Message acknowledgement

Every session that is not transacted has an acknowledgement mode that determines how messages received by the application are acknowledged. Three acknowledgement modes are available, and the choice of acknowledgement mode affects the design of the application.

The information in this topic is relevant only if an application connects to a WebSphere® MQ queue manager or a WebSphere Service Integration Bus. The information is not relevant for a real-time connection to a broker.

XMS uses the same mechanism for acknowledging the receipt of messages that JMS uses.

If a session is not transacted, the way that messages received by the application are acknowledged is determined by the acknowledgement mode of the session. The three acknowledgement modes are described in the following paragraphs:

XMSC_AUTO_ACKNOWLEDGE

The session automatically acknowledges each message received by the application.

If messages are delivered synchronously to the application, the session acknowledges receipt of a message every time a Receive call completes successfully.

If the application receives a message successfully, but a failure prevents acknowledgement from occurring, the message becomes available for delivery again. The application must therefore be able to handle a message that is redelivered.

XMSC_DUPS_OK_ACKNOWLEDGE

The session acknowledges the messages received by the application at times it selects.

Using this acknowledgement mode reduces the amount of work the session must do, but a failure that prevents message acknowledgement might result in more than one message becoming available for delivery again. The application must therefore be able to handle messages that are redelivered.

XMSC_CLIENT_ACKNOWLEDGE

The application acknowledges the messages it receives by calling the Acknowledge method of the Message class.

The application can acknowledge the receipt of each message individually, or it can receive a batch of messages and call the Acknowledge method only for the last message it receives. When the Acknowledge method is called all messages received since the last time the method was called are acknowledged.

In conjunction with any of these acknowledgement modes, an application can stop and restart the delivery of messages in a session by calling the Recover method of the Session class. Messages whose receipt was previously unacknowledged are redelivered. However, they might not be delivered in the same sequence in which they were previously delivered. In the meantime, higher priority messages might have arrived, and some of the original messages might have expired. In the point-to-point domain, some of the original messages might have been consumed by another application.

An application can determine whether a message is being re-delivered by examining the contents of the JMSRedelivered header field of the message. The application does this by calling the Get JMSRedelivered method of the Message class.