Service and Boundaries

Architectural boundaries do not fall between services. Rather, those boundaries run through the services, dividing them into components.

To deal with the cross-cutting concerns that all significant systems face, services must be designed with internal component architectures that follow the Dependency Rule. Those services do not define the architectural boundaries of the system; instead, the components within the services do.

As useful as services are to the scalability and develop-ability of a system, they are not, in and of themselves, architecturally significant elements.

The architecture of a system is defined by the boundaries drawn within that system, and by the dependencies that cross those boundaries. That architecture is not defined by the physical mechanisms by which elements communicate and execute.

A service might be a single component, completely surrounded by an architectural boundary. Alternatively, a service might be composed of several components separated by architectural boundaries. In some cases, clients and services may be so coupled as to have no architectural significance whatsoever.