Data failures
As well as failing to get any response, a much broader class of issues arises from receiving a message or input containing unexpected data. It may be surprising because your specification was wrong, or due to a gray area that was poorly specified. If the message’s sender is another part of your system, you can choose where to fix it – should the sender change the message, or should the receiver accept it correctly? If the sender is a third party, your application must handle that input, regardless of the specification.
The most basic failure is receiving an error in reply. Each message you send out needs to be tested for a variety of possible error responses. The pass mark here, as elsewhere, is that these errors are expected so they can be handled gracefully and as early as possible. Both the message and the error should be comprehensively logged, so you can replay each situation and check new code against it. By catching errors early, you can return...
