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 our 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 WebSphere Application Server. 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

• RC = JDBC Read Committed

• RR = JDBC Repeatable Read

• S = JDBC Serializable

• * Oracle does not support JDBC Repeatable Read (RR). Therefore, wsPessimisticUpdate-weakestLockAtLoad and wsPessimisticUpdate behave the same way on Oracle as do wsPessismisticRead and wsOptimisticRead. Because of an Oracle restriction, the OracleXADataSource JDBC class cannot run with an S transaction isolation level. Therefore, we cannot use this class to run an application containing enterprise beans with access intent policies configured to cause the bean to load with S isolation.

• Set access intent policies per EJB method support is deprecated for Version 6.0. IBM recommends set access intent only for the entire bean.

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:

• Read Committed with Snapshots

• Transaction Snapshot (for Serializable)

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, Version 5 Release 4. In the Contents navigation area, click Database > Reference > SQL Reference.

Related concepts

Concurrency control