Use the dead-letter (undelivered message) queue

 

When a queue manager cannot deliver a message, it attempts to put the message on its dead-letter queue. This queue should be defined when the queue manager is installed.

Your programs can use the dead-letter queue in the same way that the queue manager uses it. We can find the name of the dead-letter queue by opening the queue manager object (using the MQOPEN call) and inquiring about the DeadLetterQName attribute (using the MQINQ call).

When the queue manager puts a message on this queue, it adds a header to the message, the format of which is described by the dead-letter header (MQDLH) structure, in the Application Programming Reference. This header includes the name of the target queue and the reason that the message was put on the dead-letter queue. It must be removed and the problem must be resolved before the message is put on the intended queue. Also, the queue manager changes the Format field of the message descriptor (MQMD) to indicate that the message contains an MQDLH structure.

 

MQDLH structure

You are recommended to add an MQDLH structure to all messages that you put on the dead-letter queue; however, if you intend to use the dead-letter handler provided by certain WebSphere MQ products, you must add an MQDLH structure to your messages.

The addition of the header to a message might make the message too long for the dead-letter queue, so always make sure that your messages are shorter than the maximum size allowed for the dead-letter queue, by at least the value of the MQ_MSG_HEADER_LENGTH constant. The maximum size of messages allowed on a queue is determined by the value of the MaxMsgLength attribute of the queue. For the dead-letter queue, make sure that this attribute is set to the maximum allowed by the queue manager. If your application cannot deliver a message, and the message is too long to be put on the dead-letter queue, follow the advice given in the description of the MQDLH structure.

Ensure that the dead-letter queue is monitored, and that any messages arriving on it get processed. The dead-letter queue handler runs as a batch utility and can be used to perform various actions on selected messages on the dead-letter queue. For further details, see WebSphere MQ System Administration Guide for WebSphere MQ for AIX, HP-UX, Linux, Solaris, and Windows systems; for WebSphere MQ for z/OS see WebSphere MQ for z/OS System Administration Guide; for i5/OS see WebSphere MQ for iSeries System Administration Guide.

If data conversion is necessary, the queue manager converts the header information when you use the MQGMO_CONVERT option on the MQGET call. If the process putting the message is an MCA, the header is followed by all the text of the original message.

Messages put on the dead-letter queue might be truncated if they are too long for this queue. A possible indication of this situation is the messages on the dead-letter queue being the same length as the value of the MaxMsgLength attribute of the queue.

• Dead-letter queue processing
  

 

Parent topic:

Remotely determined errors

fg11350_