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One of very common use cases is the requirement to check payload of the processed message before and after mapping execution (for example, in order to identify correctness of the mapping execution for the particular message executed at runtime). Please note that editing or restart of the logged message is not possible. Thus, in case of pure monitoring purposes, staging will bring more overhead to message processing, so logging is recommended in this case.
If its next processing step throws an exception, the processor reschedules the message from this storage and returns. This can be noticed when examining audit log of the message.