Monday, December 17, 2007

Premature Conclusions with Services and REST

Damn, I wish we were at a white board. Oh well.

JJ, you've drawn a lot of conclusions from talking with a few people in the REST community. I think you've gotten ahead of yourself.

To defend the "REST community": What we are talking about isn't where most of the value is. Not yet anyway. Maybe someday this issue will really be an issue, but right now just GETing info with declarative representation data is so unusual that we're on the fringe.

Really, I believe this on the internet at large and in many organizations as well (enterprisey doesn't even get funny when sharing a database is so common.) . How much value does a definition language really have? Well, some. Just like I appreciate static languages for documentation and _some_ checking, having a WSDL or something is useful. None of the things being thrown around for "interface definition language" are much more useful than being able to organize documentation, generate code (a bad idea), and create pictures (sometimes useful for humans). That last point, the human part, is where you started this conversation... Let's not pretend that WSDL allows machines to process things for us.

To your recent post. I don't have much time tonight, but hear is my reaction.

Shared Understanding.
This isn't actually denied by the REST. It's just relegated to the media type.

The "contract" that is specific to a particular domain in a RESTful system is buried in the Representation. That means a) we all collectively don't know much about it, b) every system can use the standard VERBS to explore and partially integrate. The first point means we need to have this very conversation, but the value of that struggle is more common ground between all services.

Result Sets aren't Resources
Huh? Why not? I'll need to read up on the posts, but everything interesting is a resource. I don't understand why you conclude this.

Some of your recent posts have indicated what I think may be a confusion regarding the "uniform interface" and resources. Not every resource must support and expose each method. Part of the discovery of REST is learning (at runtime) what methods are available for each resources. Results sets don't need to expose PUT.

How many people are in this discussion?
Not many, I'm afraid. You're assuming that people won't discuss these issues with you, but it's probably that it doesn't matter to most of the RESTful systems out there.

As an example: the Amazon service exposed this weekend. It is people (not machine processes) that are writing the wrapper layers around the new API. It almost doesn't matter what they had exposed it in, people need to spend thought understanding it and wrapping it. Then, other people, need to consume the wrappers to write applications.

Had Amazon used some magically fantastic definition language it wouldn't change but a part of the total human/computer processing cost and value proposition. Again, I think there is value in understanding and exposing that fantastic definition language. I'm not assuming it will magically compose my systems. (I'm not necessarily claiming you make that declaration either, but the value you place on the formality seems too strong. I will read your book on composition to really understand what you are saying.)

REST and actions
There is something critically important about the distinction between "action interfaces" and "document exchange". You took Subbu to task for trying to remove actions, but I think there is a truly subtle reason that REST motivates that thinking.

I'm not sure I can express it well, but this is my intuitive view:
  • contract negotiation is an (endless?) cycle of document exchange
  • reserving a _good_ hotel room is an cycle of question/answer...
  • bartering is a cycle of bid exchange
  • resolving a speeding ticket is ...
what these real ways of getting things done tell me is that an interface contract doesn't model the real world.

REST models "action" as one or more transfer of document representation. That is more like the world than interface actions.

REST and versioning.
Really bad at versioning? Seriously? If you'd said "efficiency", or "tool support", or "machine clients" I'd be right there saying "Yeah, maybe this would improve that..."

Versioning is hard because something wants to change - and the rest of the system isn't yet ready to change as well. REST has from the beginning been about supporting that change (across organizations, and between the client and server). The very reason that interface definition languages are so hard to reconcile with REST is because of the radically different view on how to support evolution.

(I'm posting this without all the links...)

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