and you've documented the XML vocabulary, you've invented an API. Yes, you could dress it up with additional layers like XML-RPC or SOAP, and that might be a good idea, but there's really not that much need; an HTTP-XML interface is one of the easiest things in the world to do application integration with. ... Most important, you need to create some well-written human-readable documentation explaining what the tags and attributes mean and what goes inside them. Once you've done this, you've provided an interface that any reasonably-competent programmer in the world can deal with." see also :
XML structures for existing databases.
