15 Using the Transaction Service

Using JDBC Transaction Isolation Levels

Not all database vendors support all transaction isolation levels available in the JDBC API. The GlassFish Server permits specifying any isolation level your database supports. The following table defines transaction isolation levels.

By default, the transaction isolation level is undefined (empty), and the JDBC driver's default isolation level is used. You can specify the transaction isolation level in the following ways:

• Select the value from the Transaction Isolation drop-down list on the New JDBC Connection Pool or Edit Connection Pool page in the Administration Console. For more information, click the Help button in the Administration Console.

• Specify the --isolationlevel option in the asadmin create-jdbc-connection-pool command. For more information, see the Oracle GlassFish Server Reference Manual.

• Specify the transaction-isolation-level option in the asadmin set command. For example:

  asadmin set domain1.resources.jdbc-connection-pool.DerbyPool.transaction-isolation-level=serializable
  

  For more information, see the Oracle GlassFish Server Reference Manual.

Note that you cannot call setTransactionIsolation during a transaction.

You can set the default transaction isolation level for a JDBC connection pool. For details, see "To Create a JDBC Connection Pool" in Oracle GlassFish Server Administration Guide.

To verify that a level is supported by your database management system, test your database programmatically using the supportsTransactionIsolationLevel method in java.sql.DatabaseMetaData, as shown in the following example:

InitialContext ctx = new InitialContext();
DataSource ds = (DataSource)
ctx.lookup("jdbc/MyBase");
Connection con = ds.getConnection();
DatabaseMetaData dbmd = con.getMetaData();
if (dbmd.supportsTransactionIsolationLevel(TRANSACTION_SERIALIZABLE)
{ Connection.setTransactionIsolation(TRANSACTION_SERIALIZABLE); }

For more information about these isolation levels and what they mean, see the JDBC API specification.

Setting or resetting the transaction isolation level for every getConnection call can degrade performance. So by default the isolation level is not guaranteed.

Applications that change the transaction isolation level on a pooled connection programmatically risk polluting the JDBC connection pool, which can lead to errors. If an application changes the isolation level, enabling the is-isolation-level-guaranteed setting in the pool can minimize such errors.

You can guarantee the transaction isolation level in the following ways:

• Check the Isolation Level Guaranteed box on the New JDBC Connection Pool or Edit Connection Pool page in the Administration Console. For more information, click the Help button in the Administration Console.

• Specify the --isisolationguaranteed option in the asadmin create-jdbc-connection-pool command. For more information, see the Oracle GlassFish Server Reference Manual.

• Specify the is-isolation-level-guaranteed option in the asadmin set command. For example:

  asadmin set domain1.resources.jdbc-connection-pool.DerbyPool.is-isolation-level-guaranteed=true
  

  For more information, see the Oracle GlassFish Server Reference Manual.

Using Non-Transactional Connections

You can specify a non-transactional database connection in any of these ways:

• Check the Non-Transactional Connections box on the New JDBC Connection Pool or Edit Connection Pool page in the Administration Console. The default is unchecked. For more information, click the Help button in the Administration Console.

• Specify the --nontransactionalconnections option in the asadmin create-jdbc-connection-pool command. For more information, see the Oracle GlassFish Server Reference Manual.

• Specify the non-transactional-connections option in the asadmin set command. For example:

  asadmin set domain1.resources.jdbc-connection-pool.DerbyPool.non-transactional-connections=true
  

  For more information, see the Oracle GlassFish Server Reference Manual.

• Use the DataSource implementation in the GlassFish Server, which provides a getNonTxConnection method. This method retrieves a JDBC connection that is not in the scope of any transaction. There are two variants.

  public java.sql.Connection getNonTxConnection() throws java.sql.SQLException
  
  public java.sql.Connection getNonTxConnection(String user, String password) 
     throws java.sql.SQLException
  

• Create a resource with the JNDI name ending in __nontx. This forces all connections looked up using this resource to be non transactional.

Typically, a connection is enlisted in the context of the transaction in which a getConnection call is invoked. However, a non-transactional connection is not enlisted in a transaction context even if a transaction is in progress.

