Whenever ThoughtWorks rashly lets me out in front of a client, one question I'm bound to be asked is "what do you think of SOA (Service Oriented Architecture)?" It's a question that's pretty much impossible to answer because SOA means so many different things to different people.
- For some SOA is about exposing software through web services. This crowd further sub-divides into those that expect the various WS-* standards and those that will accept any form of XML over http (and maybe not even XML).
- For some SOA implies an architecture where applications disappear. Instead you have core services that supply business functionality and data separated by UI aggregators that apply presentations that aggregate together the stuff that core services provide.
- For some SOA is about allowing systems to communicate over some form of standard structure (usually XML based) with other applications. In it's worse form this is "CORBA with angle brackets". In more sophisticated forms this involves coming up with some form of standard backbone for an organization and getting applications to work with this. This backbone may or may not involve http.
- For some SOA is all about using (mostly) asynchronous messaging to transfer documents between different systems. Essentially this is EAI without all the expensive EAI vendors locking you in.
I've heard people say the nice thing about SOA is that it separates data from process, that it combines data and process, that it uses web standards, that it's independent of web standards, that it's asynchronous, that it's synchronous, that the synchronicity doesn't matter....
I was at Microsoft PDC a couple of years ago. I sat through a day's worth of presentations on SOA - at the end I was on the SOA panel. I played it for laughs by asking if anyone else understood what on earth SOA was. Afterwards someone made the comment that this ambiguity was also something that happened with Object Orientation. There's some truth in that, there were (and are) some divergent views on what OO means. But there's far less Object Ambiguity than the there is Service Oriented Ambiguity.
So what do we do? For a start we have to remember all the time about how many different (and mostly incompatible) ideas fall under the SOA camp. These do need to be properly described (and named) independently of SOA. I think SOA has turned into a semantics-free concept that can join 'components' and 'architecture'. It's beyond saving - so the concrete ideas that do have some substance need to get an independent life.