I hope to post my thoughts on protocol state next week. In the meantime, you can check out Isolation Support for Service-based Applications as well as Consistency for Web Services Applications. The papers (from the same authors) deal with the C and I of ACID, but in service scenarios where ACID transactions aren’t feasible. Both papers have dramatically impacted my thinking in this space.

Since David Ing responded to my REST/CRUD/CRAP entry on his blog, I guess that means I respond to his response here, right?

Actually, this is going to be very short^[Wow, this post wasn’t that short at all. Can you imagine how long this would have been if I had mostly disagreed with David?] because I mostly agree with what David wrote. For example:

If we say stuff like ‘REST shall/must replace all foolish WS-* SOAP systems (insert Nelson-style HaHa here)’ then we are just repeating the same lessons as before – One size won’t fit all and there is no One Ring to Bind Them illusion in REST as well. If you have something complex that fits the WS-* style then maybe REST isn’t the droid you are looking for.

Of course, this begs the question “what is complex?” David described complex as “long running transaction updating multiple distributed stores”. Personally, I’d replace “stores” with “systems” and I hate term “long running transaction” so I would have written “long running operation” or “business process”, but grammatical nitpicking aside that certainly seems like a fair description. Certainly, most of the scenarios I’m looking at in my day job could be described as being long running and updating multiple systems. David says later that he thinks “a lot of apps don’t need to be modeled that way”, but I’m not aware of the alternatives so I’ll let him expand on this on his own blog. I’ll try to get to writing about my thoughts around protocol state next week.

I did disagree with this one point:

We may say REST is really about a protocol state and not CRUD, but unless the rest of the world gravitates to that view then I’m afraid it just won’t be. If, say, through some ongoing groundswell of common usage, people start modelling entities as dereferenceable URI’s and using POST to do Create and Updates, then REST will be about CRUD by default. This is all very unacademic and unjust, but thems the breaks peachy. The lesson from SOAP is not to fight it by trying to re-educate the masses after they get a perception in their head.

I think it doesn’t really matter how the rest of the world gravitates, it only matters how you as the the service provider choose to expose your service. If you’re doing something simple like ATOM publishing, you can probably get away with REST as CRUD. (Would that be hi or lo REST?) If you’re doing something more complex, either in terms of being long running or needing multiple updates in one atomic operation like Tim’s airline example, you’ll probably need to gravitate towards REST as protocol state. But can’t the two models can co-exist nicely, even in the same app? No re-education required.

Update: – From Fielding’s Dissertation: “REST relies instead on the author choosing a resource identifier that best fits the nature of the concept being identified”. My point exactly.

• Dottie Shaw – PM on Rome – has a post on Durable Messaging and why she cares. Needless to say, I care about durable messaging too, for all the same reasons…
• PDC07 has been “rescheduled“. Feel free to theorize amongst yourselves the reasons why.
• Friend and ex-boss John deVadoss speaks on Software+Services @ the Enterprise Architect Summit. He finds time to present, but his blog remains quiet…
• JD Meier announces a p&p twofer: beta 1 of both their TFS Guide and their Perf Testing Guide. The TFS guide looks like a packaged doc version of their VSTS Guidance Project wiki.
• Facebook announces their Facebook Platform and Microsoft is supporting it with PopFly and Silverlight. TechMeme has much more.
• Pandora on the Go delivers their service to Sprint cell phones for $3 a month. Raging deal, but where’s the support for Windows Mobile phones? (via Knowing.NET)