There's been a lot of interest in Service-Oriented Architecture (SOA) at my company recently. Whenever I try to see how we might use it, I always run up against a mental block. Crudely:
Object-orientation says: "keep data and methods that manipulate data (business processes) together";
Service-orientation says: "keep the business process in the service, and pass data to it".
Previous attempts to develop SOA have ended up converting object-oriented code into data structures and separate procedures (services) that manipulate them, which seems like a step backwards.
My question is: what patterns, architectures, strategies etc. allow SOA and OO to work together?
Edit: The answers saying "OO for internals, SOA for system boundaries" are great and useful, but this isn't quite what I was getting at.
Let's say you have an Account object which has a business operation called Merge that combines it with another account. A typical OO approach would look like this:
Account mainAccount = database.loadAccount(mainId);
Account lesserAccount = database.loadAccount(lesserId);
mainAccount.mergeWith(lesserAccount);
mainAccount.save();
lesserAccount.delete();
Whereas the SOA equivalent I've seen looks like this:
Account mainAccount = accountService.loadAccount(mainId);
Account lesserAccount = accountService.loadAccount(lesserId);
accountService.merge(mainAccount, lesserAccount);
// save and delete handled by the service
In the OO case the business logic (and entity awareness thanks to an ActiveRecord pattern) are baked into the Account class. In the SOA case the Account object is really just a structure, since all of the business rules are buried in the service.
Can I have rich, functional classes and reusable services at the same time?
I really think SOA is only useful for external interfaces (generally speaking, to those outside your company), and even then, only in cases when performance doesn't really matter, you don't need ordered delivery of messages.
In light of that, I think they can coexist. Keep your applications working and communicating using the OO philosophy, and only when external interfaces (to third parties) are needed, expose them via SOA (this is not essential, but it is one way).
I really feel SOA is overused, or at least architectures with SOA are getting proposed too often. I don't really know of any big systems that use SOA internally, and I doubt they could. It seems like more of a thing you might just use to do mashups or simple weather forecast type requests, not build serious systems on top of.