RECOVERABLE EXECUTION

[0001] This application constitutes a continuation of U.S. application Ser. No. 11/247,973, entitled “RECOVERABLE EXECUTION” and filed on Oct. 10, 2005. The entire content of the aforementioned U.S. Patent Application is hereby expressly incorporated by reference in its entirety.

[0002] The invention relates to computer systems, and more particularly to a method and mechanism for providing recoverability for a set of atomic operations.

[0003] In database systems, a “transaction” normally refers to an atomic set of operations performed against a database. The transaction may access, create, modify, or delete database data or database metadata while it is being processed. A “commit” occurs when the transaction has completed its processing and any changes to the database by the transaction are ready to be permanently implemented in the database system. Because the transaction is atomic, all actions taken by the transaction must be committed at the same time. If any operation taken by the transaction cannot be performed, then the entire transaction must be aborted—not just the particular operation that failed. When the transaction is aborted, any changes made by that transaction to the database are “rolled back” such that the database is returned to its pre-existing state from immediately prior to the aborted transaction.

[0004] Transaction log records can be maintained to allow suitable recovery operations in the event of a system failure or aborted transaction. Some common problems that could cause a system failure or aborted transaction include hardware failure, network failure, process failure, database instance failure, data access conflicts, user errors, and statement failures in the database access programs (most often written in the structured query language or SQL).

[0005] Different types of transaction log records can be maintained in a database system. A common transaction logging strategy is to maintain “redo” records that log all changes made to the database. With “write ahead logging,” each change to data is first recorded in the redo log, and only afterwards is that change actually made to the database block corresponding to the changed data. This protects against the situation when a system failure occurs and the version of the database data that is immediately restored from disk does not accurately reflect the most recent state of the database. This may occur because of changes to the data that have only occurred in cache, and have not been recorded to disk before the failure. If the redo log has been properly maintained for these cache-only changes, then recovery can be performed by applying the redo records to roll the database forward until it is consistent with the state that existed just before the system failure.

[0006] Another type of log record that may be maintained is the “undo” record, which can also be referred to as a “rollback” segment. Undo records contain information about database actions that should be undone during certain database operations. For example, if the rolling forward process during recovery has applied uncommitted changes to the database, then undo records can be applied to remove uncommitted changes, thereby ensuring that only committed changes exist in the database after recovery. In addition, if a transaction is aborted, then undo records can be applied to return the database to its pre-existing state from prior to the aborted transaction. If a database uses multi-versioning to allow different transactions to view database data from different points in time, then undo records can be used to create multiple versions of the database that are consistent with the different points in time.

[0007] In some cases, a particular series of atomic transactions may be used to perform actions in a database system. Each of these atomic transactions may generate and store transaction logs, such as a redo log and an undo log. Managing the plurality of transaction logs associated with each atomic transaction in a series of atomic transactions may prove difficult and cumbersome.

[0008] The present invention may therefore include systems and methods for grouping transaction logs. Grouping transaction logs may assist a user in managing the transaction logs associated with a series of atomic transactions.

[0009] Further details of aspects, objects, and advantages of the invention are described below in the detailed description, drawings, and claims.

[0021] The present invention provides a method and mechanism for implementing transaction logs in a database system. For the purpose of explanation, throughout this document the term “disk” or “disk system” is used to refer to data storage systems, but the inventive concepts disclosed herein may also be applied to other types of storage systems besides disk-based systems. In addition, the following description will be made with respect to the storage/retrieval of relational data from a database. It is noted, however, that the present invention is applicable to managing other types and granularities of data in a computing system, and thus is not to be limited to management of just relational data. In particular, it is contemplated that the present invention may be used to replicate or otherwise manage text files, images, audio/video data, or other types of files and/or data.

[0022] One embodiment of the invention provides systems and methods for grouping a plurality of atomic transactions into a one-step operation. A series of atomic transactions may be executed to perform an operation or set of operations on data. Each of the atomic transactions may be associated with one or more transaction logs. In order to create a one-step operation from a plurality of atomic transactions, groupings of transaction logs may be created. The grouping of transaction logs may contain transaction logs sufficient to roll forward or roll back changes made by the series of atomic transactions.

