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Channel message exit programs

 

We can use the channel message exit for the following:

• Encryption on the link

• Validation of incoming user IDs

• Substitution of user IDs according to local policy

• Message data conversion

• Journaling

• Reference message handling

On WebSphere MQ for iSeries, UNIX systems, z/OS, and Windows systems, and with WebSphere MQ clients, we can specify a list of message exit programs to be run in succession.

Channel message exit programs are called at the following places in an MCA’s processing cycle:

• At MCA initiation and termination

• Immediately after a sending MCA has issued an MQGET call

• Before a receiving MCA issues an MQPUT call

The message exit is passed an agent buffer containing the transmission queue header, MQXQH, and the application message text as retrieved from the queue. (The format of MQXQH is given in the Application Programming Reference book.) If you use reference messages, that is messages that contain only a header which points to some other object that is to be sent, the message exit recognizes the header, MQRMH. It identifies the object, retrieves it in whatever way is appropriate appends it to the header, and passes it to the MCA for transmission to the receiving MCA. At the receiving MCA, another message exit recognizes that this is a reference message, extracts the object, and passes the header on to the destination queue. See the WebSphere MQ Application Programming Guide for more information about reference messages and some sample message exits that handle them.

Message exits can return the following responses:

• Send the message (GET exit). The message may have been changed by the exit. (This returns MQXCC_OK.)

• Put the message on the queue (PUT exit). The message may have been changed by the exit. (This returns MQXCC_OK.)

• Do not process the message. The message is placed on the dead-letter queue (undelivered message queue) by the MCA.

• Close the channel.

• Bad return code, which causes the MCA to abend.

1. Message exits are called just once for every complete message transferred, even when the message is split into parts.

2. In UNIX systems, if you provide a message exit for any reason the automatic conversion of user IDs to lowercase characters does not operate. See User IDs on UNIX systems.

3. An exit runs in the same thread as the MCA itself. It also runs inside the same unit of work (UOW) as the MCA because it uses the same connection handle. Therefore, any calls made under syncpoint are committed or backed out by the channel at the end of the batch. For example, one channel message exit program can send notification messages to another and these messages will only be committed to the queue when the batch containing the original message is committed.

   Therefore, it is possible to issue syncpoint MQI calls from a channel message exit program.

• Message conversion outside the message exit
  

 

Parent topic:

What are channel-exit programs?

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