message exchange --- {excerpt}a communication between the ODE engine and a partner {excerpt}. Essentially, a message exchange is a web service invocation. Each message exchanges is associated with a WSDL [port type] and operation. There are two types of message exchanges: my role message exchanges and partner role message exchanges. The former represents invocations on the server, while the latter represents invocations by the server (this is BPEL terminology, so a look at the spec may be helpful).