Service oriented vs API oriented

Question

I've seen several questions revolving around that theme on SO, but no answer that really satisfies me.

I'm trying to put words on things I feel without always being able to express them clearly enough to convince people around me. Might be that I'm wrong. Might be that my understanding is not deep enough to find proper arguments.

How would you contrast developing applications according to a "service oriented approach" instead of a "traditional" API approach?

Let's be totally clear here that, by services, I don't necessarily mean Web Services.

Here are some differences I see. Please correct me if I'm wrong:

• a service is a "living thing" that you can talk to, according to a given and explicit protocol. A service has its own runtime while a library uses the runtime of your application. You can move that "living thing" wherever you want
• a library allows code-based integration, while services traditionally use a message-based integration (however, nothing really prevents you to write a library based on exchanging messages)
• services are discoverable
• contracts are explicit and expressed "outside" the running code
• services are autonomous (but here again, you could write autonomous APIs, couldn't you?)
• boundaries are explicit

What am I missing here? What else really distinguishes services from a high-level API?

Answer 1

Service oriented architecture implies that the exposed interface does not live on the same host where the client runs and the service is completely decoupled from the client code (loose coupling). You can easily call an API by loading the necessary library and executing your code, on the same node. Rather than defining the API, service oriented architecture is focusing on the functionality, many times you can access the same feature using different protocols.

I would go for the loose code coupling if there was anything which would distinguish SOA and AOA.

Answer 2

You have covered most important points. I would add one :

Usually, a Service is stateless. Each Service request is independent. This is in contrast to a library interface where you may make certain calls in a sequence to get the desired result.