A FINE-GRAINED CONSISTENCY MECHANISM FOR OPTIMISTIC CONCURRENCY CONTROL USING LOCK GROUPS

Abstract: A method and system for database concurrency control are provided that allow lock groups to contain columns of different tables and allow an individual column of a table to be in more than one lock group. While using optimistic concurrency control for monitoring multiple transactions modifying the same database, it allows the concurrent access of a single table when the individual columns of the table are accessed by separate users or applications. This, in turn, reduces the delay of waiting for a table to be free for access and decreases the delay of rolling back transactions that are concurrently accessing a table. The reduction of these delays increases the overall data processing efficiency for the system.

Description: A FINE-GRAINED CONSISTENCY MECHANISM FOR OPTIMISTIC CONCURRENCY CONTROL USING LOCK GROUPS RELATED APPLICATIONS The following identified U.S. patent applications are relied upon and are incorporated in their entirety by reference in this application. Provisional U.S. Patent Application No. 60/068,415, entitled System and Method for Mapping Between Objects and Databases," filed on December 22, 1997. U.S. Patent Application No. , entitled "Object Relational Mapping Tool That Processes Views," bearing attorney docket no. 06502.0136-00000, and filed on the same date herewith. U.S. Patent Application No. , entitled "Evolution of Object- Relational Mapping Through Source Code Merging" bearing attorney docket no. 06502.0137-00000, and filed on the same date herewith. U.S. Patent Application No. , entitled "Integrating Both Modifications To Source Code and Modifications To A Database Into Source Code By An Object-Relational Mapping Tool," bearing attorney docket no. 06502.0138-00000, and filed on the same date herewith. U.S. Patent Application No. , entitled "Rule-Based Approach to Object-Relational Mapping Strategies," bearing attorney docket no.06502.0139-00000, and filed on the same date herewith. U.S. Patent Application No. , entitled "User Interface for the Specification of Lock Groups," bearing attorney docket no.06502.0142-00000, and filed on the same date herewith. U.S. Patent Application No. , entitled "User Interface for the Specification of Index Groups Over Classes," bearing attorney docket no. 06502.0144-00000, and filed on the same date herewith. U.S. Patent Application No. , entitled "Method and Apparatus for Creating Indexes in a Relational Database Corresponding to Classes in an Object-Oriented Application," bearing attorney docket no.06502.0145-00000, and filed on the same date herewith. U.S. Patent Application No. , entitled "Method and Apparatus for Loading Stored Procedures in a Database Corresponding to Object-Oriented Data Dependencies," bearing attorney docket no. 06502.0146-00000, and filed on the same date herewith. U.S. Patent Application No. , entitled "An Integrated Graphical User Interface Method and Apparatus for Mapping between Objects and Databases," bearing attorney docket no. 06502.0147-00000, and filed on the same date herewith. U.S. Patent Application No. , entitled "Methods and Apparatus for Efficiently Splitting Query Execution Across Client and Server in an Object-Relational Mapping," bearing attorney docket no. 06502.0148-00000, and filed on the same date herewith. Field of the Invention The present invention generally relates to data processing systems and, more particularly, to a method and system for using lock groups to implement fine-grained consistency for optimistic concurrency control. Background of the Invention Computer systems often use databases to store and manage large amounts of data. These databases usually have multiple clients that use and manipulate this data. Clients typically manipulate data in the form of various "read" and "write" operations that either allow the client to retrieve data from the database or allow the client to modify data in the database. A typical client transaction may have several read and write operations, and in a database, it is common for many transactions to concurrently execute to improve processing efficiency. In such a case, if access to the database by each transaction is allowed freely, the consistency among the data can be jeopardized. It is common for many database clients, or multiple procedures on a single client machine, to seek concurrent access to a database system in order to read data from the database and write data to the database. However, each client needs to interact with the database free from the interference of others. That is, during the course of a client's transaction to the database— reading and modifying a set of data from the database— that portion of the database must remain unchanged by other clients. To prevent this problem, concurrency control is used by database systems to govern concurrent access to the database system. It ensures the integrity of each transaction in a multi-user environment. One type of concurrency control is "optimistic concurrency control." Optimistic concurrency control means that the check point for collisions between transactions is at the end of a transaction. With optimistic concurrency control, it is assumed that a transaction will finish before another transaction attempts to change the same data. The transaction reads the necessary data, internally makes its data updates, and finally determines if any of the data has changed before writing the updates to the database. When the client is ready to actually commit its data modifications to the database, the system checks if that data has been modified by any other transaction since the time the client's transaction first read the data. If