the form: relationjiame (25,30) for SRO's and relation_name (>= 50) for MRO's. The following are only of relevance if relational_string 1602 is.not null; multiple_records 1603: true if the graph object is a multiple record object (MRO), false if it is a single record object (SRO); The following are only of relevance if multiple_records 1603 is true: mro_dep_weight 1604: the weight assigned to an implicit dependency from another relational object tool; and mro_threshold_incremcnt 1605: for each implicit dependency to ol, the amount the threshold_weight should be incremented. Referring again to Figure 12, the object manager preferably also maintains a counter numjupdates 125 (initially zero) which tracks the number of updates the object manager has propagated through the graph. The object manager also maintains a data structure (initially empty) called the consistency stack 128.5 (Figure 12c) which is used to preserve consistency among objects in caches. The application program 97 preferably communicates with the object manager via a set of API functions. Figure 17 depicts examples of several APIs in accordance with the present invention. Those skilled in the art will appreciate that other APIs can be implemented that are straightforward extensions in view of the present invention. Figure 18 depicts an example of the object manager 120 logic for handling different API functions. These functions will be described in detail later. By way of overview, nodes in the object dependence graph G 121 can be created via the API call to the object manager create_nodc (obj_id, initial_version_num, thresh_weight) 181. Dependencies between existing nodes in the graph can be created via the API call: addjdependency (source_object_id. target_object_id, dep_weight) 182. The consistencyjist 169 - corresponding to an object "obj_id" - can be set via the API call: define_consistency_list (obj_id, list of_objects) 183. Nodes can be deleted from G via the API, deletc_node (obj_id) 184. The API cache_latest_version (obj_id, cache) 185 adds the latest version of an object to a cache. The API copy_object (objjd, to_cache_id, from_cache_id) 186 attempts to copy a version of an object from one cache to another cache. Objects are deleted from a cache via the API call: delete_object (obj_id, cache) 187. An application program which changes die value of underlying data must inform the object manager. Two API calls for achieving this are: object_has_changed (obj_id) 188 where the obj_idparameter identifies a graph object; and objects_have_changed (list_of_objects) 189 where the list_of_objects parameter includes a list of (pointers to) graph objects. A node corresRpnding to an SRO is created via the API call create_sro_nodc (obj_id. initial_version_num. thresh_\veight. relation_name. list_of_attribute_ydlues) 190. MRO's are created via the API: create_mro_node (obj_id, initial_version_num, thresh_weight, relation_name, list_of_attribute_condittons, rel_default_weight,rel_defaultjhreshold) 191. The API compare_objects (obj_id, cache_idl, cache_id2) 192 can be used to determine how similar the versions of obj_id in cacheJdl and cache_id2 arc. The API updatc_cache (cache) 193 ensures that all items in the cache are current The API define_relation (relation_name. list_of_flttributes) 194 identifies relations to die object manager. When one or more records change, the object manager can be informed of this via the APIs record_has_changed (relation_name, list_of_attribute_yalues) 195 and rccords_have_changed (relation_name, list_of_attribute_conditions) 196. Nodes in the object dependence graph G 121 are created via the API call to the object manager creatc_nodc (obj_id, initial_version_num, thresh_weight) 181. Those skilled in the art that many additional APIs can be implemented in a straightforward manner within the spirit and scope of the present invention. For example, APIs can be added for modifying theobject_id 160, version_num 161, and thrcshold_wcight 168 fields after a node has been created. Dependencies between existing nodes in the graph are created via an API call: add_dependency (source_object_id, target_object_id, dep_weight) 182. Those skilled in the art that many additional APIs can be implemented in a straightforward manner within the spirit and scope of the present invention. For example, APIs can also be added to delete dependencies and modify dependency weights. The consistencyjist 169 - corresponding to an object "obj_id" - is set via an API call: define_consistency_list (obj_id, list_of_objects) 183. The consistency list for the obj_id is preferably not allowed to include the obj_id as a member. The APIs prevent this from occurring. APIs can similarly be added within the spirit and scope of the present invention to modify die consistency lists 169 after their creation. Changes to the dependency information block (DIB) 128 (Figure 16) for an object, after an object has been cached may require updates to one or more caches 127. These are straightforward. In the event of a new dependency to a cached object ol from a new graph object 02, the new dependence is obsolete if the object manager doesn't know when o2 was created, or the DIB timestamp 162 for o2 > Offi timestamp 142 for ol. Nodes can be deleted from G via the API, delete_nodc (obj_id) 184. Objects can be explicitly added to caches via the APIs: cachc_latcst_version (obj_id, cache) 185; and copy_object (obj_id, to_cache_id, from_cache_id) 186. These APIs create new Offi's 135 in the cache directory if they dont already exist for the object. Figure 19 depicts an example of the API, cachejatest_version (obj_id, cache) 185. As depicted, in step 2030, it is verified that the obj_id and cache parameters specify existing objects and caches, respectively. If so, processing proceeds to step 2040. If not, an appropriate status message is returned and processing proceeds to step 2010. In step 2040, it is determined if the latest version of an obj_id is in the cache. If so, processing continues with step 2010. If not, in step 2050 an attempt is made to obtain the latest version of obj_id from the latest_object field 1601 in the dependency information block (DIB) 128 (Fig. 16). If this field is null, in step 2050, the latest value of obj_id (and possibly makes its value accessible through the latest_objcct field 1601 of the DIB) is calculated, and the version_num field 161 in the dependency information block (DIB) 128 (Fig. 16) is updated. In step 2050, either the new version of obj_id is recalculated entirely, or just portions of it, and the new parts merged with parts from existing versions. The latter method is often more efficient than the former. An OB 10' for obj_id is created in the directory 130 for the cache, if one doesn't already exist If the cache previously contained no version of the obj_id, the cache is added to the storage_list 163 of obj_id. The version_num 141 and timestamp 142 fields of the Offi 10' (FIG. 14) are set to the version_num 161 and timestamp 162 fields of the dependency information block (DIB) 128 (Fig. 16). The actual_weight field 143ofthcOIB 10' (FIG. 14)is set to the sum_weight field 167 of the DIB. For each (o2, wcight_act, version_num) triplet belonging to the depjist 144 of the OIB 10' (FIG. 14), the weight_act 152 is set to the weight 165 for the corresponding edge on the incoming_dep 164 of the DEB. The version_num 153 is set to the version_num 161 field contained in the DIB for o2. In step 2060, it is insured that consistency is preserved. This function recursively insures that all noncurrent objects obj2 on the consistency list 169 for obj_id are updated or invalidated whenever the timestamp 142 in the Offi 10' for obj2 is before the timestamp 162 in the DIB 128 for obj_id. If any such objects obj2 are updated in this process, a similar procedure is applied recursively to the consistency lists 169 for each said obj2. The ordering of Steps 2050 and 2060 is not critical to the correctness of this embodiment Figure 20 depicts an example of the API, copy_object (obj_id, to_chche_idfrom_chche_id) 186. As depicted, in step 2100 it is verified that the obj_id, to_chche_id, and from_cache_id parameters are all recognized by the object manager. If so, in step 2110 it is determined if from_cache_id has a copy of obj_id. If not, nothing happens and processing proceeds to step 2010. A status variable is set appropriately for this (and other cases as well) and is returned to the application program to indicate what happened. Otherwise, processing continues to step 2120, in which it is determined \fto_chche_id andfrom_chche_id include identical versions of obj_id. If so, no copying needs to take place, and processing continues to step 2010. Otherwise, step 2130 determines if from_cache_id contains the latest version of the obj_id. If so, in step 2140, the object is copied to the object storage 132 area of to_chche_id and the cache directory 130 is updated. An OIB 10' for obj_id is created in the directory 130 for to_chche_id if one doesn't already exist lfto_chche_id previously contained no version of obj_id, to_chche_id is added to the storagejist 163 of obj_id. In step 2170, consistency is preserved by insuring that all noncurrent objects on consistency lists 169 with OIB time stamps 142 prior to die DIB timestamp 162 of obj_id are either updated or invalidated. Otherwise, if the result of step 2130 is negative, in step 2150 the object will not be allowed to be copied unless: (1) all objects on the consistency list 169 for obj_id for which noncurrent versions are stored in to_chche_id have the same timestamp 142 as the timestamp 142 for the version of obj_id in from_chche_id; and (2) all objects on the consistency list 169 for obj_id for which current versions are stored in to_cache-id have the same or earlier timestamp 142 as the timestamp 142 for the version ofobj_id \nfrom_chche_id. If these conditions are satisfied, in step 2160 objj'd is copied from from_chche_id to to_chche_id. An OIB 10' for the obj_id is created in the directory 130 for to_chche_id if one doesnt already exist, \fto_cache_id previously contained no version ofobj_id, to_cache_id is added to the storagejist 163 ofobj_id. A straightforward extension to the copy_object and cache_latest_version APIs would be flags which could prevent an object from being stored if other objects on the consistency list would also need to be updated. Another straightforward extension would be additional flags which would only place theobject_id in a cache if the cache did not include any version of the object_id. Another straightforward extension would be a system where the object manager maintained all previous versions of an object We could then have APIs for adding a specific object identified by a particular (object_id, version_num) pair to a cache. Objects are deleted from a cache via the API call: delcte_object (obj_id, cache) 187. One skilled in the art will appreciate that it is straightforward to implement this function in accordance with this detailed description. An example of a function performed by