[0023] In one implementation, the atomic transactions may be implemented, for example, as APIs in a database system, and the one-step operation may be implemented as a one-step API. In this implementation, the one-step API may execute a series of atomic transactions by making calls to the APIs for the atomic transactions. Because the APIs for the atomic transactions may be called by the one-step API, they may be referred to as “lower-level APIs.” The transaction logs for the low-level APIs may be associated into grouping of transaction logs. The one-step API may include calls to roll forward or roll back changes made during execution. In order to roll forward or roll back changes, the one-step API may execute transaction logs in the grouping.

[0024] FIG. 1A is a flow chart depicting a method for associating a plurality of atomic transactions. The method of FIG. 1A may be used, for example, to create a one-step operation from a plurality of atomic transactions. Each of the atomic transactions may commit separately upon successful completion, and each of the atomic transactions may generate a separate set of transactions logs. It may therefore prove difficult or cumbersome for a user to search through the transaction logs to locate and execute the appropriate logs. The method of FIG. 1A may be used to associate the logs for each atomic transaction, such that a user may quickly and easily execute a series of atomic transactions, or recover from a failure by rolling forward or rolling back changes to the data.

[0025] As shown in FIG. 1A, the method may begin in step 10, wherein a group of atomic transactions may be identified. The atomic transactions identified may be, for example, a series of atomic transactions that are to be grouped together to generate a one-step operation.

[0026] The method may continue in step 12, wherein transaction logs for each transaction may be identified. The transaction logs identified may include, for example, a redo log, an undo log, and a state log for each atomic transaction. The transaction logs will be described further below.

[0027] In step 14, the transaction logs may be associated into an atomic grouping. This includes, for example, associating the transaction logs such that any changes made to the data during the execution of the series of atomic transactions may be rolled forward or rolled back using transaction logs in the grouping. For example, if a failure occurs during the execution of the series of atomic transactions, changes to the data may be rolled forward or rolled back using transaction logs in the grouping. The groupings of transaction logs will be described further herein below.

[0028] FIG. 1B is a flow chart depicting a method for executing a plurality of atomic transactions. The method of FIG. 1B may be used, for example, to enact changes to a database through a series of atomic transactions, and to perform recovery operations in case a failure occurs during execution of the series of atomic transactions.

[0029] As shown in FIG. 1B, the method may begin in step 20, wherein a group of atomic transactions may be executed. In step 22, it may be determined whether a failure occurred during the execution of the atomic transactions. If a failure occurred, recovery may be performed for the group of transactions in step 24. In one implementation, performing recovery for a group of transactions is performed using transaction logs in a grouping.

[0030] In order to form groupings of transaction logs, a system of recoverable operations may be used. A recoverable operation may be any operation that may be reversed to return the data to its original state. In one implementation, the one-step operation may itself be or include a recoverable operation, and each atomic transaction called by the one-step operation may also be associated with or include a recoverable operation.

[0031] A recoverable script may be or include code which is executed to roll a recoverable operation forward or back. In one implementation, each atomic transaction may be implemented as a recoverable operation, and a recoverable script may be generated and executed for each atomic transaction. The system of recoverable operations may allow the one-step operation to be rolled forward or rolled back as a whole.

[0032] A recoverable script may access one or more transaction logs to roll forward or roll back changes made to a database. In one implementation, a recoverable script associated with a one-step operation may access a plurality of transaction logs. These transaction logs may be the grouping of transaction logs formed for the one-step operation. The references to the transaction logs contained in the recoverable script may constitute the grouping of transaction logs.

[0033] A method for executing a one-step operation is illustrated in FIG. 1C. As shown in FIG. 1C, the one-step operation may be implemented as a one-step API, and the atomic transactions may be implemented as lower-level APIs. However, the invention is not limited to this particular implementation.

[0034] As shown in FIG. 1C, the method may begin in step 50, wherein one or more recoverable scripts may be generated. Each recoverable script may be or include, for example, executable code describing a recoverable operation. A recoverable script may be generated for each lower-level API called by the one-step API. In one implementation, a recoverable script may also be generated for the one-step API. A method for generating recoverable scripts will be discussed further with reference to FIG. 3.

[0035] The recoverable scripts may contain references to one or more transaction logs. The references to the transaction logs contained in the recoverable scripts may constitute the grouping of transaction logs.

