Having looked at the language involved in this discussion between applications the next question is how
the applications talk to each other.
11.1.2 Producing and consuming structured data
The earlier chapter on email serves as a good example of how applications talk to each other using a structured
data format (Internet e-mail format) that is produced at one end, sent by a mail server (Mail Transfer Agent)
and consumed by the receiving email client (Mail User Agent) at the other. The application conversations in
this chapter take that basic concept a step further by using that exchange to trigger actions at either end,
through what is known as a remote procedure call (RPC).
Later in this chapter we??™ll be covering Zend_XmlRpc which uses Zend_XmlRpc_Server to send and
receive XML encoded remote procedure calls over HTTP. Originally created by Dave Winer, XML-RPC is a
surprisingly simple and flexible specification which allows it to be used in a wide variety of situations.
As functionality was added, XML-RPC evolved into SOAP (originally an acronym but now just a word in
itself) and has been adopted by the World Wide Web Consortium. However, you don??™t need to look too hard to
find complaints about how this added functionality also added a lot more complexity, which partly explains
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