Apparently OASIS has decided to tackle reliable messaging, with help from the usual non-IBM/MS Web services suspects.

I think “reliable messaging” is a huge waste of time. It’s akin to saying that the network is unreliable, so let’s just make a reliable network on top (which is different than “reliable data stream” ala TCP). Sorry, it just doesn’t work that way. “reliable network” is an oxymoron, for any number of reliability layers you might try to build on top.

As with most problems over the Internet, reliability is a coordination problem. That is, how do two or more independant pieces of software distributed on an unreliable network, coordinate to achieve some goal in a reliable manner (such that both know that the goal has been achieved or failed, etc..)? Unfortunately, you can’t coordinate “reliability” in a vacuum, like the typical reliable messaging approach of store/forward/timeout; you have to look at what task is being coordinated in the first place, and then figure out how to augment your coordination semantics such that the necessary degree of reliability can be achieved. In the context of the Web, that means using the uniform coordination semantics that are made available through HTTP.

Simple example. I want to turn on a lightbulb, and do it reliably such that I know if my attempt succeeded or not. I would use PUT. If I got back a 2xx, I would know the lightbulb was on. If I didn’t get back a response at all – say if the connection died after the request was sent – then I don’t know. But if I needed to know, I could do a GET. Perfectly reliable, no reliable messaging solution in sight.

That example doesn’t work for everything of course, because PUT is idempotent and not all operations you might want to perform are idempotent. POST is different, but the requirements on it are different too, since if you use POST, you accept that you won’t know the state of the resource after a successful submission (getting deeper into that is a topic too large for a blog, sorry).

Anyhow, I acknowledge that some work needs to be done to HTTP to help with reliability (as Paul describes). But that is in no way “reliable messaging”.


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