[0036] The method may continue in step 52, wherein the recoverable scripts may be executed. For example, the recoverable scripts corresponding to the lower-level APIs may be executed to call the lower-level APIs in sequence or to otherwise effect the changes described by the lower-level APIs. During execution of the recoverable scripts, one or more lower-level APIs may make changes to data and, upon successful completion, may commit these changes. A method for executing recoverable scripts will be discussed further with reference to FIG. 4.

[0037] In step 54, it may be determined whether a failure occurred. If no failure occurred, in some implementations, the one-step API may commit. The method may then terminate.

[0038] If a failure occurred, in step 56, it may be determined whether to roll forward or roll back. In one implementation, a user may be notified that a failure occurred, and the user may provide input specifying whether to roll forward or roll back. In another implementation, a user may specify in advance whether to roll forward or to roll back. In yet another implementation, it may be electronically determined whether to roll forward or to roll back.

[0039] If the changes are to be rolled forward, the changes are rolled forward in step 58. If the changes are to be rolled back, the changes are rolled back in step 60. Methods for rolling forward and rolling back will be discussed further with reference to FIGS. 5-6.

[0040] After rolling forward 58 or rolling back 60, the one-step API may commit, and the method may end.

[0041] The method for generating a one-step operation may be used for any series of atomic transactions. For example, the method may be used to generate a one-step operation for configuring data replication. This particular application of the invention will be described below, although those skilled in the art will recognize that many other applications are possible.

[0042] In database systems, data may be replicated from one database to another, or may be replicated within the same database. Replicating data may ensure, for example, that data remains consistent over multiple databases, or within the same database. Methods for configuring data replication may involve calling a series of atomic transactions that perform various functions involved in setting up data replication. Each atomic transaction may automatically perform a “commit” that makes any changes to the databases permanent.

[0043] The present invention may provide a single transaction that is capable of configuring replication from one database to another, or within a single database. In one implementation, this transaction may perform only one commit that makes changes to the database(s) permanent. In another implementation, this transaction may comprise several lower-level atomic transactions, each of which may perform a commit upon successful execution. In the latter case, the transaction may include mechanisms for rolling forward or rolling back all transactions made by the lower-level atomic transactions.

[0044] FIG. 1D shows a block diagram of a system that may be used in accordance with an embodiment of the present invention. As shown in FIG. 1D, data may be stored, for example, in a first database 100. The data may be stored within a table 102 within the database 100 or may be stored elsewhere. Data from the database 100 may be replicated, for example, to a second table 106 within a second database 104 or elsewhere. In replicating data from a first database 100 to a second database 104, any changes made to data in the first database 100 will be reflected in the second database 104. This replication ensures that data stored in the databases 100, 104 remains consistent.

[0045] In one implementation, tables 102 and 106 may be stored within the same database. In another implementation, data may be replicated from one file system to another, or from one memory block to another. Other implementations involving other types of data and other storage systems are possible.

[0046] When changes are made to data within the first database 100, the changes are also written to a log 110 in the form of redo records 108. The redo records 108 contain sufficient information to reenact the changes in the database 100 or to effect the changes in the database 104. In order to replicate the changes to the second database 104, a capture engine 112 retrieves redo records 108 from the redo log 110. In retrieving redo records 108, the capture engine 112 may apply one or more capture rules 116, which specify which redo records 108 to extract from the redo log 110. The capture rules 116 may be specified in advance by a user based on the changes that should be propagated to the second database 104. For example, a user may wish to replicate only specific types of data from the first database 100 to the second database 104. Alternatively, a user may wish to replicate all data from the first database 100 to the second database 104. The capture rules 116 may specify the particular subset of data that is to be replicated.

[0047] The capture engine 112 may then translate the retrieved redo records 108 into a logical format and places the logical representation of redo records 108 into a streams queue 114. A propagation process 115 may read from the streams queue 114 and from one or more propagation rules 120. The propagation rules 120 may specify, for example, what changes made in the first database 100 should be propagated in the second database 104. The propagation process 115 may apply the propagation rules 120 to the logical representation of the redo records that is stored in the streams queue 114, and place the result in an apply queue 118.