this API call is the removal of cache from the storagejist field 163 of the dependency information block (DIB) 128 (Fig. 16) for the object identified by obj_id. An application program which changes the value of underlying data must inform the object manager. Two API calls for achieving this are: object_has_changed (obj_id) 188 where the obj_id parameter identifies a graph object; and objects_have_changed (list_of_objects) 189 where the list_of_objects parameter includes a list of (pointers to) graph objects. If the graph objects on list_of_objects affect many other graph objects in common, the objects_have_changed API will be more efficient than invoking the object_has_changed API, once for each graph object on a list. Figure 21 depicts an example of the API, objectsi_havc_changed (list_of_objects) 189. Those skilled in the art will appreciate that it is straightforward to then implement the API, object_has_changed (obj_id). For ease of exposition, we assume that each element of list_of_objects corresponds to a valid node in G and that no two elements on the list_of_objects refer to the same node. It would be straightforward to adapt this function from the detailed description for situations where this is not the case. As depicted, in step 2400 increment the counter numjupdates 125 (Figure 12c) by 1. In step 2402, it is determined if all nodes corresponding to the graph objects specified by the list_of_objects parameter have been visited. If so, in step 2403, the update propagation phase (see Figure 22) is followed, in step 2404, by the consistency check phase (see Figure 26). If not, in step 2405, a new node corresponding to a graph object on the list_of_objects is visited. Let obj_id be the object_id 160 for the node. The object manager increments the version_num field 161 in the dependency information block (DIB) 128 (Fig. 16) for obj_id by 1 and sets the timestamp field 162 to the value of numjupdates 125. Steps 2406 and 2408 represent a loop which notifies each cache cl containing obj_id (obtained from storagejist 163) to update or invalidate its version of obj_id. In step 2406, a function update_or_invalidate (cl, obj_id) (see Figure 25) is invoked to cause this to happen. Those skilled in the art will appreciate that it is straightforward to apply selectivity in step 2406 in deciding which caches must update or invalidate their copies of obj_id. Figure 25 depicts an example of the update_or_ invalidate (cacheid, objectid) logic. It is called whenever the version of objectid currently in cacheid must either be updated or invalidated (see e.g., step 2406, Figure 21). As depicted, in step 2407 it is determined whether the objectid should be updated in the cacheid. If the answer is no, the objectid is invalidated from the cache in step 2440 and the procedure returns, in step 2441. If the answer is yes, in step 2442 the following changes are made to the OIB 10' (FIG. 14) for objectid: \. The version_num 141 and timestamp 142 fields are set to the current version_num 161 and timestamp 162 fields contained in the dependency information block (DEB) 128 (Fig. 16). 2. The actual_weight field 143 is set to the sum_weight field 167 in the DEB. 3. The depjist 144 (Fig. 15) is updated. Each member of the list 144 corresponds to a graph object o2 which has a dependency to the object identified by objectid. The weight_act 152 is set to the weight 165 field in the dependency information block (DEB) 128 (Fig. 16) corresponding to the same edge in G if these two quantities differ. In addition, vcrsion_num 153 is set to the version_num field 161 contained in the DEB foro2 if these two quantities differ. In step 2444, the actual value of objectid contained in the object storage area 132 is updated. First, an attempt is made to obtain the updated version of objectid from the latest_object field 1601 in the dependency information block (DEB) 128 (Fig. 16). If this succeeds, step 2444 is over. If this fails (i.e., this pointer is nil), the updated version of objectid is calculated, e.g., by either calculating the new version of objectid entirely or just recalculating portions of it and merging the new parts with parts from existing versions. The latter method is often more efficient than die former. In either case, the object manager then has the option of updating the latest_object field 1601 in the DIB so that other caches which might need the latest version of the objectid can simply copy it instead of recalculating it. In some cases, in step 2444 the actual value of the objectid can be updated with a later version of the objectid, preferably the latest easily accessible one (which would generally be the cached version with the highest vcrsion_num 141) which is not actually current. This is advantageous if calculating the current value of objectid is prohibitively expensive. Preferably, this type of update would not be allowed if either of the following are true: 1. the objectid is one of the objects on the list passed to objects_have_changed (list-pf_objects); or 2. For the later version of objectid, it is still the case that actual_weight 143 168. In step 2443, (object_id 160, cacheid) pairs are added to the consistency stack 128.5 (Fig. 12) for each object on the consistencyjist 169 which is in the cache identified by cacheid. The object manager 120 makes sure that all cached items on the consistency stack 128.5 are consistent in the consistency check phase (Fig. 26). The consistency stack could be implemented in several fashions; two possible structures are lists and balanced trees (Reference: Aho, Hopcroft, Ullman). Lists have the advantage that insertion is constant time. The disadvantage is that duplicate copies of items could end up on them. Trees have the advantage that no duplicate items need be stored. The disadvantage is that insertion is 0(log(n)), where n is the number of items on the consistency stack. Step 2443 may optionally apply more selectivity before adding an object to the consistency stack. Let object_id2 be an object on die consistency list 169 which is in cacheid. If cacheid contains a current version of object_id2, (object_id2, cacheid) doesn't have to be added to the consistency stack. The version is current if both of the following are true: 1. The vertex corresponding to object_id2 has already been visited in processing the current call to objccts_have_changed (list _of_objects) 189. This is true if and only if the timestamp field 162 in the dependency information block (DIB) 128 (Fig. 16) for object _id2 is equal to numjupdates 125;and 2. The version_num field 141 in the OB 10' (FIG. 14) and 161 in the DIB forobjecud2 are the same. If step 2443 determines that both (1) and (2) are true, (object_id2, cacheid) is not added to the consistency stack. If (1) is true but (2) is false, step 2443 could recursively invoke update_or_invalidate on object_id2 and cacheid which would obviate the need for adding (object_id2, cache_id) to the dependency stack. One skilled in the art could easily implement Steps 2442,2443, and 2444 in any order or in parallel from the description. Figure 22 depicts an example of the update propagation phase for objects_have_changed (list_of_objects) 189. The basic function performed by Steps 2403 and 2416 is to traverse all edges of the graph G accessible from the list_of_objects. The preferred technique is analogous to a depth-first search ("dfs") (reference: Aho, Hopcroft, Ullman). However, one skilled in the art could easily adapt the technique to work with other graph traversal methods such as a breadth-first search. Figure 23 depicts an example of a part of a method for propagating changes through the object dependence graph in a depth first manner, in response to changes to underlying data (dfs). Suppose an edge from a first node objl to a second node obj2 has just been traversed. In step 24 1 6, it is determined if the node ob/2 has been visited yet The answer is yes if and only if the timestamp 162 (Fig. 16) forobj2 = num_updates 125 (Fig. 12). If the result from step 24 1 6 is true, processing continues at step 24 1 7. This step is part of a loop where all caches on storagejist 163 (Fig. 16) are examined to see if they include a copy ofob/2. Recall that each object preferably has an object_id field and a version_number field. The objecUd field corresponds to something which an application program would use to identify the object (e.g., the URL), while the version_number field allows multiple objects with the same object_id to be maintained and uniquely identified. For each such cache cacheid, in step 2420 it is determined if the version ofob/2 is current by comparing the version_num field 141 in the OIB 10' (FIG. 14) with the version_num field 161 in the dependency information block (DIB) 128 (Fig. 16). If the result from step 2420 is affirmative, in step 2421 it is ensured that on the dcpjist 144 for ob/2t the element corresponding to objl has a version_num 153 = version_num 161 in the DIB If the result from step 2420 is negative, i.e., the version of obj2 is not current, a function decrease_weight (cacheid, objl. obj2) is invoked (See Figure 24). Recall that each edge can have a non negative number associated with it known as die weight which represents the importance of the data dependence. For example, high numbers can represent important dependencies, while low numbers represent insignificant dependencies. Recall also that objects can also have a value known as the threshold _weighl associated with them. Whenever the sum of the weights corresponding to incoming data dependencies which are current falls below the threshold_weight, the object is considered to be highly obsolete. Such objects should be updated or invalidated for applications requiring recent versions of objects. If the result of step 2416 is false, in step 2423 the version_num field 161 for obj2 is incremented and the timestamp field 162 is set to numjupdates 125 (Fig. 12) which indicates that obj2 has been visited. Step 2424 is part of a loop where all caches which on the storagejist 163 are examined to see if they include a copy of obj2. For each such cache cacheid, in step 2425 the decrease_weight (cacheid, objl. obj2) function is invoked. After this loop exits, in step 2426 the dfs logic (Figure 23) is recursively invoked on all outgoing edges from obj2. Figure 24 depicts an example of the decrease_weight (cacheid. from_obj, to_obj) logic. As depicted, in step 2425 the actual_weight field 143 for to_obj is decremented by w where \v is the weight_act field 152 corresponding to the edge fromfrom_objtoto_obj. In step 2435, it is determined if the actual_weight 143 After the update propagation phase, the object manager must ensure that the consistcncyjists 169 are in fact consistent. This is done in the consistency check phase depicted in Figure 26. As depicted, step 2404 is part of a loop which examines each (object_id 160, cache_id 135) pair in the consistency stack 128.5 (fig. 12c). For each such pair, in step 2451 it is determined if the version of object_id in the cache cache_id is current by