Services Aren’t Stateless

My teammate Dale and I are going to an SOA Workshop in Vancouver in mid September. The workshop is put on by SOA Systems, which was founded by “top-selling SOA author” Thomas Erl. I have a copy of his first book, but I’ve never really opened it. Dale let me borrow Erl’s second book. I figured since I was going to see him speak, I should at least flip through his books.

I was looking thru the chapter 9 “Principles of Service-Orientation”. Most of them are spot on, if not exactly news. Services are loosely coupled, autonomous and share a formal contract. Yep, with you so far. But then I got to this one:

Services are Stateless
Services should not be required to manage state information, as that can impede their ability to remain loosely coupled. Services should be designed to maximize statelessness even if that means deferring state management elsewhere.

This seems way wrong to me on several levels. Now I’m really looking forward to going to Erl’s workshop, so I can discuss this with him face-to-face.

First off, his terminology is confusing. I have a hard time believing he really think services in general should have no state at all. I’m sure there are some examples of completely state-free services, but I believe they are both very rare and fundamentally uninteresting. A simple calculator service has no state, but why would you actually build or use one (except as a demo)? I assume Erl means that service should be stateless in the same way HTTP is stateless. IMO, stateless is poor description of HTTP. Connectionless or sessionless would be more accurate.

Regardless of my opinions on poor terminology, the problem with stateless services is that many – perhaps most – business operations aren’t stateless. And while HTTP is stateless, as soon as you use cookies, it becomes a stateful protocol. If you don’t believe business operations are stateful, try buying something on your favorite ecommerce site with your cookies disabled. Most sites will give you a “Your computer must have cookies enabled” error message. Sites that still work are embedding a session ID in the URL instead of in a cookie (ASP.NET has built in support for this type of Cookieless Session State). Either way, state is required for even the simple task of ordering something from a web site.

If most business operations aren’t stateless, why should services that implement business operations be stateless? This seems like a violation of the “but no simpler” part of Einstein’s famous paraphrased quote.


I reckon the word you might prefer is 'transaction'. With those you can have 'Business transactions' (sagas lasting weeks, because they involve things out of your control, like people) or you can have 'Database/State/Entity transactions' (which hopefully won't last for weeks). If you see a Service end-point as needing to be representative of a Business transaction then it has to know about the 'transaction context' (or workflow, or state, or whatever you like calling it). Something has to pass that in or it has to be keyed/stored. If you see a Service end-point as being representative of a 'Database/State/Entity transaction' then you probably want it to (a) use it in a larger 'thing' to make a business transaction and (b) be atomic, consistent, isolated and durable, plus you'd have to ask yourself if HTTP is really the application protocol you can put up with. So the thing is we use the word Service to mean both styles of transaction, hence confusion for all. Let us know what Erl says...
Interesting points. I have addressed some of these issues on my blog at : I think there is a distinction to made between service and process state.
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