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Access intent -- isolation levels and update locks

WebSphere Application Server access intent policies provide a consistent way of defining the isolation level for CMP bean data across the different relational databases in the environment.

Within a deployed application, the combination of an access intent policy concurrency definition and access type signifies the isolation level value that Application Server sets on a database connection. See the articles, Concurrency control, and Access intent and isolation, for more information on concurrency and access type. This combination of properties also signifies the update lock flag that Application Server passes to the database through a JDBC prepared statement.

Databases do not provide as many isolation level definitions as WAS. Databases define an isolation level as one of only three types. Furthermore, only one parameter indicates the type of isolation level that the databases set on incoming connections. Each of the three types can be represented by a different parameter value, as determined by each database vendor. For example, one database might define an isolation level as RR (JDBC Repeatable read), whereas a different database might define the same isolation level as RC (JDBC Read committed).

Because of this inconsistency, WAS does not map access intent policies to the parameter values. Instead, Application Server maps access intent policies to the types of isolation level that are common across all database vendors.

Access Intent profile Isolation level Update lock implementation

DB2 Oracle* SyBase Informix Apache Derby SQL Server
wsPessimisticUpdate- Weakest LockAtLoad (Default policy) RR RC RR RR RR RR No (*Oracle, Yes)
wsPessimisticUpdate RR RC RR RR RR RR Yes
wsPessimisticRead RR RC RR RR RR RR No
wsOptimisticUpdate RC RC RC RC RC RC No
wsOptimisticRead RC RC RC RC RC RC No
wsPessimisticUpdate No-Collisions RC RC RC RC RC RC No
wsPessimisticUpdate- Exclusive S S S S S S Yes

New for MS SQL Server 2005: MS SQL Server 2005 offers a new option for the Read Committed isolation level and a new option for the Serializable isolation level:

Both options use optimistic locking. To use Read Committed with Snapshots instead of Read Committed, enable the READ_COMMITTED_SNAPSHOT setting for the database according to the MS SQL Server 2005 documentation. To use Transaction Snapshot instead of Serializable, configure the custom data source property, snapshotSerializable, to "true" and enable the ALLOW_SNAPSHOT_ISOLATION setting for the database according to the MS SQL Server 2005 documentation.


Structured Query Language (SQL) keywords and restrictions

Database SQL syntax used for locking update join restrictions order by restrictions subselect restrictions aggregation restrictions
DB2 FOR UPDATE OF not allowed not allowed not allowed not allowed
DB2 UDB for iSeries (V5R3 and earlier) FOR UPDATE OF not allowed allowed with limitations* allowed with limitations* not allowed
DB2 UDB for iSeries (V5R4 and later) WITH RS/RR USE AND KEEP EXCLUSIVE LOCKS not allowed allowed with limitations* allowed with limitations* not allowed
DB2 on z/OS V8.x WITH RS/RR USE AND KEEP UPDATE LOCKS none none none none
DB2 UDB workstation V8.2 WITH RS/RR USE AND KEEP UPDATE LOCKS none none none none
Oracle FOR UPDATE none none none none
Apache Derby FOR UPDATE OF not allowed not allowed not allowed not allowed
Informix FOR UPDATE not allowed not allowed not allowed not allowed
Sybase FOR UPDATE not allowed not allowed not allowed not allowed
Sqlserver UPDLOCK not allowed not allowed not allowed not allowed

For details on the limitations for these permitted SQL restrictions, refer to the DB2 Universal Database™ for iSeries SQL Reference. We can find this document in the iSeries Information Center, v5 Release 4. In the Contents navigation area, click Database > Reference > SQL Reference.


Related:

  • Concurrency control