The below is a bunch of hand-waving and comment-grade writing. I’m sure I say some nutty things, but it’s a comment, eh?

On the naive/simple side, SOAP is just XML over HTTP. That said, it’s intention it to be protocol independent. So, you could send SOAP over email, XMPP, FTP, or whatever. But, there’s been some playing around with HTTP headers and other things in the past.

SOAP is “fuller” than XML-RPC and, through all fo the WS-* standards, has more data types, semantics, and agreed on ways to “do things” than XML-RPC.

From that (we would hope) comes the ability to programatically integrate without a person gluing together services. Runtime integration is one of the huge goals of any a priori standards, right?

For example, a person is required to integrate the del.icio.us, flickr, and Technorati APIs together. If they were using some sort of SOAP/WS-* standard (the likes of which doesn’t exist as far as I know: I’m not ever sure what it would be beyond identity), then you could just plug in those different services to do integration.

Now, that said, there’s plenty of things like FOAF, microformats, etc. that allow you to do all this. I’m just saying that one of the goals of SOAP and WS-* is to know the data structures/formats and semantics ahead of time (a priori) such that less coder-time is needed to integrate the two together. Also, of course, the idea of a standard is to create a sort of “data platform” that can be extended and layered on. Thus, you can Russian doll different things to say, encrypt.

All that said, the question of the REST and XML-RPC-like crowds is always: do we need all that? What’s wrong with XML over HTTP? Further, the acusation from the REST crew is that SOAP and, in particular, the WS-* formats have “Gone Astronaut Architect Committee Grade” or, worse “enterprisey” and are, thus, too complex and impracticable to use. A favorite bomb-throwing hobby here is to dig up comments about SOAP and WS-* never being intended for use by humans, but requiring tools.

In one way — pulling from the microformat motto — SOAP is for computers first and humans second, where-as REST is for humans first, and computers second.

Which brings up back to Ye Olde Pragmatic Programmer: use plain text. Or, generalized, be as clear as possible.

One could say it’s Hemingway XML vs. Joyce (or Dickens?) XML…over HTTP. Of course, the SOAP/WS-* crowd would probably say REST is more like e. e. cummings.

And most firewalls are already setup for HTTP tunneling, and HTTPS and such.

I dunno, I never really understood the necessity for SOAP given XML-RPC in the first place.