What is a Service in an SOA?

A Service in an SOA meets the following criteria:

·                            Multiple-use

·                            Non-context-specific

·                            Composable with other components

·                            Encapsulated i.e., non-investigable through its interfaces

·                            A unit of independent deployment and versioning

A Service is written to a specification. It does not matter what the specification is: COM, Java Beans, WSDL, etc., as long as it adheres to the specification. It is only by adhering to the specification that it becomes a Service and gains features like reusability and so forth.
The idea in OO (Object Orientation) is that software should be written according to a mental model of the actual or imagined objects it represents.
Services, by contrast, makes no such assumptions, and instead state that software should be developed by gluing prefabricated components together much like in the field of electronics or mechanics.
When a Service is to be accessed or shared across execution contexts or network links, some form of serialization (also known as marshalling) is employed to turn its interfaces into a bitstream.
It takes significant effort and awareness to write a Service that is effectively reusable. The Service needs:

·                            to be fully documented;

·                            more thorough testing;

·                            robust input validity checking;

·                            to pass back useful error messages as appropriate;

·                            to be built with an awareness that it will be put to unforeseen uses;

·                            a mechanism for compensating developers who invest the (substantial) effort implied above.

No big deal to agree with these statements, is it? Now read, for fun, the definition of a Software Component where I took these text fragments from while replacing "software component" by "Service".

So, is a software component a Service and is CBD (Component Based Development) the same as building a Service Oriented Architecture? Not quite, it is the other way around: A Service in an SOA (as SOA is currently understood) is a software component, but a component with a strong business function constraint. SOA is an instance of a component based architecture. So building an SOA has always to do with CBD, but CBD has not always to do with building an SOA as we understand an SOA.

The idea that software should be componentized, built from prefabricated components (like Services), was first published in Douglas McIlroy's address at the NATO conference on software engineering in Garmisch, Germany, 1968 titled Mass Produced Software Components. A very interesting reading. Some quotes:

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