*Complexity is the Enemy! *

This is the message driven home repeatedly by Roger Sessions in his book *Simple Architectures for Complex Enterprises*.

Sessions recommends tackling a complex enterprise architecture by identifying the subcomponents of a complex system and dividing that system into autonomous subsystems. He refers to these subsystems as Autonomous Business Capabilities (ABCs) and the process of dividing them as a Simple Iterative Process (SIP).

Before describing how to approach this process, Sessions presents a mathematical proof that subdividing a complex system into a set of subsystems reduces the complexity of the system as a whole. This seems intuitive to many of us, but the mathematics allow us to be more forceful in our commitment to this process. The mathematics is relatively simple (nothing beyond high school math) and he even recommends training team members in this mathematics before beginning any SIP.

A large part of an Enterprise Architect's job is to define the optimal way to partition the complex system. By applying mathematics to his model, he removes the emotions that so often dictate how a project is broken up.

The process of splitting a complex system into appropriate subsystem isn't overwhelming, but it is critical to managing complexity. According to Sessions, Each ABC should contain only elements that relate to one another; and the elements of one ABC should not relate directly to or communicate directly with any element in another ABC. Once partitioned, each ABC should be roughly the same size, although it is possible to split a subsystem further into sub-subsystems. It is also critical that communication between each subsystem take place only at a few clearly-defined points.

If this sounds like a recipe for Service Oriented Architecture, this is no coincidence. Sessions concludes his book with recommendations on moving from business partitions (ABCs) to software partitions, which he describes as "fortresses". These software partitions follow many of the same rules as ABCs created with the SIP, so making this transition is straightforward.

This is a good book for anyone who aspires to be an Architect (Enterprise or otherwise) and wants to apply a systematic approach to managing complexity.