(WMB) Error handling

When we design any message flow, we often do not give more emphasis on error handling. Well as per my experience I found this error handling techniques & design principles more crucial than designing the happy path.
So here I am with few details on how to handle unhappy path in WMBV6.0 with information about the message flow error behavior.


Design Consideration

• Connect the Failure terminal of any node to a sequence of nodes that processes the node's internal exception (the Failure flow).
• Connect the Catch terminal of the input node or a TryCatch node to a sequence of nodes that processes exceptions that are generated beyond it (the Catch flow).
• Insert one or more TryCatch nodes at specific points in the message flow to catch and process exceptions that are generated by the flow that is connected to the Try terminal.
• Ensure that all messages that are received by an MQInput node are processed within a transaction or are not processed within a transaction.

Understanding the Flow Sequence

• When an exception is detected within a node, the message and the exception information are propagated to the node's Failure terminal ( Diagnostic information is available in the ExceptionList). 
• If the node does not have a Failure terminal or if it is not connected, the broker throws an exception and returns control to the closest previous node that can process the exception. This node can be a TryCatch node  (Root & LocalEnvironment are reset to the values they had before) or the MQInput node .
• If the catch terminal of the MQInput node is connected, the message is propagated there (ExceptionList entries are available; Root & LocalEnvironment are reset to the values they had before).Otherwise if is not connected, the transactionality of the message is considered.
• If the message is not transactional, the message is discarded. Otherwise , if it is transactional, the message is returned to the input queue, and it is read again, whereupon the backout count is checked.
• If the backout count has not exceeded its threshold, the message is propagated to the output terminal of the MQInput node for reprocessing. Otherwise if it is exceeded & if the failure terminal of the MQInput node is connected then the message is propagated to that path. (Root is available but ExceptionList is empty)
• If the failure terminal of the MQInput node is not connected, the message is put on an available queue, in order of preference; message is put in backout queue, if one is defined; otherwise in dead-letter queue, if one is defined. If the message cannot be put on either of these queues, it remains on the input queue in a retry loop until the target queue clears.(It also records the error situation by writing errors to the local error log)

For more info : http://www.redbooks.ibm.com/redpapers/pdfs/redp4043.pdf