there has been no change to the particular data, then the client's transaction can complete. If any data has been changed, then the system rolls back the client's updates and notifies the client so that it can retry its transaction, if desired. A "roll back" refers to the transaction being canceled and all data being restored to its original state before the transaction started. While optimistic concurrency control can provide significant performance improvement by avoiding the delay of initially securing the data, it is also possible that the system must roll back a transaction if another process modifies the same data. Referring now to object-oriented systems and programming languages, such systems use "classes" containing both data members that store data and function members (or methods) that act upon the data. Classes form templates for the creation of "objects," representing instances of a class. Classes also define methods, which are procedures that operate on objects of the same class. An exemplary object-oriented programming language is the Java™ programming language described in "The Java Programming Language," Ken Arnold, James Gosling, Addison- Wesley, 1996, which is incorporated herein by reference. For further description of the Java language, refer to "The Java Language Specification," James Gosling, Bill Joy, Guy Steele, Addison- Wesley, 1996, which is incorporated herein by reference. When accessing databases in object-oriented systems, some systems use "lock groups" to implement optimistic concurrency control. With reference to optimistic concurrency control, a lock group, despite its name, does not refer to locking blocks of data. Instead, a lock group provides a collision detection mechanism. In particular, lock groups specify one or more fields to be modified by only one client during a transaction by that client. Lock groups are thus useful for managing data in a database. For example, if a customer object includes fields for a customer name and address, a lock group may specify the name and address fields so that, for example, one client does not change the data in the name field while another client changes data in the corresponding address field. In other words, the data in the name and address fields can only be changed by one client during a transaction to avoid, for example, an incorrect address being entered for a particular name. Lock groups are used to determine conflicts when a client commits the transaction. When a client performs a transaction modifying data in a particular object and attempts to commit the transaction to save the modifications, the system determines if any lock groups include any of the modified fields, and if so, it further determines if any other application modified any field of those lock groups. The system does not commit the transaction and modify the database if another client made such modifications. A conventional lock group includes all fields in an object. However, objects may include many fields and a client will often only modify a small set of fields in an object. Many conflicts occur when a lock group contains a large number of fields because only one client may modify data in an entire object during a transaction, which effectively prevents other clients from modifying any data in the object during the transaction. Conventional object-oriented systems using optimistic concurrency control to access a database do not allow the client to modify some fields of an object without stopping other transactions from simultaneously modifying other fields in the same object. In conventional systems, if one transaction modifies a field of an object, no other transaction may modify another field in the same object. Consequently, conventional systems do not allow a field to simultaneously belong to more than one lock group. Additionally, conventional lock groups do not contain fields of different classes. These limitations slow the system by stopping different applications from simultaneously accessing different fields of the same object. It is therefore desirable to improve such systems. SUMMARY Methods and systems consistent with the present invention satisfy this and other desires by providing a method for allowing lock groups to contain fields in different classes and allowing a field to be in more than one lock group. In accordance with methods consistent with the present invention, a method is provided in a data processing system having lock groups and classes with fields. This method receives an indication of a lock group specifying that the lock group contains a first field from a first class and a second field from a second class and creates the specified lock group. It further receives a request to commit a transaction affecting a value for one of the fields in the lock group, and determines whether to commit the transaction based on whether at least one of the fields in the lock group was modified during the transaction. Methods and systems consistent with the present invention allow the concurrent access of a single object when the individual fields of the object are accessed by separate clients. This, in turn, reduces the delay of waiting for an object to be free for access and decreases the delay of rolling back transactions that are concurrently accessing an object. The reduction of these delays increases the overall data processing efficiency for the system. The above features, other features and advantages of the present invention will be readily appreciated by one of ordinary skill in the art from the following detailed description of the preferred implementations when taken in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of the invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings, Figure 