For example, if your system posts a message and then acts on it, wrap the raw message-sending technology with an interface of your own. Then you can confirm that the call is made as you expect in one test case, perhaps using a simple stub that implements the messaging interface or using mocking as described in the next section. You can add a second test that verifies the behavior of the message handler, simply calling the point that would be normally called by the messaging infrastructure. Sometimes, though, depending on your architecture, this is not possible without a lot of work.