[0048] An apply engine 122 may use one or more apply rules 124 that specify how the redo records 108 should be applied to the second database 104. The apply engine 122 may then apply the redo records to the second database 104 to replicate the changes to the first database 100 in the second database 104.

[0049] In one particular implementation, a method for replicating data involves a series of calls made to lower-level APIs. Such a method is described further with reference to FIG. 2.

[0050] As shown in FIG. 2, a method for configuring data replication may include a series of steps 200, 202, 204, 206, 208, 210,212, 214, 216, 218. Each of the steps 200, 202, 204, 206, 208, 210, 212, 214, 216, 218 may be an atomic transaction. Furthermore, each of the steps 200, 202, 204, 206, 208, 210, 212, 214, 216, 218 may be implemented as a call to a lower-level API. Each call to a lower-level API may result in a separate commit upon successful completion.

[0051] In particular, a method for configuring replication from a first database to a second database may include initiating a streams queue 200 and initiating a propagation process 202. The method may further include specifying one or more propagation rules 204. Specifying one or more propagation rules 204 may include, for example, accepting user input and creating one or more propagation rules based on the user input. The method may further include initiating a capture process 206 and specifying one or more capture rules 208. Specifying one or more capture rules 208 may include, for example, examining the structure of the first database and the second database and creating one or more capture rules based on those structures.

[0052] Steps 200, 202, 204, 206, 208 may be implemented, for example, as calls to lower-level APIs, and may be executed within the first database.

[0053] The method may continue in step 210, wherein an apply queue may be initiated. In step 212, an apply process may be initiated. The method may further include specifying one or more apply rules 214. Specifying one or more apply rules 214 may include, for example, examining the structure of the second database and creating one or more apply rules based on that structure. In step 216, redo records may be captured and propagated at the first database. The method may continue in step 218, wherein changes may be applied to the second database.

[0054] Steps 210, 212, 214, and 218 may be implemented, for example, as calls to lower-level APIs, and may be executed within the second database. Step 216 may be implemented, for example, as a call to a lower-level API, and may be executed within the first database.

[0055] In one implementation, a one-step operation may be provided for data replication. This one-step operation may configure data replication, for example, by performing atomic transactions as illustrated in steps 200, 202, 204, 206, 208, 210, 212, 214, 216, 218. Each of the atomic transactions may commit separately upon successful execution. Thus, in the case of a failure, some of the changes involved in configuring data replication may be committed, while other changes may not yet be committed. In this case, it may be difficult and cumbersome for a database administrator or other user to determine which step caused the error and to recover from the error by rolling forward or rolling back the changes to the data.

[0056] To aid a user in recovering from such a failure, the one-step operation may use a system of recoverable operations, each associated with an atomic transaction.

[0057] In order to aid a user in recovering from a failure, a one-step operation may use a system of recoverable operations. A recoverable operation is, for example, any operation that may be reversed to return the data to its original state. In one implementation, the one-step operation may itself be or include a recoverable operation, and may further make calls to one or more atomic transactions, each of which may also be or include a recoverable operation.

[0058] A recoverable script may be executable code describing a recoverable operation. Each recoverable script may include a set of “forward blocks” of code that may be executed to perform the recoverable operation, a set of “undo blocks” of code that may be executed to undo changes made to the data, and a set of “state blocks” of code that may be used to set the states of various variables to mimic successful completion of the recoverable operation.

[0059] FIG. 3 illustrates a method for generating a block within a recoverable script associated with a recoverable operation, in accordance with an embodiment of the present invention. In generating or executing a one-step operation, the method of FIG. 3 may be performed for each atomic transaction. The method of FIG. 3 may result in a recoverable script that includes a forward block, an undo block, and a state block associated with the atomic transaction. If an error occurs when executing the one-step operation, these blocks may be used to roll forward or roll back. Methods for rolling forward and rolling back will be discussed further with reference to FIGS. 5-6.

[0060] The first time a recoverable script is generated for a particular atomic operation, it may be necessary to generate various blocks of code, such as forward blocks, undo blocks, state blocks, location blocks, and the like. However, in the case of a failure, the method of generating a recoverable script for a particular atomic transaction may be performed more than once. In this case, many of the blocks of code may already exist. In this case, it may not be necessary to generated duplicate blocks of code when generating a recoverable script.