The main advantage of using non-transactional connections is that the overhead incurred in enlisting and delisting connections in transaction contexts is avoided. However, use such connections carefully. For example, if a non-transactional connection is used to query the database while a transaction is in progress that modifies the database, the query retrieves the unmodified data in the database. This is because the in-progress transaction hasn't committed. For another example, if a non-transactional connection modifies the database and a transaction that is running simultaneously rolls back, the changes made by the non-transactional connection are not rolled back.

Here is a typical use case for a non-transactional connection: a component that is updating a database in a transaction context spanning over several iterations of a loop can refresh cached data by using a non-transactional connection to read data before the transaction commits.

Flat Transactions

The Enterprise JavaBeans Specification, v3.0 requires support for flat (as opposed to nested) transactions. In a flat transaction, each transaction is decoupled from and independent of other transactions in the system. Another transaction cannot start in the same thread until the current transaction ends.

Flat transactions are the most prevalent model and are supported by most commercial database systems. Although nested transactions offer a finer granularity of control over transactions, they are supported by far fewer commercial database systems.

Global and Local Transactions

Both local and global transactions are demarcated using the javax.transaction.UserTransaction interface, which the client must use. Local transactions bypass the XA commit protocol and are faster. For more information, see The Transaction Manager, the Transaction Synchronization Registry, and UserTransaction.

Commit Options

The EJB protocol is designed to give the container the flexibility to select the disposition of the instance state at the time a transaction is committed. This allows the container to best manage caching an entity object's state and associating an entity object identity with the EJB instances.

There are three commit-time options:

• Option A - The container caches a ready instance between transactions. The container ensures that the instance has exclusive access to the state of the object in persistent storage.

  In this case, the container does not have to synchronize the instance's state from the persistent storage at the beginning of the next transaction.

  Note:

  Commit option A is not supported for this GlassFish Server release.

• Option B - The container caches a ready instance between transactions, but the container does not ensure that the instance has exclusive access to the state of the object in persistent storage. This is the default.

  In this case, the container must synchronize the instance's state by invoking ejbLoad from persistent storage at the beginning of the next transaction.

• Option C - The container does not cache a ready instance between transactions, but instead returns the instance to the pool of available instances after a transaction has completed.

  The life cycle for every business method invocation under commit option C looks like this.

  ejbActivate   ejbLoad   business method   ejbStore   ejbPassivate
  

  If there is more than one transactional client concurrently accessing the same entity, the first client gets the ready instance and subsequent concurrent clients get new instances from the pool.

The glassfish-ejb-jar.xml deployment descriptor has an element, commit-option, that specifies the commit option to be used. Based on the specified commit option, the appropriate handler is instantiated.

Bean-Level Container-Managed Transaction Timeouts

The transaction timeout for the domain is specified using the Transaction Timeout setting of the Transaction Service. A transaction started by the container must commit (or rollback) within this time, regardless of whether the transaction is suspended (and resumed), or the transaction is marked for rollback. The default value, 0, specifies that the server waits indefinitely for a transaction to complete.

To override this timeout for an individual bean, use the optional cmt-timeout-in-seconds element in glassfish-ejb-jar.xml. The default value, 0, specifies that the Transaction Service timeout is used. The value of cmt-timeout-in-seconds is used for all methods in the bean that start a new container-managed transaction. This value is not used if the bean joins a client transaction.

Transactions and Non-Persistent Messages

During transaction recovery, non-persistent messages might be lost. If the broker fails between the transaction manager's prepare and commit operations, any non-persistent message in the transaction is lost and cannot be delivered. A message that is not saved to a persistent store is not available for transaction recovery.

Using the ConfigurableTransactionSupport Interface

The Java EE Connector 1.6 specification allows a resource adapter to use the transaction-support attribute to specify the level of transaction support that the resource adapter handles. However, the resource adapter vendor does not have a mechanism to figure out the current transactional context in which a ManagedConnectionFactory is used.

If a ManagedConnectionFactory implements an optional interface called com.sun.appserv.connectors.spi.ConfigurableTransactionSupport , the GlassFish Server notifies the ManagedConnectionFactory of the transaction-support configured for the connector connection pool when the ManagedConnectionFactory instance is created for the pool. Connections obtained from the pool can then be used with a transaction level at or lower than the configured value. For example, a connection obtained from a pool that is set to XA_TRANSACTION could be used as a LOCAL resource in a last-agent-optimized transaction or in a non-transactional context.