Strict message ordering using activation specifications or ASF listener ports connected to WebSphere MQ Version 7.x

Strict message ordering can be achieved when deploying message driven bean applications to the WebSphere® MQ messaging provider when no special facilities have been coded into the application to handle messages arriving out of order.

For WebSphere Application Server Version 7 and later, listener ports are stabilized. For more information, read the topic about stabilized features. Plan to migrate your WebSphere MQ message-driven bean deployment configurations from using listener ports to using activation specifications. However, you should not begin this migration until you are sure the application does not have to work on application servers earlier than WebSphere Application Server Version 7. For example, if you have an application server cluster with some members at Version 6.1 and some at a later version, you should not migrate applications on that cluster to use activation specifications until after you migrate all the application servers in the cluster to the later version.

The following assumptions have been made in this scenario:
  • The message-driven bean (MDB) application is transactional.
  • The back-out threshold (BOTHRESH) on the WebSphere MQ queue has been set to 0.
  • You are using WebSphere MQ Version 7.0 or later.

WebSphere Application Server configuration for ordered delivery

  • The WebSphere MQ queue manager must be running on WebSphere MQ Version 7.0 or later.
  • The connection to the queue manager must use the WebSphere MQ messaging provider normal mode. See the Rules for selecting the WebSphere MQ messaging provider mode topic of the WebSphere MQ information center.
  • If you are using listener ports Maximum sessions on the listener ports in WebSphere Application Server must be set to 1.
  • If you are using activation specifications Maximum server sessions on the activation specifications in WebSphere Application Server must be set to 1.

Important information about this configuration

  • ASF listener ports and WebSphere MQ activation specifications contain two separate parts, which together perform message delivery. These two parts are seen as separate applications by the queue manager:
    • Part one detects messages as they arrive, but does not consume them. Instead it dispatches them to the second part.
    • Part two is a server session pool which allocates a thread to process the message within the application's transaction, and deliver it to the onMessage() method of the MDB.
  • From Version 7.0, WebSphere MQ provides a push model for detection of the messages, which is more efficient than the polling model used in previous versions of WebSphere MQ, and provides better ordering of message under normal operation.

Circumstances in which messages can be delivered out of order

Messages can be delivered out of order with this configuration in the following circumstances:

  • After a transaction rollback, the next message available on the queue might be delivered before the rolled back message is re-delivered:
    • For an ASF listener port, setting Maximum retries to zero prevents out of order delivery after a rollback by stopping the listener port when a rollback occurs. However, the listener port must then be restarted manually.
    • For an activation specification, selecting Stop endpoint if message delivery fails and setting Number of sequential delivery failures before suspending endpoint to 0 prevents out of order delivery after a rollback by pausing the message endpoint when a rollback occurs. However, the message endpoint for the MDB must then be resumed manually. For more information, see the WebSphere MQ documentation.
  • Messages can be delivered out of order during a transaction recovery:
    Note: A specific set of events must occur in a specific order for this scenario to be encountered, and as such it is uncommon. However, if ordered message delivery is critical to the operation of your application, the you must consider it.
    • Out of order message delivery can occur with this deployment option during recovery from a failure of one of the following components:
      • The application server hosting the MDB
      • The WebSphere MQ queue manager
      • A network connecting the application server and queue manager
    • If one of these components fails in the middle of a two-phase commit of an MDB transaction, the application server transaction manager reestablishes its connection to the queue manager to resolve the transaction when the component is available again.
    • This recovery process is asynchronous, and it is possible for delivery of new messages to the MDB to begin before the transaction recovery process is complete. If the outcome of the transaction recovery is to roll back the transaction, then the message will be returned to the WebSphere MQ queue and re-delivered to the application, possibly after new messages have already been delivered.

Considerations for a clustered deployment

  • You must activate the MDB on one cluster member only, as the application server does not have a facility which can manage this activation automatically.
  • You can set the startup state of listener ports to stopped, separately to setting the startup state of an application.
  • You can manually start and stop applications, ASF listener ports, and message endpoints with MBean interfaces by using wsadmin scripting, or by using the com.ibm.websphere.management.AdminClient interfaces from Java™ code.