For most of this book, my examples are going to focus on XML protocols. These are XML applications used
for machine-to-machine exchange of information exchange across the Internet over HTTP.
Three such XML protocol applications are of particular interest: RSS、XML-RPC、SOAP.
RSS: is used to exchange headlines and abstracts between different Web news sites.
XML-RPC: supports remote procedure calls across the Internet by passing method names and arguments
embedded in an XML document over HTTP.
SOAP: a more complex implementation of this idea called SOAP. Whereas XML-RPC uses only elements,
SOAP adds attributes and namespaces as well. SOAP even lets the body of the message be an XML
element from some other vocabulary, so it opens up a host of other interesting examples.