Common closure principle

The common closure principle (CCP) is a package design principle that restates the single responsibility principle (SRP) for larger components (packages). The principle states that, ideally, all the components of a package should all have the exact same reasons to change. Or, to put it another way, a change request that affects one component of a package should affect all of the other components of the same package, but it should not affect anything outside the package.

Thereby, each expected change request should affect a minimal number of components. To explain it another way, each component should be closed to a maximum number of expected change requests.

The term "expected" here signifies a few important implications.

1. The inherent concepts/responsibilities of a system run deeper than a surface-level description of its behavior.
2. The deeper concepts/responsibilities of a system are not entirely objective, but can be modeled in different ways.
3. Determining the concepts/responsibilities of a system is not just passive describing but also active strategizing.

This principle leads to highly cohesive components. It also implies loosely coupled components, because related concepts that do change together end up enclosed together in the same wrapper. When each single concept is expressed by a single component, there are no unnecessary couplings between components.

This is one of Robert C Martin’s six package principles.