Transaction Propagation in Bean-Managed Transactions in Java

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Transaction Propagation in Bean-Managed Transactions
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With stateless session beans, transactions that are managed using UserTransaction must be started and completed within the same method. In other words, UserTransaction transactions cannot be started in one method and ended in another. This makes sense because stateless session bean instances are shared across many clients; while one stateless instance may service a client s first request, a completely different instance may service a subsequent request by the same client. With stateful session beans, however, a transaction can begin in one method and be committed in another because a stateful session bean is used by only one client. Therefore, a stateful session bean can associate itself with a transaction across several different clientinvoked methods. When a client that is already involved in a transaction invokes a bean-managed transaction method, the client s transaction is suspended until the method returns. This suspension occurs regardless of whether the BMT bean explicitly started its own transaction within the method or the transaction was started in a previous method invocation. The client transaction is always suspended until the BMT method returns.
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Transaction control across methods is strongly discouraged because it can result in improperly managed transactions and long-lived transactions that lock up or even leak resources. It s best practice to let the method defining the transaction start also dictate where the transaction ends.
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Message-driven beans and bean-managed transactions
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Message-driven beans also have the option of managing their own transactions. In the case of MDBs, the scope of the transaction must begin and end within the onMessage() method. It is not possible for a bean-managed transaction to span onMessage() calls. It is important to understand that in a BMT, the message consumed by the MDB is not part of the transaction. When an MDB uses container-managed transactions, the message it handles is a part of the transaction, so if the transaction is rolled back, the consumption of the message is also rolled back, forcing the JMS provider to redeliver the message. But with bean-managed transactions, the message is not part of the transaction, so if the BMT is rolled back, the JMS provider will not be aware of the transaction s failure. However, all is not lost, because the JMS provider can still rely on message acknowledgment to determine whether the message was delivered successfully.
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The EJB container will acknowledge the message if the onMessage() method returns successfully. If, however, a RuntimeException is thrown by the onMessage() method, the container will not acknowledge the message, and the JMS provider will suspect a problem and probably attempt to redeliver the message. If redelivery of a message is important when a transaction fails, your best course of action is to ensure that the onMessage() method throws an EJBException so that the container will not acknowledge the message received from the JMS provider.
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Vendors use proprietary (declarative) mechanisms to specify the number of attempts to redeliver messages to BMT/NotSupported MDBs that fail to acknowledge receipt. The JMS-MDB provider may provide a dead message area into which such messages will be placed if they cannot be processed successfully according to the retry count. Administrators can monitor the dead message area so that delivered messages can be detected and handled manually.
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Other than the message, everything between the UserTransaction.begin() and User Transaction.commit() methods is part of the same transaction. If a transaction failure occurs, these operations will be rolled back and the message will not be sent.
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Heuristic Decisions
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Transactions are normally controlled by a transaction manager (often the EJB server), which manages the ACID characteristics across several enterprise beans, databases, and servers. The transaction manager uses a two-phase commit (2-PC) to manage transactions. 2-PC is a protocol for managing transactions that commits updates in two stages. 2-PC is complex, but basically it requires that servers and databases cooperate through an intermediary the transaction manager in order to ensure that all of the data is made durable together. Some EJB servers support 2-PC and others do not, and the value of this transaction mechanism is a source of some debate. The important point to remember is that a transaction manager controls the transaction; based on the results of a poll against the resources (databases, JMS providers, and other resources), it decides whether all the updates should be committed or rolled back. A heuristic decision takes place when one of the resources makes a unilateral decision to commit or roll back without permission from the transaction manager. When a heuristic decision has been made, the atomicity of the transaction is lost and data-integrity errors can occur.
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UserTransaction throws a few different exceptions related to heuristic decisions, dis-
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