Settling Some Accounts with REST Advocates

(Couldn't resist a cheeky title again, although in deference to those who find the term RESTafarian trivialising, I won't use it this time.)

Stu Charlton posted a lengthy "rebuttal" to my post on "Paying the RESTafarians Back in their Own Coin", and I've now answered his points. You may find that nothing he said really invalidated any of my points, and I think I may have unearthed a couple of his misconceptions around the application domain as well.

I must say that it appears people don't read what I have written carefully enough. They assume I'm saying something and then "refute" it. Others find such "refutations" convincing! Still others like to call me names without reading my post carefully and seeing that I haven't actually said a single negative thing about REST! (Now I'm being thin-skinned :-).

Anyway, I hope these points lay to rest (no pun intended) such arguments.

The Good in WS-* (Update)

See post below for my initial, entirely positive reactions to Stu Charlton's critique of the WS-* family of Web Services technologies.

Now for some qualifiers.

I wonder why Stu didn't include SOAP in the list of "good" technologies. I don't mean the overloaded meaning of SOAP (the RPC style of usage, for example), but quite simply the SOAP message, the envelope. All the WS-* stuff is useless without the primary vehicle of the SOAP message which carries them from endpoint to endpoint. One has to admit there is a certain elegance in its simplicity, analogous to the humble IP packet that is the workhorse of the Internet (read more here).

The envelope has just two parts - a header and a body. All the domain-neutral WS-* tags that Stu talks about go into the headers. The body contains domain-specific XML payloads that hopefully conform to some defined schema. Unlike with the TCP stack, where packets are recursively enveloped within other packets (e.g., HTTP inside TCP inside ESP (IPSec)), ultimately being packaged inside IP for transport over the "wire", SOAP has a single-level layering approach. All quality-of-service tags represented by the WS-* standards are placed on a co-equal basis within the SOAP header, which is OK because they largely represent orthogonal concerns. (There is still some interdependence, e.g., you can't just have WS-Security if you're using WS-ReliableMessaging; you need WS-SecureConversation as well, because the system now has to be cognisant of conversation state.)

The SOAP message represents, at once, a domain-neutral wire protocol that supports any application and a domain-specific application protocol that defines the verbs, nouns and conversation grammar of a particular application domain.

I see both SOAP and REST as ways to move domain-specific XML documents around. From that angle, they're both defining a core application wire protocol that can support any application domain (HTTP or SOAP-with-headers). Each application has to define the nouns, verbs and conversation grammar appropriate to it and exploit this "wire protocol" to move the XML noun payload in the context of an interaction verb (GET/POST/PUT/DELETE being pre-defined within the REST wire protocol or a bespoke one for SOAP/WS-*) and thereby build up a domain-meaningful conversation, what the REST folk call a state machine and what the SOAP/WS-* folk call a process.

I think we're all in violent agreement over the need to move XML around in a series of interactions that occur within the larger bounded context of a state machine or process. As has been said in many other contexts, our similarities are greater than our differences, so let's not read too much into the SOAP-REST debate.

William Vambenepe has more to add on this. It's an earlier post, but very topical. There's a lot more promise in SOAP than most of us realise, and the standards themselves sometimes belie that promise. Read William's post to see what needs to be fixed, hopefully in the next version of the WS-* standards.

The Good in WS-*

Stu Charlton has posted a fantastic and in-depth critique of SOAP/WS-* standards on his blog:

This is exactly the kind of analysis I was looking for. It's pretty humbling to realise the depth of knowledge that's out there. I guess I've lots of learning to do...

Too bad there doesn't seem to be any provision for comments on his blog. I wanted to say some nice things :-(.

Thanks, Stu!