Message Exchange Patterns Although all SOAP messages carry the same structure, the ways in which use them can be combined into a number of different "message exchange patterns", or MEPs. The two major message exchange patterns are: • In-Out: in this MEP, the client sends a soap message to the server, in which processes the message and sends a response back. This is probably the most commonly used MEP, and is useful for tasks such as searching for information, or submitting information in situations in which acknowledgment is important. • In-Only: In this MEP, the client sends a message to the server without the expectation of receiving a response. You may use this MEP for activities such as pinging a server to wake it up, reporting logging information for which you need no acknowledgment, and so on. Within these two MEPs, you also have several variables to consider: • Blocking versus non-blocking: When the client sends a message, the application may wait to receive a response before moving on, or it may simply send a message and then move on, specifying a callback action to be completed when the response is received. • Number of parameters: Ultimately, a message sent from a client to server is intended to execute a particular action. That action may require no parameters, or it may require one or more parameters. These parameters must be encoded as part of the payload of the message. Taking all of these options into consideration, you can create virtually any MEP. For example, you can create an Out-Only system by reversing roles for the In-Only MEP. isCOBOL for Axis2 also includes support for less prominent MEPs, such as Robust-In-Only.
