Granularity of Services

I notice a misconception spreading around at a large scale between SOA-bloggers and article-writers, even by the great gurus. They keep a discussion alive about the optimal granularity of a service. But they are mixing up services which are functional components, and "Web Services" which is an interface implementation technology.

To me it makes more sense to discuss at which levels of granularity the use of web services technology is appropriate. And that depends on system implementation characteristics such as degree of distribution and heterogeneity and desired openness.

The granularity of functional components "goes to the bottom". It's just common modular and structured design and programming practice, originating from the 70's, every developer should now about. A nice pdf on this subject is available here. So nothing new to SOA. Read, for instance, "Reliable Software through Composite Design" by Glenford J. Myers, 1975.

Or study Yourdon's DFD (Dataflow Diagrams) method; very interesting is part III and chapter 22.