1 is a diagram of a computer system in accordance with methods and systems consistent with the present invention; Figure 2 is a flowchart of a method for committing a transaction in accordance with methods and systems consistent with the present invention; and Figure 3 is a diagram of an example of concurrent transactions in accordance with methods and systems consistent with the present invention. DETAILED DESCRIPTION Introduction Methods and systems consistent with the present invention use optimistic concurrency control and allow multiple clients to simultaneously modify the same obj ect provided that they modify fields in different lock groups. They further allow an optimistic concurrency control lock group to contain fields of different classes. Additionally, they allow a single field to be in more than one lock group. These systems allow "fine-grained" consistency for concurrent access to a database. Fine-grained consistency refers to the fact that individual fields of an object may be locked without locking the entire obj ect, thereby allowing other transactions to concurrently access other fields of the object. Methods and systems consistent with the present invention allow increased concurrent access to objects. They reduce the delay of waiting for all fields of an object to be free for access and, in turn, reduce transaction roll backs. Generally, lock groups are used to specify related data items within an object. While a user modifies data within an object, the user may not affect the entire object. If the data is also part of a lock group, the transaction only impacts the set of fields indicated by the lock group. Other processes are free to access and update data within the same object, as long as that data is outside of the lock group. For example, it might be possible for some other process to change fields in the obj ect without conflicting with the transaction's changes. When ready to commit data changes, the system checks the original, unmodified copy of the data against the current data (at commit time) in the database. If the data is the same, the transaction commits its changes to the database. However, if it detects a difference, then the system scans the lock groups for this object and checks for a collision. That is, it checks to determine whether it is about to commit a change to a field that is specified within a lock group, and during the time since it read the data and prepared to commit its changes, some other client has modified either the same field or another field also specified within the lock group. If so, this is a collision condition and the process does not commit its changes. Note that there can be multiple lock groups for an object, and these lock groups may overlap. In one embodiment, a client using a system consistent with the present invention establishes the lock group as a group of fields that are interdependent. A lock group can correspond to a single field or any combination of fields in any number of classes. The user can specify those fields that together should be treated as a lock group, and often those fields whose values depend on each other are grouped together. When an application modifies a field, its transaction may be adversely affected if another application modifies or updates any field that belongs to any lock group that contains the specified field. In one implementation of the present invention, the system operates in an object- relational mapping tool that maps relational database information into objects and vice versa where the techniques described below are performed when transactions are committed. Such a mapping tool is further described in co-pending U.S. application no. , entitled "User Interface for the Specification of Lock Groups," which has previously been incorporated by reference. Such a mapping tool generates source code from a schema in a relational database containing tables. The generated source code contains classes that contain fields, and these classes and fields are generated from the tables and columns of the relational database. The mapping tool maps a table to a class and maps each column of the table to a field in the class. Users can use the mapping tool to define lock groups on the fields of the classes, however, these lock groups are actually effectuated by lock groups on the corresponding columns in the tables corresponding to the objects. For purposes of clarity, the lock groups will be described below as being defined on the columns of tables in the database. However, one skilled in the art will appreciate that the lock groups can also be viewed as being implemented on fields of a class or other elements in a data structure. System Details Figure 1 is a block diagram of a data processing system 100 suitable for use with methods and systems consistent with the present invention. The data processing system 100 comprises a computer system 101 connected to the Internet 103. Computer system 101 includes a central processing unit (CPU) 104, a main memory 106, and a secondary storage device 110 interconnected via bus 102. Additionally, the computer system 100 includes a display 112, and an input device 114. The main memory 106 contains a virtual machine (VM) 120 such as the Java™ Virtual Machine (JVM), a hash table 122, a client 126, and a transaction manager 128. The transaction manager 128 also contains a cache 130 used to temporarily store updated data before committing it to the database. As described below, the hash table 122 may be implemented as any form of temporary storage and is used to temporarily store columns of tables belonging to lock groups. The Java™ Virtual Machine is a well-known execution vehicle for computer programs and is described in greater detail in Lindholm and Yellin, The Java Virtual Machine Specification. Addison- Wesley, 1996, which is incorporated herein by reference. The VM 120 is contained a Java Runtime Environment (JRE) 121. The client 126 utilizes the database 124 by using the transaction manager 128. The transaction manager 128 performs transactions on the database 124 using fine-grained concuπency control as will be further described below. The database 124 stores data against which transactions may be performed. This database 124 may reside on a server computer (not shown) or, in another implementation consistent with the present invention, may also reside on the computer 101. One skilled in the art will appreciate that the hash table 122 and other aspects of methods and systems consistent with the present invention may be stored on or read from any other computer readable media besides memory like secondary storage devices, such as hard disks, floppy disks, and CD ROM, or a carrier wave from a network 103, such as the Internet. Although the hash table 122 is described as operating in a VM 120, one skilled in the art will appreciate that the hash table 122 may operate in other programs, like operating systems, and may operate in other object-oriented or non-object oriented programming environments. Additionally, one skilled in the art will also appreciate that data processing system 100 may contain additional or different components. Method Figure 2 depicts a flowchart of the steps performed by the transaction manager 128 when implementing lock group transactions consistent with the present invention. The transaction manager 128 receives a transaction request from a client 126. During a transaction, the transaction manager 128 stores local copies of the columns in memory (i.e., a cache) and performs operations on these copies of the columns, using the well- known technique of caching. Only at commit time does the transaction manager 128 update the copy of the columns in the database 124. Generally, when it is time for the transaction to commit, if a column of a lock group is to be updated, the transaction manager 128 checks whether all columns in that lock group remained unchanged in the database 124 during the transaction. If so, the transaction manager 128 commits the transaction. Otherwise, the transaction manager 128 rolls back the transaction. When it is time to commit the changes to the database 124, the first step performed by the transaction manager 128 is to determine which columns were updated in the cache 130 by the transaction (operation 200). To accomplish this, the transaction manager 128 at the beginning of the transaction cached the initial value of the columns on which the transaction is to operate and now compares these values with the current values of the columns in the cache 130; columns that do not have the same value have been updated in the cache 130. Next, the transaction manager 128 selects a lock group (operation 202) and determines if any of the columns of the lock group were updated in the cache 130 during the transaction (operation 204). If any of the columns of the lock group were changed, all of the columns of that lock group are added to the hash table 122 (operation 206), and if there are more lock groups to process (operation 208), processing continues to operation 202. Otherwise, the hash table 122 has been populated with columns from the lock groups that have changed, and the transaction manager 128 selects a column from the hash table (operation 210). The transaction manager 128 then determines whether the database value for this column has changed in the database 124 during the processing of the transaction (operation 212). To make this determination, the transaction manager 128 compares what the database value for the column was at the beginning of the transaction with the database value for the column at the end of the transaction (commit time). If the database value for the column has changed in the database 124 during the transaction, the transaction manager 128 rolls back the transaction by aborting it (operation 214). If, however, the value has not changed, another column is selected (operation 216). If none of the columns in the hash table 122 had database values that changed in the database 124 during the transaction, the transaction manager 128 commits the transaction, and then updates the columns in the database based on the value of the columns in the cache 130 (operation 218). Example For example, a customer table might consist of the columns lastname , firstname , street , city, zip_cc.de , area_code , ρhone_number , and sales_representative . It might not make sense to define a lock group that contains both lastname and sales_representative because, in most environments, changes to the values of these two columns would be independent of each other. (However, if a customer were assigned a sales representative based simply on last name, then these columns would be dependent on each other.) However, because the address components— street , city, zip_code, area_code , and phone_number— are interrelated, they might be placed together in one lock group. Having this information in the same lock group ensures that if two clients were simultaneously updating address information for the same customer, one client would not be updating the phone_numbe r while the other client was updating the customer's area_code. However, two processes can simultaneously update the same customer table provided that the data they access is not in the same lock group, thus improving concurrent access. Since the same column can be in multiple lock groups, in the above example, the area_code might be in both the Phone and Address lock groups. Figure 3 illustrates multiple transactions accessing the same table using lock groups in an optimistic concurrency control situation in accordance with methods and systems consistent with the present invention. In this example, MyCustomerTable has six data members divided into three lock groups (Phone, Address, and CustomerName) . Four transactions (T1-T4 ) simultaneously access different attributes within the same MyCustomerTable instance. Transaction TI modifies two attributes-city and street, while transaction T2 modifies lastname. They both complete because TI commits before any changes are committed by any of the other transactions, and none of the columns modified by TI belong to any lock group containing the columns modified by T2 . However, the system does not let transaction 3 complete its modification to area_code because other attributes in lock group Address changed after T3 read area_code, but before it completed its modifications. These changes were committed to the database 124 by TI. Note, though, that the system allows T4 to commit because there are no conflicting changes. If T4 had attempted to change area_code, there would have been a conflict because area_code belongs to the lock groups Address and Phone, and I already changed other attributes that belong to Address. Transaction T3 and transaction T4 potentially had a conflict because of modifications to attributes in lock group Phone, which has the attributes ar e a_code and phone_number. T4 can commit its changes to phone_number because the database values for the attributes in lock group Phone remain unchanged from when T4 read them— due to T3 ' s failed attempt to modify area_code. However, had T3 succeeded, then T4 would have failed. Conclusion Methods and systems consistent with the present invention allow a lock group to contain individual columns of different tables. They also allow a column in a table to belong to more than one lock group. This permits transactions to concurrently access a single table in the database if the individual columns of the lock groups do not overlap. The concurrent access of a single table in the database increases the overall data processing efficiency by reducing the delay of transaction roll backs and the delay of waiting for an table to be released from a lock group. The foregoing description of an implementation of the invention has been presented for purposes of illustration and description. It is not exhaustive and does not limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teaching or may be acquired from practicing of the invention. The scope of the invention is defined by the claims and their equivalents.

Claims: What is claimed is: 1. A method in a data processing system having a database and data structures with elements, the method comprising: receiving an indication of a lock group specifying that the lock group contains a first element from a first data structure and a second element from a second data structure; creating the specified lock group; receiving a request to commit a transaction to the database affecting a value for one of the elements in the lock group; and determining whether to commit the transaction to the database based on whether at least one of the elements in the lock group was modified by another transaction during the transaction. 2. The method of claim 1 , wherein the determining further includes: aborting the transaction when at least one of the elements of the lock group was modified during the transaction. 3. The method of claim 1 , wherein the determining further includes: committing the transaction when the elements of the lock group were unchanged during the transaction. 4. The method of claim 1, wherein the data structure is a class and the elements are fields, and wherein the receiving further includes: receiving an indication of a lock group specifying that the lock group contains a first field from a first class and a second field from a second class. 5. The method of claim 1, wherein the data structure is a table and the elements are columns, and wherein the receiving further includes: receiving an indication of a lock group specifying that the lock group contains a first column from a first table and a second column from a second table. 6. A method in a data processing system having a database and a data structure with elements, the method comprising: receiving an indication of two lock groups specifying that the two lock groups each share a first of the elements; creating the specified lock groups; receiving a request to commit a transaction to the database affecting a value of a second of the elements in one of the lock groups; and determining whether to commit the transaction based on whether at least one of the elements in the one lock group was modified during the transaction. 7. The method of claim 6, wherein the determining further includes: aborting the transaction when at least one of the elements of the one lock group was modified during the transaction. 8. The method of claim 6, wherein the determining further includes: committing the transaction when the elements of the one lock group were unchanged during the transaction. 9. A method in a data processing system having a database with database data structures and a hash table, the method comprising: creating a lock group with elements from different database data structures; initiating a transaction by a client such that at least one of the elements in the lock group is updated; receiving a request to commit the transaction, further including the steps of: determining which of the elements were updated by the transaction; determining which of the lock groups contain the updated elements; adding the updated elements of the determined lock groups to the hash table; determining whether the added elements were updated in the database by another client during the transaction; aborting the transaction when it is determined that at least one of the added elements was updated in the database by another client during the transaction; and committing the transaction when it is determined that the added elements remained unchanged in the database during the transaction. 