Sun Microsystems Inc

16.4.1.1 Non-transactional execution

Some enterprise beans may need to access resource managers that do not support an external transaction coordinator. The Container cannot manage the transactions for such enterprise beans in the same way that it can for the enterprise beans that access resource managers that support an external transaction coordinator.

If an enterprise bean needs to access a resource manager that does not support an external transaction coordinator, the Bean Provider should design the enterprise bean with container-managed transaction demarcation and assign the NotSupported transaction attribute to all the bean’s methods. The EJB architecture does not specify the transactional semantics of the enterprise bean methods. See Subsection 16.7.5 for how the Container implements this case.

16.4.2 Isolation levels

Transactions not only make completion of a unit of work atomic, but they also isolate the units of work from each other, provided that the system allows concurrent execution of multiple units of work.

The isolation level describes the degree to which the access to a resource manager by a transaction is isolated from the access to the resource manager by other concurrently executing transactions.

The following are guidelines for managing isolation levels in enterprise beans.

•The API for managing an isolation level is resource-manager specific. (Therefore, the EJB architecture does not define an API for managing isolation level.)

•If an enterprise bean uses multiple resource managers, the Bean Provider may specify the same or different isolation level for each resource manager. This means, for example, that if an enterprise bean accesses multiple resource managers in a transaction, access to each resource manager may be associated with a different isolation level.

•The Bean Provider must take care when setting an isolation level. Most resource managers require that all accesses to the resource manager within a transaction are done with the same isolation levels. An attempt to change the isolation level in the middle of a transaction may cause undesirable behavior, such as an implicit sync point (a commit of the changes done so far).

•For session beans and message-driven beans with bean-managed transaction demarcation, the Bean Provider can specify the desirable isolation level programmatically in the enterprise bean’s methods, using the resource-manager specific API. For example, the Bean Provider can use the java.sql.Connection.setTransactionIsolation(...) method to set the appropriate isolation level for database access.

•For entity beans with container-managed persistence, transaction isolation is managed by the data access classes that are generated by the persistence manager provider’s tools. The tools must ensure that the management of the isolation levels performed by the data access classes will not result in conflicting isolation level requests for a resource manager within a transaction.