What is the Message Exchange Patterns?

Can someone please explain what the MEP is? I can't grasp the concept.

From what I figure, is just a concept for two parties to under开发者_Go百科stand each other (i.e. agree on a type of format that the messages they exchange between them must have).

Is this it, or is there more?

In the context of SOAP, it is used to define the behavior of an operation; whether it takes an input, returns an output, and how exceptions are handled. This is defined in the wsdl. See the pattern attribute of the operation element in the example wsdl snippet below (taken from Wikipedia):

<interface name="RESTfulInterface">
  <operation name="Get" pattern="http://www.w3.org/ns/wsdl/in-out">
    <input messageLabel="In" element="tns:request"/>
    <output messageLabel="Out" element="tns:response"/>
  </operation>
</interface>

In this service, the operation takes an input and returns and output (in-out).