[0061] As shown in FIG. 3, the method may begin in step 300, wherein it is determined whether a forward block exists. In one implementation, the forward block will be specified by the lower-level API or other atomic transaction, and thus will exist. If the forward block does not yet exist, it may be generated in step 302.

[0062] In step 304, it may be determined whether the forward block location exists. The forward block location may specify the database or segment of memory in which the forward block should be executed. For example, if the forward block is executed to copy a subset of data from a first database, then the first database may be the forward block location.

[0063] In some cases, the forward block location may be explicitly specified by the forward block, and the forward block location may therefore already exist. In other cases, the forward block may be examined to determine the forward block location. In this case, the forward block location may not yet exist. In still other cases, the recoverable script for the atomic transaction may have previously been generated, and the forward block location for the atomic transaction may have previously been defined, and may therefore exist. If the forward block location does not yet exist for the atomic transaction, the forward block location may be generated in step 306.

[0064] In step 308, it may be determined whether an undo block exists for the atomic transaction. The undo block may be executed to reverse the changes made by the forward block. For example, if the forward block is executed to copy a subset of data from a first database to a second database, the undo block may be executed to delete the subset of data from the second database. If the recoverable script for the atomic transaction has previously been generated, the undo block for the atomic operation may exist. In other cases, the undo block may not yet exist.

[0065] If the undo block does not yet exist, in step 310, it may be determined whether an undo block can be created. In some cases, the undo block may be created, for example, based on the forward block. In other cases, it may not be possible to generate the undo block at this point. For example, if one or more rules affect the execution of the forward block, the undo block may not be generated until the rules are obtained during the execution of the forward block. If it is possible to generate the undo block, the undo block may be generated in step 312.

[0066] In step 314, it may be determined whether an undo block location exists for the atomic transaction. The undo block location may specify the database or segment of memory in which the undo block should be executed. For example, if the undo block is executed to delete a subset of data from the second database, then the second database may be the undo block location.

[0067] In some cases, the undo block location may be explicitly specified by the undo block, and the undo block location may therefore already exist. In other cases, the undo block may be examined to determine the undo block location. In this case, the undo block location may not yet exist. In still other cases, the recoverable script for the atomic operation may have previously been generated, and the undo block location for the atomic operation may exist. If the undo block location for the atomic operation does not yet exist, the undo block location may be specified in step 316.

[0068] In step 318, it may be determined whether the state block exists for the atomic operation. The state block may be executed to set the states of variables and set other internal states to mimic the successful operation of the forward block. The state block may be used for rolling forward and rolling back changes to the database, as will be described further with reference to FIGS. 5-6.

[0069] If the recoverable script for the atomic transaction has previously been generated, the state block for the atomic transaction may exist. In other cases, the state block may not yet exist.

[0070] If the state block does not yet exist, in step 320, it may be determined whether the state block can be created. In some cases, the state block may be created, for example, based on the forward block. In other cases, it may not be possible to generate the state block at this point. For example, if one or more rules affect the execution of the forward block, the state block may not be generated until the rules are obtained during the execution of the forward block.

[0071] If it is possible to generate the state block, the state block may be generated in step 322.

[0072] In step 324, it may be determined whether a state block location exists for the atomic transaction. The state block location may specify the database or segment of memory in which the state block should be executed. For example, if the state block is executed to set variables in a first memory block, then the first memory block may be the state block location.

[0073] In some cases, the state block location may be explicitly specified by the state block, and the state block location may therefore already exist. In other cases, the state block location may not yet exist, and the state block may be examined to determine the state block location. In still other cases, the recoverable script for the atomic operation may have previously been generated, and the state block location for the atomic transaction may exist.

[0074] If the state block location for the atomic transaction does not yet exist, the state block location may be specified in step 326.

[0075] The redo, undo, and state blocks may include references to one or more transaction logs. In one implementation, these references to the transaction logs may constitute the grouping of transaction logs.

[0076] FIG. 4 is a flow chart illustrating a method for executing a block within a recoverable script. A one-step operation may include one or more atomic transactions. In one implementation, the one-step operation may be implemented as a one-step API, and the atomic transactions may be implemented, for example, as lower-level APIs. Prior to execution of the one-step operation, a recoverable script may be generated for each atomic transaction, as discussed with reference to FIG. 3. During the execution of a one-step API, a recoverable script may be executed for each atomic transaction, as illustrated in FIG. 4.

