Whatever... [Deepak Alur's Blog]: Deepak Alur's Blog @ Sun

Service granularity is a recurring hot discussion in design teams. So, I am glad that my amigo John Crupi started this discussion on SOA Granularity. The issue of granularity is a never ending discussion, and as John mentions is an easy way to start a debate. The issue of granularity is not just an issue in SOA. We have always had to deal with object/procedure/service <insert-your-favorite-here> granularity in designing a system. I think SOA magnifies the granularity issue by bringing it to the forefront of the design discussion.

IMO, granularity is a subjective matter. What is coarse-grained for one is not coarse-grained for another. I think this subjectivity is one of the reasons that granularity becomes a hot topic debate amongst designers. While granularity is one aspect that must be paid close attention to, I think it is better not to issue edicts around it. Because the moment you define a law of granularity, someone is going to break it! Trust me.

Pragmatically speaking, a (composite) SOA application will contain services of varying levels of granularity as shown in the diagram. I think what John is pointing out is that you want to ensure that the services that are exposed externally are the ones that are on the top (i.e. relatively coarse-grained). Anyway, I do agree with John that when talking about SOA services, we should be talking about business services. But, that does not really imply that we should always make a service coarse-grained. So, can a business service be fine-grained? It depends, on the business it serves.

For instance, let's say you are a famous online auction company and many successful small business partners use/depend on your auction platform. Let's say that these users require a service to be exposed that returns the latest bid price for a given item number. Is this too fine-grained? It sure feels like it when compared to other services like Create Auction,  End Auction, etc. But what if this is indeed business speak. That is, your business managers want to provide a Latest Bid Price service. And what if your partners using this service are willing to pay for it. Will you not expose this service because it is fine-grained? Hell, no! You are going to expose that service. Why? Because it is going to make money! Whether that be a Dime, a Rupee, a Yen or a Yuan. (OK, it is not always about money, but surely about generating some business value.)

So while we lean towards making SOA services as coarse-grained as necessary, remember that whether a service is coarse-grained or fine-grained or in between, if it makes business sense, it must be a good service to expose to the world.