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Concepts
What Is a Service?
A service is a well-defined unit of work performed by a component and packaged for easy access. The idea is to implement the functionality exactly once, do it well, and then make it widely accessible. The extra effort involved in initially creating the service (and there is always extra effort) is paid for by avoiding future development costs either through the direct reuse of the service or by isolating the service user from ongoing internal implementation changes within the service.
The Layered Approach to SOA
Another key concept in SOA is the layering or classification of service types. If you consider how a “top-down” implementation approach would be tackled, you can get a better understanding of these layers. Step 1: Business challenges and processes
SOA concepts
SOA concepts
Now let's take a look at some SOA concepts to better understand what SOA is.
Definition of a service in SOA
There are a lot of different definitions of services, but I think these do the best job of explaining what a service really is.
From Web Services and Service-Oriented Architecture: The Savvy Manager's guide (see Resources for a link):