User exits enable user-provided custom software to track data passing through message flows in WebSphere Message Brokers.
The user-provided functions can be invoked at specific points during the life-cycle of a message as it passes through the message flow, and can invoke utility functions to query information about the point in the flow, and the contents of the message assembly.
In the diagram above, the MQInput node is used as an example, but the function applies to all input nodes, including user-defined nodes. However, there is a slight difference between built-in nodes and user-defined nodes, in the way in which user exits are invoked. For built-in input nodes, the user exit is invoked as soon as possible after the data has been read from the external source. For user-defined input nodes, the user exit is called just before the node propagates the message.
In the example shown above, event 4 is fired at the end of the transaction. The user exit is invoked after the transaction has completed, so the user exit processing is not part of that transaction. The user exit is invoked even if no transactional processing was completed by the flow. Where the message flow property Commit Count is greater than 1, there is a many-to-one ratio between event 1 and event 4. This is also true for some scenarios specific to the particular input node; for example, when an MQInputNode is configured with the Commit by Message Group property selected.
Multiple user exits can be registered, and, if they are, they are invoked in a defined order (see mqsichangeflowuserexits command). Any changes made to the message assembly by a user exit are visible to subsequent user exits.
The user exits can be registered on a dynamic basis, without needing to redeploy the configuration.
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