[edited version of post I made on Dr. Dobbs Portal]

Back in April I  provided a definition for "architecture" as one of my first posts on DDJ. I also promised I'll talk about the distinction betwen Architecture and Design. Well this time is now.

When I try to think about it. I see two base criteria to distinguish between the architecture and design:

• Design deals with local decisions, where architecture is broader. For instance, you "design" the interfaces for your classes, but you "architect" the division into tiers.
• Design is mostly about the functional requirements, while architecture is mostly about quality attributes. You design how a specific workflow will fulfill a certain use case, but you architect the solution to the system's availability.

It is probably quite evident that this distinction only provides blurry borders between architecture and design; for example, when you have a multi-tier solution and you "architect" the UI and say it will implement MVP pattern. Can this be considered local decision and thus design or is this the overall decision (for the UI) and thus architecture?

The way I see it the exact cross-point from architecture to design is not that important. The point in talking about two distinct activities in the development process is to maintain separation of concern. you need to handle both to make sure a solution will actually work whether you do a little design while architecting or do a little architecture while designing really doesn't matter. Also architects should be involved in both activities anyway...

Udi Dahan, which is one of the best architects I know has recently created an excellent course on SOA - you can find the details of the syllabus on Udi's site .

I had a chance to work with Udi in the past and the solution we implemented utilized many of the patterns and techniques Udi covers in his course - so these are not just nice theories but rather real stuff that works

[crossposted from DDJ]

Yesterday I attended an interesting presentation on SOA by Dr. Donald F. Ferguson, chief architect for IBM's software group.

I was happy to hear him validate some of my thoughts on SOA (e.g., workflows are better kept inside services rather then outside, transaction boundaries should be inside a service, and so on), and introduced a couple of things I didn't know much about (for example, OSGi, a SOA platform for networked devices that's not based on web services. He also presented some nice insights (for instance, looking at the middleware as an infrastructure service and thus nicely unifying SOA and EDA)

On of the insights Donald presented was the use of heuristics as an aid to modeling and validating architectures. Some of the heuristics he mentioned include:

• Occam's Razor -- avoid needless repetition
• Don't create something new if you can compose existing stuff to get the same result
• externalize volatility -- don't put in the code things that are likely to change
• Focus on "name,value" programming not "offset programming" -- make things easy to understand
• Different is hard
  

If you look at heuristics as an abstraction of experience, they can provide as a good tool for keeping yourself on the right track. Some heuristics are universal (maybe the ones mentioned above and a few others like "simplify, simplify, simplify" or "the original statement of a problem is probably not the best one or it may even not be the right one"), but the problem is, as always, deciding (in advance) which heuristics to apply to a problem.

If you interested in using heuristics as an architect tool you may want to look at " The Art of Sysytem Architecting", by Mark Maier and Eberhardt Rechtin. The book discusses the architectures of different system types (collaborative, IT, Manufacacturing, etc.) and provides heuristics for each of these systems.

Heuristics are a good tool to use when you design an architecture and in a way the different design principles (e.g., the single responsibility principle) can also be considered heuristics. Nevertheless it is very important to verify designs by additional methods like code and formal evaluation and not rely on heurisitcs as the only tool.