Messages containing incorrect data

 

When you use units of work in your application, if a program cannot successfully process a message that it retrieves from a queue, the MQGET call is backed out. The queue manager maintains a count (in the BackoutCount field of the message descriptor) of the number of times that happens. It maintains this count in the descriptor of each message that is affected. This count can provide valuable information about the efficiency of an application. Messages whose backout counts are increasing over time are being repeatedly rejected; design your application so that it analyzes the reasons for this and handles such messages accordingly.

On WebSphere MQ for z/OS, to make the backout count survive restarts of the queue manager, set the HardenGetBackout attribute to MQQA_BACKOUT_HARDENED; otherwise, if the queue manager has to restart, it does not maintain an accurate backout count for each message. Setting the attribute this way adds the penalty of extra processing.

On WebSphere MQ for iSeries, WebSphere MQ for Windows, and WebSphere MQ on UNIX systems, the backout count always survives restarts of the queue manager.

Also, on WebSphere MQ for z/OS, when you remove messages from a queue within a unit of work, we can mark one message so that it is not made available again if the unit of work is backed out by the application. The marked message is treated as if it has been retrieved under a new unit of work. You mark the message that is to skip backout using the MQGMO_MARK_SKIP_BACKOUT option (in the MQGMO structure) when you use the MQGET call. See Skipping backout for more information about this technique.

 

Parent topic:

Locally determined errors

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