[0077] A method for executing a recoverable script may begin in step 400, wherein the forward block may be executed. In step 402, it may be determined whether an undo block exists for the atomic transaction. In some cases, the undo block may already exist, for example, because it has been generated during step 312 of FIG. 3. However, in other cases, it may not be possible to generate the undo block until the execution of the forward block. In this case, the undo block may not yet exist, and in step 404, generation of the undo block may begin.

[0078] In step 406, it may be determined whether a state block exists for the atomic transaction. In some cases, the state block may already exist, for example, because it has been generated during step 326 of FIG. 3. However, in other cases, it may not be possible to generate the state block until the execution of the forward block. In this case, the state block may not yet exist, and in step 408, generation of the state block may begin.

[0079] In step 410, it may be determined whether all processes are complete. This may include, for example, determining whether execution of the forward block is complete, whether generation of the undo block is complete, and whether generation of the state block is complete. When all processes are complete, the atomic transaction may commit in step 412.

[0080] Prior to executing a one-step operation, a recoverable script may be generated for each atomic transaction included in the one-step operation, as shown in FIG. 3. During execution of a one-step operation, a recoverable script may be executed for a set of atomic transactions. This may include executing a plurality of blocks, executing each block according to a method shown in FIG. 4. In the case of a failure in execution of the one-step operation, a user may choose to roll forward or roll back changes to the database. A method for rolling forward changes associated with the one-step operation is illustrated in FIG. 5, and a method for rolling back changes associated with the one-step operation is illustrated in FIG. 6.

[0081] FIG. 5 is a flow chart illustrating a method for rolling forward after a failure. As shown in FIG. 5, the method may begin in step 500, wherein the error causing the failure may be noted, if possible. In some cases, the process may fail before the error can be noted, and the method will begin in step 501.

[0082] In step 501, the state blocks for each successful committed transaction may be executed, in the order in which the transactions were executed. Executing the state block for each successfully committed transaction may ensure that all internal states are set to the values they had prior to the execution of the failed transaction. This may ensure that the execution of the undo and forward blocks in steps 502, 504, and 506 proceeds appropriately.

[0083] In step 502, the undo block corresponding to the failed atomic transaction may be executed. Executing the undo block for the failed step may reverse any changes that may have been made in the failed step.

[0084] In step 504, the forward block for the failed transaction may be executed. In step 506, processing may continue. Processing 506 may include, for example, executing the forward block for any transactions following the failed transaction.

[0085] FIG. 6 is a flow chart illustrating a method for rolling back after a failure. As shown in FIG. 6, the method may begin in step 600, wherein the error causing the failure may be noted, if possible. In some cases, the process may fail before the error can be noted, and the method will begin in step 601.

[0086] In step 601, the state blocks for each successful committed transaction may be executed, in the order in which the transactions were executed. Executing the state block for each successfully committed transaction may ensure that all internal states are set to the values they had prior to the execution of the failed transaction. This may ensure that the execution of the undo blocks in steps 602, 604, and 606 proceeds appropriately.

[0087] In step 602, the undo block corresponding to the failed atomic transaction may be executed. Executing the undo block for the failed step may reverse any changes that may have been made in the failed step.

[0088] In steps 604 and 606, undo blocks may be executed for each prior transaction, in reverse order. For example, if a failure occurred during the third atomic transaction, the undo block for the second atomic transaction would be executed, followed by the undo block for the first atomic transaction.

[0089] In step 604, it may be determined whether a previous transaction exists. If a previous transaction exists, the undo block for the previous transaction may be executed in step 606. The method may then return to step 604. Rolling back the changes made by each atomic transaction in reverse order may return the databases to their original state.

[0090] The execution of the sequences of instructions required to practice the invention may be performed in embodiments of the invention by a computer system 1400 as shown in FIG. 7. In an embodiment of the invention, execution of the sequences of instructions required to practice the invention is performed by a single computer system 1400. According to other embodiments of the invention, two or more computer systems 1400 coupled by a communication link 1415 may perform the sequence of instructions required to practice the invention in coordination with one another. In order to avoid needlessly obscuring the invention, a description of only one computer system 1400 will be presented below; however, it should be understood that any number of computer systems 1400 may be employed to practice the invention.