comparing the vcrsion_num field 141 with the version_num field 161. If the answer is yes, processing returns to step 2404.. Otherwise, the object must either be updated or invalidated. In step 2455 it is determined whether the object should be updated. If the answer is no, the object is invalidated in step 2440 described earlier (see Figure 25). If the answer is yes, die latest value is added to the cache in step 2050 and the new consistency constraints are satisfied in step 2060 which were both described earlier (see Figure 19). The ordering of steps 2050 and 2060 is not critical to the correctness of mis example. Another API, update_cache (cache) 193, ensures that all items in the cache are current. It does so by examining the OIB's for every object in the cache and invalidating or updating obsolete items. It ignores consistency lists because all objects will be current and therefore consistent after the function completes. Relations The present invention has special features for handling records (These records are not synonymous with records used in the preferred embodiment) which may be part of a relational database (see "Understanding the New SQL: A Complete Guide" by J. Melton and A. R. Simon, Morgan Kaufmann, 1993). For example, suppose that a relation rel_name has die attributes age and weight, both of type integer. For the following: rel_name(age = 25, weight = 34) represents a single record; while rel_name (age = 25) is a multirecord specifier (MRS) and represents all records belonging to rel_name for which age=25. The present invention has features allowing objects which correspond to either single or multiple records to be managed. Such objects are known as relational objects. A single object can represent multiple records from the same relation. Such an object is known as a multiple record object (MRO) while an object corresponding to a single record is known as a single record object (SRO). An MRO objl contains (includes) another relational object obj2 if the set of records corresponding to obj2 is a subset of the set of records corresponding to objl. The object manager automatically adds dependencies from a relational object to an MRO which contains it The object manager maintains a balanced tree known as the multiple record tree (MRT) 122 which contains pointers to all MRO nodes in G and is indexed alphabetically by the relational_string field 1602 in the dependency information block (DEB) 128 (Fig. 16). A balanced tree known as the single relation tree (SRT) contains pointers to all SRO nodes in G and is also indexed alphabetically by the relational_string field 1602 in the DIB. An alternative approach which is easy to implement from this description would be to maintain a single balanced tree for both single and multiple relations. Another variation would be to use data structures other than balanced trees for maintaining this information. According to the present invention, before a relational object is created, the relation must be identified to the object manager via the API: define_relation (relation_name, list_of_attributes) 194 Each element of the list_of_attributes argument is a pair containing the name and type of the attribute. The API define_relation 194 stores information about the relation in the relation info area 129 (Fig. 12). Figure 27 depicts an example of the logic for creating a node corresponding to a single record object (SRO). Recall that an object corresponding to a single record is known as a single record object (SRO). A balanced tree known as the single relation tree (SRT) contains pointers to all SRO nodes in G and is also indexed alphabetically by the relational_string field 1602 in the DIB (Fig. 16). A node corresponding to an SRO is created via the API create_sro_node (objjd, initial_version_num. thresh_weight, relation_name, list_of_attribute_values) 190 (Fig. 18a). Referring now to Figure 27, in step 2300 it is determined if all input parameters are valid (e.g., they are of the right type, etc). It is also verified that the relation "relation_name" was previously defined via a call to define_relation 194 by examining the relation info area 129. It is also verified that the list_of_attribute_values contains the correct number of values and that all values are of the correct type. It is also verified that a node for objjd or a node corresponding to the same record doesn't already exist (it would be easy to modify the design so that if a node for objjd already existed, the old node would be overwritten. It would also be easy to modify the design so that multiple nodes with the same obj_id could exist It would also be easy to allow multiple nodes to correspond to the same record. If it is determined that all parameters are valid, processing continues with step 2305. Otherwise, create_sro_node returns at step 2320 with an appropriate status message. In step 2305 a new node is created in G by initializing theobject_id 160 to objjd; version_num 161 to initial_version_num; threshold_weight 168 to thresh_weighf, and relational_string 1602 to relation_name concatenated with all of the attribute values. The relation and attribute values comprising relational_string 1602 are preferably all separated by delimiters. That way, it is easy to identify the relation and each attribute value easily from the relational_string 1602. A multiple_records 1603 field (fig. 16) is set to false. In step 2310, a pointer to the node is added to the SRT. The position of the new pointer in the SRT is determined from relational_string 1602. In step 2315 dependencies are added from the obj _LJd to each multiple record object (MRO) containing it Such MRO's are found by examining the multiple record tree MRT 122. The MRT is preferably a balanced tree which contains pointers to all MRO nodes in G and is indexed alphabetically by the relationaLstring field 1602 in the dependency information block (DIB) 128 (Fig. 16). It is only necessary to examine MRO's for relation_name. All such MRO's can be identified in O(log (n) + m) instructions where n is the total number of MRO's and m is the number of MRO's for the relation_name. For each MRO "obj2_id" containing objjd, a dependency from objjd to obj2_id is created. Referring again to Figure 16, the dependency is preferably initialized with a weight of the mro_dep_weight 1604 for obj2_id. The threshold_wcight 168 for obj2_id is incremented by mro_threshold_incrementl605 for obj2 _id. A straightforward extension to the algorithm would be to use other techniques for assigning weights to the dependency and modifying the threshold_weight 168. Returning now to Figure 27, in step 2320, the process returns with a status message. The order of steps 2305,2310, and 2315 can be varied. Furthermore, these steps can be executed concurrently. Figure 28 depicts an example of logic for creating multiple record objects (MROs). MRO's are created via the API: create_mro_node (objjd, initial_yersion_num. thresh_weigh, relation_name, list_of_attribute_conditions,rel_default_weight,rel_defaul _threshold) 191 (Fig. 18a); attribute conditions are of the form: = 25; > 96; > 45 and Recall that a single object can represent multiple records from the same relation. Such an object is known as a multiple record object (MRO) while an object corresponding to a single record is known as a single record object (SRO). An MRO objl contains another relational object obj2 if the set of records corresponding to obj2 is a subset of the set of records corresponding to objl. The object manager automatically adds dependencies from a relational object to an MRO which contains it. The object manager also preferably maintains a balanced tree known as the multiple record tree (MRT) 122 which contains pointers to all MRO nodes in G and is indexed alphabetically by the relational_string field 1602 in the dependency information block (DIB) 128 (Fig. 16). A balanced tree known as the single relation tree (SRT) contains pointers to all SRO nodes in G and is also indexed alphabetically by the relational_string field 1602 in the DIB. As depicted, in step 2600, it is determined if all input parameters are valid (e.g., they are of the right type, etc). In addition, it is verified that the relation "relationjiame" was previously defined via a call to define_relation 194 API (Fig. 18a) by examining the relation info storage area 129 (Fig. 12). It is also verified that the list_of_attribute_conditions is valid; and that a node for objjd or a node corresponding to the same set of records doesn't already exist Those skilled in the art will appreciate that it would be easy to modify the design so that if a node for obj_id already existed, the old node would be overwritten. It would also be easy to modify the design so that multiple nodes with the same objjd could exist. It would also be easy to allow multiple nodes to correspond to the same set of records. If the result of step 2600 is a determination that all parameters are valid, processing continues with step 2605. Otherwise, create_mro_node returns at step 2620 with an appropriate status message. In step 2605, (with reference also to Figure 16) a new node is created in G (Fig. 17) by initializing the object_id 160 to objjd, version_num 161 to initial_version_num, threshold_weight 168 to thresh_weight, and relational_string 1602 to relationjiame concatenated with all of the attribute conditions. The relation and attribute conditions comprising the relational_string 1602 are all separated by delimiters. That way, it is easy to identify the relation and each attribute condition easily from the relational_string 1602. The multiple_records 1603 field is set to true; the mro_dcp_weight 1604 is set to rel_default_weight, and the mro_threshold_increment 1605 is set to rel_default_threshold. In step 2610, a pointer to the node is added to the MRT. The position of the new pointer in die MRT is determined by relational_string 1602 . In step 2615 dependencies are added from obj_id to each MRO containing it, in the same manner as step 2315. For each object obj2_id contained by objjd, in step 2625 a dependency is added from obj2_id to obj_id. Such dependent objects are found by searching both the MRT 122 and SRT 123 and considering all other relational objects for relation_name. Each dependency is assigned a weight of the mro_dep_weight 1604 foT obj_id. For each such dependency, the threshold_weight 168 for objjd is incremented by the mro_thrcshold_increment 1605 for objjd. Those skilled in the art will appreciate that other techniques can be used for assigning weights to the dependency and modifying the threshold_weight 168. In step 2620, crcate_mro_node returns with a status message. The order of steps 2605,2610,2615, and 2625 can be varied. Furthermore, these steps can be executed concurrently. Alternatively, the weight of a dependency from a relational object objl to an MRO obj2 which contains it could be based on the proportion and importance of records corresponding to obj2 which are also contained in objl. This variant could be applied to Steps 2315,2615, or 2625. Another alternative would be to selectively add dependencies between MRO's when neither MRO is a subset of the other but the two MRO's have one or more records in common. Returning now to Figure 16, those skilled in the art will appreciate that within the spirit and scope of the present invention APIs can be added to modify the relational_string 1602, multiple_rccords 1603, mro_dep_weight 1604, and mro_threshold_incremcnt 1605 for a relational object after the object has been defined via die create_sro_node 190 or the create_mro_node 191 APIs. When one or more records change, the object manager can be informed of this via the APIs (Fig. 18a) record_has_changed (relation_name, list_of_attrtbute_yalues) 195 and records_have_changed (relation_name, list_of_attribute_conditions) 196. These APIs automatically propagate changes throughout the dependence hierarchy. Figure 29a depicts an example of how the records_have_changed (relation_name, list_of_attribute_conditions) 196 API can be implemented. Those skilled in the art will appreciate that it is straightforward to implement the rccord_has_changed (relation_jiame, list_of_attribute_values) 195 API therefrom. As depicted, in step 2700 it is determined if the input parameters are valid. It is also verified that the relation relation_name was previously defined (via a call to the define_relation 194 API (Fig. 18a)) by examining the relation info area 129 (Fig. 12). It is also verified that the list_of_attribute_conditions is valid. If die input parameters are valid, processing proceeds to step 2710. Otherwise, in step 2730 the procedure is aborted with an appropriate status message. In step 2710, all relational objects are found which include at least one record which has changed. This can be done by examining all relational objects on the MRT 122 and SRT 123 (Fig. 12) which correspond to the relation_name. In step 2720, the changes can be propagated to other nodes in G by invoking the objects_have_changed 189 API on the list of all objects identified in step 2710. Finally, in step 2730, records_have_changed returns an appropriate status message. A straightforward variant of the records_have_changed API would be to consider the proportion and importance of records in a relational object which have changed in determining how to propagate change information throughout G. The API compare_objects (obj_id, chche_idl. cache _id2) 192 (Figure 18b) can be used to determine how similar the versions ofobj_id in chche_idl and cache_id2 are. For example, the version_num 141 fields can be compared to see if the two versions are the same. If they are different, an indication can be provided of how much more recent one object is from the other, for example, by the difference in the vcrsion_num 141 and timestamp 142 fields (Fig. 14). If the two versions of the object are different, a similarity score can be computed ranging from 0 (least similar) to scores are preferably based on the sum of weights of incoming dependencies to obj_id from graph objects obj_id2, for which the version of obj_id2 consistent with obj_id" in chche_idl, is identical to the version of obj_id2 consistent with obj_id in cache_id2. The similarity score (SS) can be calculated using the formula: SS = common_weight / sum_weight 167 where common_weight = sum of weight 165 corresponding to edges from graph objects obj_id2 to objj'd where the version_num 153 fields corresponding to the edges are identical for both versions of obj_id. The compare_objects logic can also be used to determine whether the two versions are highly dissimilar or not They are highly dissimilar if and only if common_weight Extensions A straightforward extension to the present invention would be to include threshold_weight fields in OIBs (Fig. 14) and to let caches 2' (Figure 13) set these fields independently. Another straightforward extension to would be to allow different consistency lists for the same object corresponding to different caches. A further extension would be a system where multiple dependencies from a graph object objl to another graph object obj2 could exist with different weights. Application programs could independently modify these multiple dependencies. Still another extension would be to use other algorithms for determining when an object is obsolete based on the obsolete links to the object When a graph object changes, the preferred embodiment does not consider how the graph object changes when propagating the information through the dependence graph G. It only takes into account the fact that the graph object has changed. An extension would be to also consider how a graph object changes in order to propagate the changes to other graph objects. This could be done in the following ways: 1. By providing additional information about how a graph object has changed via parameters to functions such as the object_has_changed. This information would be used to modify links from the graph object to other graph objects which depend on its value and would be subsequently used to determine how successors to the graph object have changed. 2. When the object manager 120 determines that a graph object o2 has changed, the object manager could consider both: which predecessors of it have changed; and any information that it has recursively collected on how the predecessors have changed. The object manager would then use this information to determine how o2 has changed. The information on how o2 has changed would be used to modify links to other graph objects which depend on o2 and would be subsequently used to determine how successors to o2 have changed. For example, consider Figure 29b. u2 and u3 are underlying data which have changed. The object manager propagates the change information to ol and o3. When the object manager propagates change information to o2, it not only considers the weights of the edges from ol and o3 to o2 in determining how to update or invalidate cached copies of o2. It also considers the nature of the changes to u2, u3, ol. and o3. This information may also be used to determine how to update or invalidate cached versions of o4. Other Applications The present invention can also be used in a system where an application has to make a decision on whether or not to update underlying data. By examining the object dependence graph, the system can determine the other objects affected by the changes to the underlying data. If this set is satisfactory, the changes could be made. Otherwise, the system could refrain from making the changes to the underlying data. Those skilled in the art will appreciate that the present invention could also be used by a compiler, run-time system, or database in order to efficientiy schedule operations. Different schedules could result in different changes to underlying data. By analyzing the object dependence graph, the program making scheduling decisions could determine a favorable method to schedule operations. Detailed Description of a Scaleable Method for Maintaining and Consistently Updating Caches This embodiment of the present invention is designed to function on a collection of one or more physical (computer) systems connected by a network. There may be more than one instance of the present invention residing in this collection of systems. Although dictionary are also implied, the following definitions are provided for guidance to distinguish among multiple instances of the present invention. Object Sources. Object Sources include one or more products such as are sold by IBM under the trademark DB2 and by Lotus under the trademarks LOTUS NOTES and DOMINO Server, or Other Sources 3030 including data or objects from which more complex objects (such as HTML pages) are built. Trigger. Any means which can be used to cause actions to occur automatically in response to modification in the data. A trigger is a standard feature of many standard Object Sources such as are sold by IBM under the trademark DB2 and by Lotus under the trademarks LOTUS NOTES and DOMINO Server to cause actions to occur automatically in response to modification in the data. One embodiment of the present invention uses triggers in a novel way to keep objects built from data stored in an Object Source synchronized with the data. Trigger Notification. This is a message sent to the present invention in response to a trigger being invoked within an Object Source. Cache transactions. Include requests to a cache manager to read, update, or delete cache objects. Trigger Monitor. An example of logic in accordance with the present invention for keeping the objects in a cache managed by a Cache manager synchronized with associated remote data. The Trigger Monitor can be a single long running process monitoring remote data sources for the purpose of keeping complex objects stored in a cache managed by a Cache manager synchronized the underlying data. Master Trigger Monitor. This an instance of a Trigger Monitor which receives Trigger Notifications. Slave Trigger Monitor. This is an instance of a Trigger Monitor to which Trigger Notifications are forwarded from a Master trigger monitor 3000' (that is, not from Object Sources directly). Local Cache. This is a cache (or other standard object store such as a file system) which is updated by an instance of a Trigger Monitor residing on the same physical machine as the cache itself. Remote Cache. This is a cache (or other standard object store such as a file system) which is updated by an instance of a Trigger Monitor residing on a different physical machine from the cache itself. It is possible for the present invention to play the role of both Master 3000 (if it receives trigger events) and Slave 3000a (if it receives notifications of trigger events from some master). Referring now to the drawings, Figure 30a depicts a block diagram example of a system having features of the present invention. As depicted, the system includes (one or more) remote nodes 3108. The nodes 3108 can be servers providing Web pages to clients via Web servers (denoted as httpd 3080). Each Web server can provide a significant percentage of dynamic Web pages which are constructed from a database 3010. Each such server node 100, because of the cost involved in generating Web pages, caches one or more objects 3004 including complex objects such as dynamic Web pages. Multiple requests for the same dynamic page can be satisfied from the cache 3003, thus reducing overhead. The use of multiple server nodes 3108 increases the volume of requests that the system can service. It is possible, although not a requirement, that the servers nodes 3108 can be separated geographically by long distances. In accordance with the present invention, when a change to an object source such as the database 3010 occurs which might affect the value of one or more objects 3004 stored in a cache 3003, a trigger monitor 3000 notifies each cache manager 3001 of the objects whose values have changed. The trigger monitor 3000 might inform a cache manager 3001 that an object 3004 in its cache 3003 has changed. In this case, the cache manager 3001 could invalidate its copy of the object 3004. Alternatively, the trigger monitor 3000 could inform a cache manager 3001 that an object 3004 has changed and also provide the new value of the object 3004. Those skilled in the art will appreciate that the new value for the object 3004 could be computed on the data server node 3102 as well as the remote node 3108 or some intermediate, e.g., proxy node. In either alternative case, the cache manager would also have the option of dynamically updating the object 3004, e.g., storing the new version, without having to invalidate it. Figure 30b depicts a more detailed example of the Trigger Monitor 3000. Here, the Trigger Monitor 3000 is instantiated as a Master Trigger Monitor 3000'. As depicted, the maintenance of caches 3003 including complex object 3004s is done by a process (or collection of processes) according to the present invention called the Trigger Monitor 3000. The Trigger Monitor 3000 is preferably a single long running process monitoring data sources 3050 for the purpose of keeping the contents of a Cache manager 3001 synchronized with the underlying data. A Master trigger monitor 3000' is an instance of a Trigger Monitor 3000 which receives Trigger Events 3020. The Master Trigger Monitor 3000' includes: a Trigger Monitor Driver 3040; Object Id Analysis 3041 logic; Object Generator 3042 logic; and a Distribution Manager 3043. The Master Trigger Monitor 3000' works in conjunction with Object Sources-3050, cache manager 3001 (known as a local cache manager), and zero or more other (Slave) Trigger Monitors 3000" (Fig. 30c) and a remote cache manager 3002, which reside on other physical machines. Object Sources 3050 include one or more entities; for example a database 3010 such as is sold by IBM Corp. under the trademark DB2; or any Other Sources 3030 such as a server sold by Lotus Corp. under the trademark DOMINO, from which more complex objects (such as HTML pages) are built. When an Object Source 3050 detects a change, a trigger is invoked. The trigger, which is a standard feature of many standard Object Sources 3050 such as the above, is typically used to cause actions to occur automatically in response to modification of the data. The present invention uses triggers in a novel way to keep object 3004 built from data stored in an Object Source synchronized with the data. Associated with the trigger is a sendjtrigger 3026 API (see Figure 30d) which causes a message to be sent to the Trigger Monitor Driver 3040. In response, the Trigger Monitor Driver 3040 can then generate a transaction (see Figure 30e) called a Trigger Event 3020. The Trigger Event 3020 can be translated (by conventional means) into a Record ID 3012 and forwarded to a Cache Manager 3001 for translation. The Cache Manager 3001 returns a corresponding list of Object IDs 3009 which are enqueued to the Object Id Analysis (OIA) component 3041. The OIA 3041 generates, by well known means, a set of Object Disposition Blocks (ODB) 3100 (described below), one for each Object ID 3009. Figure 31 depicts an example of the Object Disposition Block (ODB) 3100. The Object ID 3009 is used to identify an object 3004 in the cache 3003 when subsequently replacing or deleting the objects. The Cache Id 3200 is used to identify which of the caches 3003 the objects 3004 belongs in. The External ID 3101 is an additional identifier by which the Object Generator 3042 might know the object. The Request Disposition 3103 is used by the Object Generator to generate an Update Object Request 3022 or a Delete Remote Object Request 3025 (Figure 30e). If the request disposition 3103 is a DispRegcnerate 3130, the objects 3004 represented by the ODB 3100 are regenerated by the system and distributed. If the request disposition 3103 is a Displnvalidate 3131, the objects 3004 are deleted from all systems. Figure 32 depicts an example of the cache ID 3200. As depicted, the Cache ED preferably includes a cache name 3201, a cache host 3202 identifier and cache port 3203 identifier. Returning now to Figure 30b, the ODB 3100 is sent to the Object Generator 3042. The Object Generator examines the ODB 3100 and does one of the following: a) generates a Delete Remote Object Request"3025; b) establishes connections with the Object Sources 3050, rebuilds the object 3004, and creates an Update Object Request 3022. The TMD 3040 then passes the Delete Remote Object Request 3025 or the Update Object Request 3022 to the Distribution Manager 3043. The Distribution Manager 3043 establishes a connection with each configured Remote Cache Manager 3002 or Slave Trigger Monitor 3000" (Figure 30c), and delivers each the request If the request is a Forward Trigger Request 3021, the request is sent to the Slave Trigger Monitor 3000" (Fig. 30a). If the request is an Update Object Request 3022, the new object is sent to the Remote Cache manager 3001 via the cache_object 410 API (Fig. 4). If the request is a Delete Remote Object Request 3025 the object 3004 is purged from each Remote Cache manager 3001 via the delete_object 420 API (Fig. 4). Figure 30c depicts another example of the Trigger Monitor 3000. Here, the Trigger Monitor 3000 is instantiated as a Slave Trigger Monitor 3000". If the Master Trigger Monitor 3000' is maintaining exactly one system, or if an object 3004 is to be regenerated (that is, not deleted), it can be fully maintained using the process described in Figure 30b. If the Trigger Monitor 3000 is maintaining multiple systems, it is possible that the object 3004 exists in some but not all caches. In particular, the object 3004 may not exist in the same cache as the Trigger Monitor 3000 which received the Trigger Event 3020. To handle this case a Slave Trigger Monitor 3000" (Fig. 30c) is run on each configured node. As depicted, the Slave Trigger Monitor 3000" receives a Forward Trigger Request 3021. This is processed identically to a Trigger Event 3020 until it arrives in the Object Generator 3042. If the Object Disposition Block 3100 has a Request Disposition 3103 equal to DispRegenerate 3130, the request is discarded. If the Request Disposition 3101 is Displnvalidate 3131 a Delete Local Object Request 3023 is built and sent to the Slave's Local Cache. Referring again to Figure 30, the trigger monitor 3000 is preferably embodied as a single long running process, monitoring the object sources 3050. One skilled in the art could easily adapt the present invention to consist of one or more processes per component, some of which may overlap in time to improve throughput of the system. One skilled in the art could also easily adapt the present invention to use multiple threads of operation in a single process, each thread implementing one or more of the components, some of which may overlap in time, if the underlying system provides support for threaded processes. Conventional mechanisms such as multiphase commit and persistent data objects are preferably used when receiving Trigger Events 3020 and Forward Trigger Requests 3021 to provide a guarantee to the object sources 3050 that these requests, once delivered, remain in the system until completion. Conventional mechanisms such as retry, and multiphase commit are preferably used to provide a guarantee that enqueued outbound requests (depicted in Figure 30e) remain in the system until completion. The Object Id Analysis (OIA) component 3041 translates the Object IDs 3009 into Object Disposition Blocks 3100 (Fig. 31). The OIA 3041 may be specified and interfaced as a configuration option, an API,, or in any other standard way. One skilled in the art could easily build such a mechanism. The Object Generator 3042 translates the information in an Object Disposition Block (3100) into the transaction types depicted in Figure 30c and described below. The trigger monitor 3000 provides an interface to this component using configuration options, APIs, or any other standard technique. Examples of Object Generators 3042 are the products sold by: IBM under the trademark NET.DATA; Lotus Corporation under the trademark DOMINO Server, or any Web server from which HTML pages can be fetched. Figure 30d depicts an example of the send_trigger API. As depicted, the sendjtrigger 3026 API enables the Object Sources 3050 to communicate with the Trigger Monitor Driver 3040. The sendjtrigger 3026 API sends a message including sufficient information (message parameters) to uniquely identify the trigger and construct a Trigger Event 3020. One skilled in the art could easily define and specify that information using standard techniques (such as variable-length parameter lists). Figure 30e depicts examples of transaction types used in accordance with the present invention. As depicted, several transactions 3020... 3025 can be generated within the system: A Trigger Event 3020 is generated in response to receipt of a message sent via the sendjtrigger 3026 API. The Trigger Event 3020 is a structure which maintains sufficient information to translate the data sent by the send_trigger 3026 API into one or more Show Dependent Object Requests 3024 and to properly track and guide itself through the system. A Forward Trigger Request 3021 is generated in response to receipt of a Trigger Event 3020 sent via the send_trigger 3026 API. The Forward Trigger Request 3021 is a structure which maintains sufficient information to generate one or more Show Dependent Object Requests 3024 and to properly track and guide itself through the system. An Update Object Request 3022 is generated by the Object Generator 3042 to cause new objects to be distributed to Remote Cache Managers 3002 via the Distribution Manager 3043. The Update Object Request is a structure which maintains sufficient information to replace an object 3004 in any arbitrary cache 3003. A Delete Local Object Request 3023 is generated by the Object Generator to cause a local Cache 3003 to delete an object 3004. The Delete Local Object Request 3023 is a structure which maintains sufficient information to delete an object 3004 from the Local Cache manager 3001. A Show Dependent Object Request 3024 is generated by the Trigger Monitor Driver 3040 in response to a Trigger Event 3020 to request the dependency information from the Local Cache Manager 3001. The Show Dependent Object Request 3024 is a structure which maintains sufficient information to analyze a Trigger Event 3020 or a Forward Trigger Request 3021 and invoke the API show_dependent_objects 3024 to acquire Object IDs 3009 from the Local Cache Manager 3001. A Delete Remote Object Request 3025 is generated by the Object Generator 3042 to cause an object 3004 to be deleted from remote cache managers 3002 via the Distribution Manager 3043. The Delete Remote Object Request 3025 is a structure which maintains sufficient information to delete an object 3004 from an arbitrary cache 3003. Figure 33 depicts an example of a high-level organization and communication paths of the Trigger Monitor Driver 3040 and the Distribution Manager 3043. The preferred organization consists of several independently executing threads of control: A Receiving Thread 