Harry Pierson posted a story about Windows Communication Foundation and his "epiphany" (my words) on the usefulnes of WS-ReliableMessaging in Web Services. I found several comments here that made me think. Sam Gentile recently commented as well on the importance of WS-RM, and I did a few quick posts there. Let me first start this discussion that I believe that the basic concept behind reliable messaging (and WS-RM) is indeed very important and very needed in the Web Services World to make it easier to implement reliable services and integrate disparate applications. That said, I've been frank in the past and say that I believe that the WS-RM spec, as it exists right now, is remarkably lacking. Harry mentioned the core point: It does not demand persistence of the message and conversation state from the endpoints involved in the communication. Thus, as WS-RM currently exists, and how platforms such as WCF implement it, it means you don't really have the guarantee of reliablity. At best, all you have is the illussion of reliability. The only thing you can be sure of is that one of your endpoints is gonna fail, sooner or later. If there is one thing that I have learned from my work on Application Integration (with and without BizTalk), is that ensuring reliable communication across a distributed application infrastructure is a tough nut to crack. There's a lot of very significant issues that can arise here, depending on the type of problem domain you're solving: Message delivery, delayed deliver, synchronous and asynchronous interchanges, ordered deliver (and even tougher, in-order processing), lost and duplicated messages and so on. A spec like WS-RM doesn't try to solve all this issues, for obvious reasons, and that really is fine. A lot of these issues cannot be taken into account by themselves, but only in the context of the overall architecture and requirements. In particular, how the individual endpoints are designed can make a huge impact in which of the issues mentioned before are significant and how significant. Some architectural choices will make some of these problems go away without having to write more code for it. For example, a given service might have a set of messages created in such a way that the operations they represent are idempotent. In that case, duplicated messages should not be a problem, and spending a lot of time to avoid them would not be an efficient use of your development budget, as they are a non issue. In other contexts, for example, missing one or two messages during high load might also not be important because perhaps the contents can be reconstructed from subsequent exchanges. So given this, why do I make so much fuss about WS-RM if it wasn't meant to be the end-all solution anyway? Because of the way it has been positioned by the marketplace: It is usually portrayed as saying that it "finally solves the reliable messaging problem for Web Services". It doesn't. Unfortunately, the WCF literature hasn't helped clear
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