The broker provides basic error handling for all your message flows. If basic processing is not sufficient, and you want to take specific action in response to certain error conditions and situations, you can enhance your message flows to provide your own error handling.
The options that you can use to do this are quite complex in some cases. The options that are provided for MQInput nodes are extensive because these nodes deal with persistent messages and transactions. MQInput is also affected by configuration options for WebSphere MQ.
Because you can decide to handle different errors in different ways, there are no fixed procedures to describe. This section provides information about the principles of error handling, and the options that are available, and you must decide what combination of choices that you need in each situation based on the details that are provided in this section.
You can choose one or more of these options in your message flows:
If you include user-defined nodes in your message flow, you must refer to the information provided with the node to understand how you might handle errors with these nodes. The descriptions in this section cover only the built-in nodes.
When you design your error handling approach, consider the following factors:
When an exception is detected within a node, the message and the exception information are propagated to the node's failure terminal. If the node does not have a failure terminal, or it is not connected, the broker throws an exception and returns control to the input node.
If an MQinput node detects an internal error, its behavior is slightly different; if the failure terminal is not connected, it attempts to put the message to the input queue's backout requeue queue, or (if that is not defined) to the dead letter queue of the broker's queue manager,
A message is propagated to a catch terminal only if it has first been propagated beyond the node (for example, to the nodes connected to the out terminal).
The general principles of error handling are:
The fail flow is also invoked if an exception is generated beyond the MQInput node (in either out or catch flows), the message is transactional, and the reinstatement of the message on the input queue causes the backout count to reach the backout threshold.
The SCADAInput node does not propagate the message to the failure terminal if an exception is generated beyond the node and you have not connected its catch terminal.
The Error Handler sample demonstrates how to use an error handling routine to trap information about errors and to store that information in a database. The error handling routine is a subflow that you can add, unchanged, to any message flow. The sample also demonstrates how to configure message flows to control transactionality; in particular, the use of globally coordinated transactions to ensure overall data integrity.
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