10. A computer-readable memory device comprising: a first data structure with a first element defined to be a member of a lock group; and a second data structure with a second element defined to be a member of the lock group such that a determination of whether to commit a transaction requested by a client is based on whether the first element or the second element were updated by another client during the transaction. 11. A data processing system having a database with database data structures and a hash table, the data processing system comprising: means for creating a lock group with elements from different database data structures; means for initiating a transaction by a client such that at least one of the elements in the lock group is updated; means for receiving a request to commit the transaction, further including: means for determining which of the elements were updated by the transaction; means for determining which of the lock groups contain the updated elements; means for adding the updated elements of the determined lock groups to the hash table; means for determining whether the added elements were updated in the database by another client during the transaction; means for aborting the transaction when it is determined that at least one of the added elements was updated in the database by another client during the transaction; and means for committing the transaction when it is determined that the added elements remained unchanged in the database during the transaction. 12. A computer-readable medium containing instructions for controlling a data processing system to perform a method, the data processing system having a database and data structures with elements, the method comprising: receiving an indication of a lock group specifying that the lock group contains a first element from a first data structure and a second element from a second data structure; creating the specified lock group; receiving a request to commit a transaction to the database affecting a value for one of the elements in the lock group; and determining whether to commit the transaction to the database based on whether at least one of the elements in the lock group was modified during the transaction. 13. The computer-readable medium of claim 12 , wherein the determining step of the method further includes: aborting the transaction when at least one of the elements of the lock group was modified during the transaction. 14. The computer-readable medium of claim 12, wherein the determining step of the method further includes: committing the transaction when the elements of the lock group were unchanged during the transaction. 15. A computer-readable medium containing instructions for controlling a data processing system to perform a method, the data processing system having a database and a data structure with elements, the method comprising: receiving an indication of two lock groups specifying that the two lock groups each share a first of the elements; creating the specified lock groups; receiving a request to commit a transaction to the database affecting a value of a second of the elements in one of the lock groups; and determining whether to commit the transaction based on whether at least one of the elements in the one lock group was modified during the transaction. 16. The computer-readable medium of claim 15 , wherein the determining step of the method further includes: aborting the transaction when at least one of the elements of the one lock group was modified during the transaction. 17. The method of claim 15, wherein the determining step further includes: committing the transaction when the elements of the one lock group were unchanged during the transaction. 18. A data processing system comprising: a secondary storage device with a database containing data structures with elements; a memory further including: a transaction manager configured to receive a definition of a lock group containing a element in a first data structure and a element in a second data structure, configured to receive from a client a request to commit a transaction wherein a first one of the elements of the lock group is modified, and configured to abort the transaction when a second one of the elements of the lock group is changed in the database by another client during the transaction; and a processor for running the transaction manager. 19. A data processing system comprising: a secondary storage device with a database containing data structures with elements; a memory further including: a transaction manager configured to receive a definition of two lock groups sharing a first one of the elements, configured to receive from a client a request to commit a transaction wherein a second one of the elements of one of the two lock groups is modified, and configured to abort the transaction when a third one of the elements of the one lock group containing the second one of the elements is changed in the database by another client during the transaction; and a processor configured to run the transaction manager.

Inventors: LEARMONT TIMOTHY R, LEARMONT, TIMOTHY, R.
Applicants: LEARMONT TIMOTHY R, LEARMONT, TIMOTHY, R.
Application year: 1998
Main classification: G06F17
IPC codes: G06F9, G06F12, G06F17