3300 receives requests including Trigger Event 3020 and Forward Trigger Request 3021 and saves them to some persistent store. An Incoming Work Dispatcher Thread 3320 dequeues incoming requests from 3300 and enqueues them for processing. A Cache Manager Communications Thread 3340 sends the Delete Local Object Request 3023 and Show Dependent Object Request 3024 requests to the Local Cache Manager 3060. An Object Generator Thread 3360 coordinates generation of the object requests: Delete Remote Object Request 3025; and Update Object Request 3022, and enqueues them for distribution. A Distribution Thread 3080 (which is a main component of the Distribution Manager 3043) dequeues requests from the Distribution Manager Queue 3370 and enqueues them to all outbound machines. The Outbound Transaction threads 3395 contact remote machines and forward the work enqueued on the Machine Outbound Queues 3390. As is conventional, these threads can communicate via several FIFO queues: the Incoming Request Queue 3310; the Cache Manager Request Queue 3330; the Object Generator Queue 3350; the Distribution Manager Queue 3370; and the Machine Outbound Queues 3390 (one per distributed cache). Figure 34 depicts an example of the Receiving Thread 3300 logic. As depicted, in step 3410, an incoming message (either the send_trigger API 3026 or a Forward Trigger Request 3021) enters the system and is converted to a Trigger Event 3020. In step 3420, the message is written by the receiving thread 3300 to a persistent queue 3450 and enqueued in step 3430 to the Incoming Request Queue 3310. In step 3440, the request type is checked. In step 3460, if it is a Trigger Evcnt 3020, a Eorward Trigger Request 3021 is enqueued to the Distribution Manager Queue 3370. In step 3490, the receiving thread 3300 returns to waiting 3490 for work. Figure 35 depicts an example of the Incoming Work Dispatcher Thread 3320 logic. As depicted, in step 3510, the incoming work dispatcher thread 3320 dequeues the work request In step 3520, a Show Dependent Object Request 3024 is enqueues to the Cache Manager Request Queue 3330. In step 3590, the receiving thread 3300 returns to waiting for work. Figure 36 depicts an example of the Cache Manager Communications Thread 3340 logic. As depicted, in step 3610, the cache manager communications thread 3340 dequeues a next request and establishes communications with the Local Cache Manager 3001. In step 3023, if the request is a Delete Local Object Request, in step 3650, the delete_object 420 API is used to delete the object from the local cache 3003. In step 3024, if the request is a Show Dependent Object Request, in step 3620 the show_dependent_objccts 460 API is used to fetch the Object IDs 3009. In step 3630, the Object IDs 3009 are passed to the Object ID Analysis 3042 component which builds an Object Disposition Block 3100. In step 3640, the Object Disposition Block 3100 is enqueued to the Object Generator 3043. Finally, in step 3690, the Cache Manager Communications Thread 3340 returns to waiting for work 3690. Figure 37 depicts an example of the Object Generator Thread 3360 logic. As depicted, in step 3710, the object generator thread 3360 dequeues a next request from the queue 3350. In step 3720, the Disposition of the object is checked. If it is a Displnvalidate 3131 proceed to step 3750; if a DispRegenerate 3130 proceed to step 3730. In step 3730, the RequestType is checked. If it is a Forward Trigger Request 3021 proceed to step 3770; if it is a Trigger Event 3020 proceed to step 3740. In step 3740, the Data Sources 3050 are contacted to regenerate the objects 3004. The new objects 3004 is enqueued with an Update Object Request 3022 to the Distribution Manager Queue 3370. The process then returns to step 3790 to wait for work. In step 3750, the RequestType is checked. If it is a Forward Trigger Request 3021 proceed to step 3780; if it is a Trigger Event 3020 proceed to step 3760. In step 3760, a Delete Remote Object Request 3024 inbuilt and enqueued to the Distribution Manager Queue 3370. The process then returns to step 3790 to wait for work. In step 3770, the request is deleted from the system. The process then returns to step 3790 to wait for work. In step 3780, a Delete Local Object Request 3023 is enqueued to the Cache Manager Request Queue 3330. The process then returns to step 3790 to wait for work. Figure 38 depicts an example of the Distribution Manager Thread 3380 logic. As depicted, in step 3810 the Distribution Manager Thread 3380 dequeues work from the Distribution Manager Queue 3370 and enqueues a copy of the request to each of the Machine Outbound Queues 3390. The process then returns to step 3790 to wait for work. Figure 39 depicts an example of the Outbound Transaction Thread 3395 logic. There is one Outbound Transaction Thread 3395 for each machine participating in the distributed update scheme. As depicted, in step 3910 the thread dequeues work from the Machine Outbound Queue 3390 and checks the request type. In step 3920, if it is an Update Object Request 3022 or Delete Remote Object Request 3025 the process continues at step 3920; if it is a Forward Trigger Request 3021, the process continues at step 3930. In step 3930, if it is a Forward Trigger Request 3021 the process continues at step 3930. In step 3920 the remote Cache manager 3001 is contacted. In step 3940, if the request is an Update Object Request 3022, the cache_object API 410 is used to send the new objects 3004 to the remote cache manager 3002. The process then returns to step 3990 to wait for work. In step 3950, if the request is a Delete Remote Object Request 3025, the delete_object API 420 is used to delete the objects 3004 from the remote cache manager 3002. The process then returns to step 3990 to wait for work. In step 3930, the remote Trigger Monitor 3000a is contacted. In step 3960, the Forward Trigger Request 3021 is sent to the remote Trigger Monitor 3000. The process then returns to step 3990 to wait for work. The process then returns to step 3790 to wait for work. Extensions and Variations Other exits not iterated here may be required for full analysis of Trigger Events 3020 and translation into actions (such as Update Object Request 3022 or Delete Remote Object Request 3025), depending on the specific application of this invention. For example, referring now to Figure 40: a) it may be useful to translate 4000 a single Trigger Event 3020 into a set of multiple Show Dependent Object Requests 3024 via an exit; b) it may be useful to modify or analyze 4010 an objects 3004 as created by the Object Generator 3042, prior to enqueing that objects 3004 in an Update Object request 3022; and c) it may be useful to write an objects 3004 to the file system instead of, or in addition to, writing the objects 3004 to cache 3003. Another use of the Trigger Monitor 3000 would be to reuse its ability to generate and distribute objects for the purpose of handling objects which may not currently exist in cache: a) a prime_cache API 4020 could be used to generate and distribute an objects 3004 given an object ID 3009, regardless of whether that objects 3004 is currently known to any cache 3003; and b) a global_delete API 4030 could be used to insure that some specific objects 3004 is removed from all caches 1 in the system without knowing whether that object actually exists anywhere. The Trigger Monitor 3000 may be implemented to enforce strict FIFO ordering and processing of requests, or to permit full asynchronous processing of requests, or to process requests according to any well known scheduling scheme, or any combination of the above. Maintaining Consistency As discussed herein before, while dictionary meanings are also implied by terms used herein, the following glossary of some terms is provided for guidance: A transaction manager is a program which manages state. Examples include: cache managers managing caches; database management systems such as DB2; and transaction processing systems such as CICS. A transaction is a request made by another program to a transaction manager. A state-changing transaction is a transaction which modifies state managed by the transaction monitor. Requests to a cache manager to read, update, or delete cache objects would constitute transactions. Reads and modifications of data are known as accesses. A lock is an entity which limits the ability of processes to read or write shared data. When a process acquires a read lock on a piece of data, other processes can access the data but no other processes may modify the data. When a process acquires a write or exclusive lock on the data, no other processes may read or modify the data. Several methods for implementing locks exist in the prior art. See e.g., "Computer Architecture: A Quantitative Approach," 2nd edition, by Hennessy and Patterson, Morgan Kaufrnann, 1996. Let S be a set of transactions which modify data d on a system containing one or more transaction managers. S is performed consistently if: (1) for any request rl not in S which accesses all or part of d, all parts of d accessed by r\ are either in a state before modification by any transaction in S or in a state after modification by all transactions in S. (2) For any requests rl and rl not in S where r2 is received by the system either at the same as rl or after rl and both rl and r2 access a subset d of d, (a) if the version of d accessed by rl has been modified by transactions in S, then the version ofd" accessed by r2 has also been modified by transactions in S. (b) if the version of d accessed by r2 has not been modified by transactions in 5, then the version of d accessed by rl has also not been modified by transactions in S. A timestamp is an attribute which can be assigned to events such as a transaction being received by a system or a lock being acquired. Common methods for implementing time stamps in the prior art include clock times and numbers which order events. Another feature of the present invention is the ability to make a set of consistent updates to one or more caches. The present invention is of use for a set of requests S to one or more cache managers 3001 where the following properties are desirable: (1) For any program p accessing die system, S must be made atomically. That is,p cannot have a view of the system where some requests in S have been satisfied and others have not (2) For any two requests rl and r2 received by appropriate cache managers 3001 at the same time, rl and rl see the same view of the system with respect to S. That is, either both rl and r2 see a view of the system before requests in S have been satisfied, or both rl and r2 sec a view of the system after requests in S have been satisfied. (3) For any two requests rl and r2 where r2 is received by a cache manager 3001 after rl is received by a cache manager, if rl has a view of the system after requests in S have been satisfied, then r2 must see the same view of the system. If r2 sees a view of the system before requests in S have been satisfied, then rl must see the same view. Figure 41 depicts an example of logic for making a set S of requests consistently to a system including one or more caches. Preferably, each request in S is directed to one cache manager 3001. The set of cache managers C receiving a request from S may have one or more members. As depicted, in step 45 00, the set of requests Sis received by the system. Each request is directed to a specific cache manager 3001. In step 4505, the cache managers lock data. For each cache managery receiving a request from S, the cache manager j acquires write locks for data modified by a request in S and read locks for data read but by a request in S but not written by a request in S. Data locked in this step will subsequently be referred to as locked data. In step 4600, the system determines the time the last lock was acquired, last_lock_time. If the set of cache managers C receiving a request from S has only one member, this step can easily be implemented using prior art. If C has multiple members, last_lock_time is determined in the manner described in Figure 42. In step 4510, requests received before lasUockjtime which are waiting on locked data are performed. In step 4520, requests in S are performed. In step 4530, locks are removed from locked data which allows requests received after last_Jock_time which are waiting on locked data to be performed. Steps 4510,4520, and 4530 must be performed in order. An alternative embodiment to that depicted in Figure 41 is to use a single lock to prevent requests from accessing data accessed by a request in S. The preferred embodiment allows much higher levels of concurrence than this alternative approach. Figure 42 depicts an example of logic for determining a last_lock_time if the set of cache managers C receiving a request from S has multiple members. As depicted, in step 4600, each member of C denoted cache mgr idetermines the time at which it acquired the last lock in step 4505, last_lock_rime_i; cache mgr i then sends last_Jock_time_i to a program known as a coordinator program. In step 4610, the coordinator program receives lasUock_time_i values from all cache managers in C and sets lastjockjtimc to the latest last_lock_timc_i value it receives. In step 4615, the coordinator program sends last_tock_time to all cache managers in C. A variation on the example depicted in Figure 42 would be for each cache mgr i in C to exchange values of last_lock_time_i with other cache managers in Cin step 4600 instead of sending last_lock_time_i to a coordinator program. In step 4610, each cache mgr / in C would determine lastjockjtime from the last_lock_time_i values it receives. Step 4615 would not be necessary. The preferred emlxxiiment requires less communication and fewer comparisons when C is large and is thus more scaleable than the variation just described. One skilled in the art could easily adopt the present invention to achieve consistency in other systems containing one or more transaction managers wherein the transaction managers do not have to be cache managers. Now that the invention has been described by way of a detailed description, with alternatives, various enhancements, variations, and equivalents will become apparent to those of skill in the art Thus it is understood that the detailed description has been provided by way of example and not as a limitation. The proper scope of the invention is properly defined by the claims. Claim; 1 In a computer system comprising one or more caches storing one or more complex objects for determining how changes to underlying data can affect values of one or more complex objects, comprising: means for identifying at least part of the underlying data, wherein the underlying data may or may not be cacheable; and means for mapping said at least part of the underlying data to one or more of said complex objects having one or more data dependencies on said at least part of the underlying data; and means for maintaining an object dependence graph (G) which may change over time and which includes a plurality of graph objects and edges indicating one or more data dependencies between graph objects. 2. The system as claimed in claim 1 wherein said means for mapping comprises a means for scheduling operations where different schedules might result in different changes to underlying data. 3. The system as claimed in claim 2 wherein said means for scheduling operations is a compiler, run-time system, or database. 4 The system as claimed in claim 1, comprising means for assigning weights to the edges wherein the weights are correlated with an importance of data dependencies. 5 The system as claimed in claim 3 wherein a weight of at least one edge does not change after it is assigned. 6. The system as claimed in claim 2 wherein said means for determining how similar said two versions are, comprises means for dividing said sum of the weights of edges (o2,01) in said set S such that a same version of said graph object o2 is consistent with said versions ol_vl and ol_v2 by said sum of the weights of all edges terminating in 01 in said set S. 7. The system as claimed in claim 1, wherein said means for mapping comprises: means for using information about how one or more complex objects would be affected by changes to said at least part of the underlying data to determine whether or how to make changes to said at least part of the underlying data. 8. The system as claimed in claim 1, wherein said means for mapping includes means for generating one or more of: multiple processes; multiple threads; and one or more long-running processes or threads managing storage for said one or more caches. 9. The system as claimed in claim 1 wherein said means for mapping comprises: means for identifying a change to said at least part of the underlying data; and means for identifying how one or more complex objects are affected, in response to said means for identifying a change to said at least part of the underlying data. 10. The system as claimed in claim 8, wherein said means for mapping includes means for generating atleast one long-running process, comprising. an application identifying a change to at least part of the underlying data; and the application communicating an identified change to at least part of the underlying data to said means for mapping. 11. The system as claimed in claim 13 comprising means for detecting a highly obsolete version of an object from the cache. 12. The system as claimed in claim 8 comprising means for replacing a highly obsolete version of the object with a more recent version. 13. The system as claimed in claim 12 wherein the more recent version is a current version of the object. 14. The system as claimed in claim 12 wherein said means for determining that a version ol_vl of an object ol stored in a cache would become highly obsolete, comprises: means for maintaining an object dependence graph (G) which may change over time and which includes a plurality of graph objects and edges indicating one or more data dependencies between graph objects; means for initializing and maintaining weight_act fields for said ol_vl corresponding to one or more edges terminating in said ol; for one or more changes to a current version of a graph object o2 such that an edge from said object o2 to said object ol exists such that ol-vl has a weight_act field, means for decrementing the weight act field corresponding to the edge; and means for determining if said oljvl would become highly obsolete based on its weight_act fields. 15. The system as claimed in claim 14 comprising: means for maintaining a threshold weight for said ol_vl; wherein said means for determining if said ol_vl would become highly obsolete based on its weight_act fiels has means for comparing a sum of the weight_act fields for said ol_vl to said threshold weight. 16. The system as claimed in claim 14 comprising: means for assigning weights to the edges of the object dependence graph wherein the wrights are correlated with an importance of data dependencies; and means for initializing one or more weight_act fields for said ol_vl to one or more weights of corresponding edges in the object dependence graph. 17. The system as claimed in claim 14 comprising means for setting the weight_act field corresponding to an edge from said o2 to said ol in the object dependence graph, that is greater than a predetermined minimum possible value, to said predetermined minimum possible value whenever the current version of o2 is updated. 18. The system as claimed in claim 1 wherein said one or more caches has one or more of: a multiple version cache; a single version cache; and a current version cache. 19. The system as claimed in claim 15 wherein said one or more caches has at least one single version cache cl, comprising for at least one of the objects ol, means for maintaining a consistency set of other objects o2 such that ol and o2 must be consistent whenever they are both contained in cl. 20. The system as claimed in any preceding claim comprising: means for adding a current version of said object ol to the single version cache, possibly replacing a previous version of the object ol; means for identifying one or more objects o2 in the consistency set for ol such that a noncurrent version o2_v of said object (s) o2 is contained in said cache cl and said o2_v was created before the current version of said ol was created; means for removing said o2_v from said cache cl or replacing o2_v with a more recent version of said object (s)o2; and means for recursively applying to objects in the consistency set for said object (s) o2, said means for identifying one or more objects and said means for removing said o2_v from said cache cl or replacing o2_v with a more recent version of said object (s). 21. The system as claimed in claim 20 comprising: means for using the consistency set for an object ol to decide whether or not to add a version of the object ol to the single version cache, possibly replacing a previous version of the object ol. 22. The system as claimed in claim 20 comprising: for multiple objects ol,..., on in a single version cache cl; means for replacing each object oi' with a different version in cl without ensuring that all constraints imposed by a consistency set for the object oi are satisfied at the time said oi is updated but instead adding objects objl from a consistency set for said oi for which an inconsistent version of objl might exist in cl to a consistency stack; and after all objects ol, ,.., on in cl have been replaced with current versions, means for traversing all objects on the consistency stack and for each such object objl for which an inconsistent version exists in cl, removing it from cl or replacing it with a consistent version. 23. The system as claimed in claim 3 wherein said G is one of: a simple dependence graph, and a multigraph. 24. The system as claimed in claim 22 wherein said G is a simple dependence graph in which proper maximal nodes are complex objects and proper leaf nodes are underlying data which are not objects. 25. The system as claimed in claim 24 wherein said means for maintaining an object dependence graph comprises: means for maintaining an outgoing adjacency list for at least one proper leaf node v, and means for maintaining an incoming adjacency list for at least one proper maximal node w. 26. The system as claimed in claim 25 comprising means for accessing one of outgoing adjacency lists and incoming adjacency lists via a hash table. 27. The system as claimed in claim 25 wherein said means for maintaining an object dependence graph comprises : when initially changed data includes some data d which could be represented by a proper leaf node changes, means for searching for the outgoing adjacency list corresponding to data d\ and if the outgoing adjacency list is found, means for examining the list to determine which complex objects are affected. 28. The system as claimed in claim 25, comprising means for deleting at least one proper maximal node w from the object dependence graph. 29. The system as claimed in claim 29 wherein said means of deleting at least one proper maximal node w from the object dependence graph comprises : means for deleting said w from the outgoing adjacency list for at least one proper leaf node on the incoming adjacency list for w; and means for deleting the incoming adjacency list for said w. 30. The system as claimed in claim 30 comprising means for deleting at least one proper leaf node v from the object dependence graph. 31. The system as claimed in claim 31 wherein said means for deleting at least one proper leaf node v from the object dependence graph, comprises : means for deleting v from the incoming adjacency list for one or more proper maximal nodes on the outgoing adjacency list for v; and means for deleting the outgoing adjacency list for said v. 32. The system as claimed in claim 3 wherein a graph object r is a relational object (RO) which has an associated relational specifier which can represent one of single (SRO) or multiple (MRO) records and wherein said means for maintaining an object dependence graph comprises: (a) means for storing the relational specifier for said r; (b) if one or more graph objects include said r, means for adding one or more dependencies from said r to one or more graph objects which include said r, and (c) if said r includes one or more graph objects, means for adding one or more dependencies to said r from said one or more graph objects included by said r 33. The system as claimed in claim 32 comprising: means for assigning weights to the edges wherein the weights are correlated with an importance of data dependencies; and with an importance of data dependencies; and wherein one of said steps (b) and (c) include means for determining the weight of at least one of the dependencies corresponding to an RO node r\ containing an RO node r2, according to one or both of: a percentage of records of said r 1 contained by said r2; and a relative importance of said records. 34. The system as claimed in claim 32 comprising at least one (d) means for adding one or more dependencies from said r to another RO r2 wherein said another RO r2 does not include said r using a metric based on one or both of: a percentage of records in said r which are also contained in said r2; and a relative importance of the records in said r which are also contained in said r2; (e) means for adding one or more dependencies from another RO r3 to r wherein said r does not include said r3 using a metric based on one or both of: a percentage of records in said r3 which are also included in said /•; and a relative importance of the records in said r3 which are also contained in said r. 35. The system as claimed in claim 34 comprising: means for assigning weights to the edges wherein the weights are correlated with an importance of data dependencies; and means for calculating the weight of said one or more dependencies added in one or both of means (d) and (e) using said metric. 36. The system as claimed in claim 32 wherein one or more relational specifiers have relation names such that pairs of ROs having a common record have a high probability of possessing a same relation name. 37. The system as claimed in claim 36 comprising means for segregating ROs using one of: relation names; and a balanced tree in conjunction with the relation names. 38. The system as claimed in claim 32 wherein said means for identifying at least part of the underlying data comprises: means for identifying a relational specifier; and means for locating one or more ROs in the object dependence graph having one or more records in common with the relational specifier; and means for locating one or more ROs in the object dependence graph having one or more records in common with the relational specifier. 39 The system as claimed in claim 3, wherein said means for mapping comprises: means for visiting a set of one or more nodes (update_set) in the object dependence graph corresponding to said at least part of the underlying data; and means for traversing the edges in the object dependence graph in order to visit nodes reachable from said update_set. 40. The system as claimed in claim 33 wherein said means for traversing comprises means for traversing the object dependence graphs in one of: a depth-first manner; and a breadth-first manner. 41. The system as claimed in claim 33 comprising: means for maintaining a timestamp which uniquely identifies each object dependence graph traversal; means for storing the timestamp at a vertex of the object dependence graph whenever the vertex is visited for the first time during a traversal; and means for determining whether a vertex has been visited during the current traversal by comparing the timestamp at the vertex to the timestamp for the current traversal. *• 42. The system as claimed in claim 41, comprising means for incrementing the time stamp for each new graph traversal. 43. The system as claimed in claim 3, wherein two versions ol_vl and ol_v2 of a graph object ol exist, and the object dependence graph G contains at least one edge (o2, ol) terminating in ol, comprising: for one or more edges (o2,ol) terminating in said ol and comprising a set S, means for maintaining a version number of said object (s) o2 which is consistent with ol_vl and a second version number of said object (s) o2 which is consistent with ol_v2; means for determining how similar ol vl and ol_v2 are based on a number of edges (o2,ol) in said set S such that a same version of a graph object o2 is consistent with said versions ol_v_l and ol_v2 and a number of all edges terminating in ol in said set S. 44. The system as claimed in claim 43 in which said means for determining how similar oljvl and ol_v2 are, comprises means for dividing a number of edges (o2, ol) in said set S such that a same version of a graph object o2 is consistent with said versions ol_vl and ol_v2 by a number of all edges terminating in ol in said set S. 45. The system as claimed in claim 13 where said means determining that a version of a complex object ol stored in a cache would become highly obsolete comprises means for determining a number of edges (o2, ol) such that said version of ol is inconsistent with the current version of said o2. 46. The system as claimed in claim 45 wherein said means for determining that a version of a complex object ol stores in a cache would become highly obsolete further is based on the total number of edges terminating in said ol. 47. The system as claimed in claim 3, comprising means for using information on how a set s_update of one or more graph objects would be affected by changes to x the underlying data, in conjunction with the object dependence graph to determine how at least one other graph object having dependencies, specified either directly or transitively by the object dependence graph, on one or more objects in said s_update would be affected. 48 The system as claimed in claim 11 comprising means for implementing the consistency stack by storing references to said objects on a linked list. 49. The system as claimed in claim 11 comprising : means for using one or more balanced trees for implementing said consistency stack; and for at least one object objl, means for verifying that a reference to said objl is not already included on the consistency stack before adding a reference to said obj 1 to the consistency stack. 50. The system as claimed in claim 26 wherein consistency set(s) are implemented using one or more linked lists. 51. A computer system having one or more caches storing one or more complex objects, and one or more remote data sources storing underlying data which may affect a current value of one or more of said objects, the system comprising: means for maintaining an object dependence graph which may change over time and which includes a plurality of graph objects and edges indicating one or more data dependencies between said graph objects. means for recognizing when at least part of said underlying data has changes; means for communicating to a cache, one or more of: information about at least part of said underlying data which has changed; and information which includes the identity of at least one object whose value has changed as the result of said underlying data which has changed; and information which allows the identity to be determined of at least one object whose value has changed as the result of said underlying data which has changed; and information which allows the identify to be determined of at least one object whose value has changed as the result of said underlying data which has changed; and means for removing an object from the cache, or updating a anew version of an object in the cache, in response to said means for communicating. 52. The system as claimed in claim 11, wherein said at least part of the underlying data is stored in one or more remote data sources comprising : means for communicating to a cache, one or more of information about at least part of said underlying data which has changed; and information which includes the identity of at least one object whose value has changed as the result of said underlying data which has changed; and information which allows the identify to be determined of at least one object whose value has changed as the result of said underlying data which has changed; and means for removing an object from the cache, or updating a new version of an object in the cache, in response to said means for communicating. 53. A computer system including one or more caches storing one or more complex objects substantially as herein described with reference to and as illustrated by the accompanying drawings.

Documents:

1962-del-1998-abstract.pdf

1962-del-1998-assignment.pdf

1962-del-1998-claims.pdf

1962-del-1998-correspondence-others.pdf

1962-del-1998-correspondence-po.pdf

1962-del-1998-description (complete).pdf

1962-del-1998-drawings.pdf

1962-del-1998-form-1.pdf

1962-del-1998-form-19.pdf

1962-del-1998-form-2.pdf

1962-del-1998-form-3.pdf

1962-del-1998-form-4.pdf

1962-del-1998-form-6.pdf

1962-del-1998-gpa.pdf

1962-del-1998-pa.pdf

1962-del-1998-petition-137.pdf

1962-del-1998-petition-others.pdf