The Messaging Protocol: SOAP
I think it's fair to say that XML's promise was not truly fulfilled until the creation of the SOAP protocol. SOAP is a messaging protocol based on XML, meaning that it specifies a way to send XML-based messages from one process to another, usually from one machine to another.
These observations won't come as a big surprise if you are reading this book sequentially. Chances are, you know more about the SOAP standard than any other. SOAP is not a protocol for remote procedure calls (RPCs). SOAP is not simple. SOAP is not just for interoperability between operating systems. SOAP accommodates interoperable remote procedure calls, but it isn't purely about this feature.
So, just what is SOAP? SOAP is about sending messages. More specifically, SOAP is a protocol that specifies an enveloping mechanism for sending data (via XML). Furthermore, it specifies how to send these messages to a final destination, and the processing model that applies if that message goes through several intermediaries. And, it specifies how to do this over HTTP. This chapter covers all of these aspects of SOAP, beginning with a brief overview of the protocol.
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