A few weeks ago I posted a reaction to a post by Pete Lacey that asked what is SOA. In a comment to my post Pete said that my definition isn't good since

"...even according to your definition, an architectural style contains constraints, and to date neither SOA nor web services have been shown to exhibit any constraints"

The idea behind this series of posts is to try to take a little more formal view at what I think SOA is. It is based on my thinking for the past few weeks but it is also still a work in progress (so any comments are welcome)

The way I see it SOA is an architectural style which is derived from the following architectural styles:

1. Client/Server
2. Layered System
3. Pipe and Filters
4. Distributed Agents
   

Note that if you add to the above statelessness, uniformed pipe and filters and a cache you can get a RESTful SOA. This is not REST as REST itself does not require distributed agent or even pipes and filters (but it does build on client/server and layered system). In other words not all RESTful systems are SOA, you can build SOAs which are not RESTful and you can build RESTful SOAs.

The main components of SOA are Service,Message, Contracts and Consumers. Policies also exists but now I tend to think they are optional. The four architectural styles mentioned above affect the definitions of the different components and the way they interact together

In the following posts on this subject I'll first take a look at each of the contributing architectural styles and how they affect SOA and later try to provide a definition that builds on them