This document will discuss how, what, and where transaction isolation levels are set in WebSphere Applications by default and for specific resource references.
Resolving the problem
Requirements for setting data access isolation levels
Please read and review the following URL first to obtain a better understanding of how to determine and change the isolation level for a WAS DataSource.
Connections used by other 2.x enterprise beans and other non-CMP components
Note: These are the most common types of connections. For other types of connections, refer to the following URL based on your version of WebSphere Application Server:
WAS v6.1 http://publib.boulder.ibm.com/infocenter/wasinfo/v6r1/index.jsp?topic=/com.ibm.websphere.express.iseries.doc/info/iseriesexp/ae/cdat_isolevel.html
For all other JDBC connection instances (connections other than those used by CMP beans), you can specify an isolation level on the data source resource reference. For shareable connections that run in global transactions, this method is the only way to set the isolationLevel for connections. Trying to directly set the isolation level through the setTransactionIsolation() method on a shareable connection that runs in a global transaction is not allowed. To use a different isolation level on connections, you must provide a different resource reference. Set these defaults through your assembly tool.
Each resource reference associates with one isolation level. When your application uses this resource reference Java Naming and Directory Interface (JNDI) name to look up a data source, every connection returned from this data source using this resource reference has the same isolation level.
Components needing to use shareable connections with multiple isolation levels can create multiple resource references, giving them different JNDI names, and have their code look up the appropriate data source for the isolation level they need. In this way, you use separate connections with the different isolation levels enabled on them.
It is possible to map these multiple resource references to the same configured data source. The connections still come from the same underlying pool; however, the connection manager does not allow sharing of connections requested by resource references with different isolation levels. Consider the following scenario:
o A data source is bound to two resource references: jdbc/RRResRef and jdbc/RCResRef.
o RRResRef has the RepeatableRead isolation level defined. RCResRef has the ReadCommitted isolation level defined.
If your application wants to update the tables or a BMP bean updates some attributes, it can use the jdbc/RRResRef JNDI name to look up the data source instance. All connections returned from the data source instance have a RepeatableRead isolation level. If the application wants to perform a query for read only, it is better to use the jdbc/RCResRef JNDI name to look up the data source.
The resource references can be identified in the WEB-INF/web.xml file of the application. The web.xml will contain something similar to the following to identify the resource reference(s):
The customization of the transaction isolation level is shown in the WEB-INF/ibm-web-ext.xmi file of the application. The ibm-web-ext.xmi will contain something similar to the following to specify the custom isolation level:
<resourceRefExtensions xmi:id="ResourceRefExtension_1" isolationLevel="TRANSACTION_REPEATABLE_READ">
<resourceRefExtensions xmi:id="ResourceRefExtension_2" isolationLevel="TRANSACTION_READ_COMMITTED">
Note the isolationLevel parameter. This is where the custom isolation level is set. Also note the bolded ResourceRef tag. This references the specific resource reference defined in the web.xml file.
If you do not specify the isolation level
The product does not require you to set the isolation level on a data source resource reference for a non-CMP application module. If you do not specify isolation level on the resource reference, or if you specify TRANSACTION_NONE, the WebSphere Application Server run time uses a default isolation level for the data source. Application Server uses a default setting based on the JDBC driver.
For most drivers, WebSphere Application Server uses an isolation level default of TRANSACTION_REPEATABLE_READ. Use the following table for quickreference:
|Default isolation level:
(for connections used by non-CMP entities)
|RR = JDBC Repeatable read (TRANSACTION_REPEATABLE_READ) = *RS (Read Stability) on the IBM i server.|
Note: These same default isolation levels are used in cases of direct JNDI lookups of a data source.
o RR = JDBC Repeatable read (TRANSACTION_REPEATABLE_READ)
o RC = JDBC Read committed (TRANSACTION_READ_COMMITTED)
Available beginning in WebSphere Application Server v6.1 all editions:
To customize the default isolation level, you can use the webSphereDefaultIsolationLevel custom property for the data source. In most cases, you should define the isolation level in the deployment descriptor when you package the EAR file; however, in certain situations, you might need to customize the default isolation level. This property will have no effect if any of the above options are used, and this custom property is provided for those situations in which there is no other means of setting the isolation level.
Use the following values for webSphereDefaultIsolationLevel custom property:
|Possible values||JDBC isolation level||DB2 isolation level|
|8||TRANSACTION_SERIALIZABLE||Repeatable Read (RR)|
|4 (default)||TRANSACTION_REPEATABLE_READ||Read Stability (RS)|
|2||TRANSACTION_READ_COMMITTED||Cursor Stability (CS)|
|1||TRANSACTION_READ_UNCOMMITTED||Uncommitted Read (UR)|
|0||TRANSACTION_NONE||No Commit (NC)|
Note : If TRANSACTION_NONE is used, the DB file does not have to be journaled.
To define this custom property for a data source, you should do the following:
|1.||Click Resources > JDBC provider > JDBC_provider.|
|2.||Click Data sources in the Additional Properties section.|
|3.||Click the name of the data source.|
|4.||Click Custom properties.|
|5.||Create the webSphereDefaultIsolationLevel custom property
a. Click New.
b. Enter webSphereDefaultIsolationLevel for the name field.
c. Enter one of the "possible values" in the value field from the table above. i.e. 0, 1, 2, 4, or 8
1. Resource reference isolation level
2. Isolation level that is specified by the access intent policy
3. Custom property that configures an isolation level
4. Application Server's default setting.