This post on service service granularity from Ross Mason (the original developer of Mule ESB) got me in retrospective mode. I've been thinking about this before and with the time, I have come up with some criteria how to break functionality into services. Unfortunately that turned out to be only half of the puzzle. The other half is the implementation and maintenance strategy that would allow you to achieve your nicely layered boxes-and-arrows vision, while delivering ROI on quarterly intervals and not falling into the JBOWS or stovepipe-system pitfalls.
I personally favor starting top-down, focusing on specific applications and task-level services, and harvesting entity and utility services from working production code. The downside of this is that it is generally more work, often seen as 'tinkering' and in case we consolidate utility code from multiple systems, will likely introduce temporary instabilities.
This makes it a difficult sell to management - what they see is the IT guys trying to get budget to work on some kind of metaphysical inner-beauty thing instead of delivering new features. Saying that this would reduce bugs is not a great argument, because you will be reminded that's why we pay the QA team and by the way, as a professional you are not supposed to write bugs in the first place.
On the other side, the PHBs of the world seems to love the idea of the Centers of Excellence, located in China or Malaysia, full of trained and cheap labour, building the utility services and coding the entity diagrams designed by the Architecture Group in their ivory tower, while the solution engineers use point and click tools to churn 'codeless' composite applications. Unfortunately this breaks at the other end - unexpected interactions are discovered at deployment time, maintenance takes too long as the whole turns out to be more than its parts, exhibiting "emergent behavior" (also known as "underspecified features") left and right.
So, there is no quick and easy way. I guess we need to become IT salesmen and we need to compete with the professional vendor scamsters and the consultancy loan-sharks (that's a topic for another post).
