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Message Exchange Patterns
The messaging flexibility provided by SOAP allows services to communicate using a variety of message exchange patterns, satisfying the requirements of distributed applications. We exploit several of them in the core building blocks of the architecture, in particular those that have proven particularly useful in distributed systems. The use of remote procedure calls, for example, popularized the synchronous request/response message exchange pattern. When message delivery latencies are uncontrolled, asynchronous messaging is needed. When the asynchronous request/response pattern is used, explicit message correlation becomes mandatory.Broadcast transports popularized one-to-many message transmissions. The original sender imposing its messages on the recipients by just sending them is referred to as the push model. While this model is effective in local area networks, it does not scale well to wide area networks nor offer recipients an option to regulate the message flow.Another useful pattern is based on an application's ability to express interest in particular kinds of messages, making the publish/subscribe pattern quite popular. By explicitly subscribing to message sources (or topics), applications have a somewhat more controlled flow relevant information.The pull model is used when a recipient explicitly requests a message from a source. This makes message flow the recipient's responsibility. The pull pattern can also be combined with publish/subscribe. It is well-suited for situations where recipients might be intermittently disconnected from the sources.