[0091] A computer system 1400 according to an embodiment of the invention will now be described with reference to FIG. 7, which is a block diagram of the functional components of a computer system 1400 according to an embodiment of the invention. As used herein, the term computer system 1400 is broadly used to describe any computing device that can store and independently run one or more programs.

[0092] Each computer system 1400 may include a communication interface 1414 coupled to the bus 1406. The communication interface 1414 provides two-way communication between computer systems 1400. The communication interface 1414 of a respective computer system 1400 transmits and receives electrical, electromagnetic or optical signals that include data streams representing various types of signal information, e.g., instructions, messages and data. A communication link 1415 links one computer system 1400 with another computer system 1400. For example, the communication link 1415 may be a LAN, in which case the communication interface 1414 may be a LAN card, or the communication link 1415 may be a PSTN, in which case the communication interface 1414 may be an integrated services digital network (ISDN) card or a modem.

[0093] A computer system 1400 may transmit and receive messages, data, and instructions, including program, i.e., application, code, through its respective communication link 1415 and communication interface 1414. Received program code may be executed by the respective processor(s) 1407 as it is received, and/or stored in the storage device 1410, or other associated non-volatile media, for later execution.

[0094] In an embodiment, the computer system 1400 operates in conjunction with a data storage system 1431, e.g., a data storage system 1431 that contains a database 1432 that is readily accessible by the computer system 1400. The computer system 1400 communicates with the data storage system 1431 through a data interface 1433. A data interface 1433, which is coupled to the bus 1406, transmits and receives electrical, electromagnetic or optical signals that include data streams representing various types of signal information, e.g., instructions, messages and data. In embodiments of the invention, the functions of the data interface 1433 may be performed by the communication interface 1414.

[0095] Computer system 1400 includes a bus 1406 or other communication mechanism for communicating instructions, messages and data, collectively, information, and one or more processors 1407 coupled with the bus 1406 for processing information. Computer system 1400 also includes a main memory 1408, such as a random access memory (RAM) or other dynamic storage device, coupled to the bus 1406 for storing dynamic data and instructions to be executed by the processor(s) 1407. The main memory 1408 also may be used for storing temporary data, i.e., variables, or other intermediate information during execution of instructions by the processor(s) 1407.

[0096] The computer system 1400 may further include a read only memory (ROM) 1409 or other static storage device coupled to the bus 1406 for storing static data and instructions for the processor(s) 1407. A storage device 1410, such as a magnetic disk or optical disk, may also be provided and coupled to the bus 1406 for storing data and instructions for the processor(s) 1407.

[0097] A computer system 1400 may be coupled via the bus 1406 to a display device 1411, such as, but not limited to, a cathode ray tube (CRT), for displaying information to a user. An input device 1412, e.g., alphanumeric and other keys, is coupled to the bus 1406 for communicating information and command selections to the processor(s) 1407.

[0098] According to one embodiment of the invention, an individual computer system 1400 performs specific operations by their respective processor(s) 1407 executing one or more sequences of one or more instructions contained in the main memory 1408. Such instructions may be read into the main memory 1408 from another computer-usable medium, such as the ROM 1409 or the storage device 1410. Execution of the sequences of instructions contained in the main memory 1408 causes the processor(s) 1407 to perform the processes described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to implement the invention. Thus, embodiments of the invention are not limited to any specific combination of hardware circuitry and/or software.

[0099] The term “computer-usable medium,” as used herein, refers to any medium that provides information or is usable by the processor(s) 1407. Such a medium may take many forms, including, but not limited to, non-volatile and volatile media. Non-volatile media, i.e., media that can retain information in the absence of power, includes the ROM 1409, CD ROM, magnetic tape, and magnetic discs. Volatile media, i.e., media that cannot retain information in the absence of power, includes the main memory 1408.

[0100] In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. For example, the reader is to understand that the specific ordering and combination of process actions shown in the process flow diagrams described herein is merely illustrative, and the invention can be performed using different or additional process actions, or a different combination or ordering of process actions. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense.