Objects are Nouns... Services are Verbs... SOA hinges on designing the right Verbs
I still find postings that express confusion about the relationship between object-oriented architectures and service-oriented architectures... so I would again like to offer my own $0.02 worth of insight...
That's it. That's all there is to it.
An Object is primarily a Thing that is manipulated (and is generally stateful), a Service is primarily an Action that is performed (and is generally stateless).
In an object-oriented architecture, the software components are primarily organized as entities (things) that are to be manipulated. These entities generally have relationships to other entities, and that leads to the emphasis on design artifacts such as UML class diagrams.
In a service-oriented architecture, the software components are primarily organized as tasks (actions) that are to be performed. These tasks are generally designed to function as steps in more complex processes, and that leads to the emphasis on design artifacts such as BPMN diagrams.
Businesses use software to automate business processes. Business processes do involve Nouns (Objects), but they are generally more concerned with Verbs (Services).
Let me explain: Most businesses process forms of one sort or another. In general, manipulation of the form (an object) is not the primary concern of the business; the primary concern of the business is to perform a set of tasks (services) based on the contents of the form.
If the software components are packaged as distinct services, the mapping between the business requirement to "process the form" and the software implementation of the process is relatively straight forward. This clean mapping between requirements and implementation results in solutions that can better respond to changing requirements over time (Trust me, all business requirements change over time).
Sun's Tim Bray has recently expressed his opinion that the term "SOA" is meaningless, and that he prefers the term "Web-style". With respect to Tim, I disagree with his opinion.
I do not agree with Tim that "Web-style" is a better term than "SOA".
I agree with Tim that the term SOA has:"become damaged, weakened by over hype, over use, over promise (and) under deliver", but unlike Tim, I think that I can "explain what the difference is between SOA and Web services". Web services are a protocol specific form of service. To implement an SOA you must eventually pick a protocol, but it needed be a Web oriented protocol.
Tim asks for "an explanation that's meaningful in terms a programmer can understand, so a programmer can go and build one of these things". Perhaps it is my own arrogance or naivete, but I think my explanation is a darn good start.
In my experiences with many developers, I have found that explaining how to design and build SOA solutions is not any more difficult or "verbose" than it was to explain how to design and build Object-Oriented solutions. I confess that I do resort to "paragraphs and paragraphs (of) prose", but everyone who knows me understands that I always resort to paragraphs and paragraphs of prose ;-)
But if Tim is right, and the term SOA has become a dead alabatross, then I'd like to offer an alternative to "Web-style": How about Process Oriented Architecture? Kind of has a nice ring to it; Don't you think?
Regardless of the acronym or term that we use to describe it, the point remains the same: If your primary task is to automate a business process, then you should package (most of) your software components as verbs... er, I mean services.
(cross-posted at Thoughtful Programmer)