05 — Package & Component Principles

Driving question: What makes a component boundary stable as a system evolves?

Learning objectives

• Explain release/reuse/common-closure/common-reuse principles.
• Use dependency cycles and stability to discuss component structure.
• Identify inappropriate “layer” dependencies.
• Relate component boundaries to ownership and deployment constraints.

Principles of cohesion

• Release/Reuse Equivalence: reusable things need a coherent versioned release boundary.
• Common Closure: group elements that tend to change for the same reasons.
• Common Reuse: avoid forcing clients to depend on things they do not use.

These principles pull in different directions; package design is a trade-off.

Principles of coupling

Acyclic dependency structures reduce coordinated release pressure. Stable dependencies should point toward components that change less frequently or contain higher-level policy.

Define instability for a component as:

\[ I = \frac{C_e}{C_a + C_e} \]

where \(C_e\) is efferent coupling and \(C_a\) is afferent coupling. The number is useful only when interpreted with system context.

Design / research exercise

Build the package dependency graph of a medium-size project. Identify cycles, highly unstable hubs, and “utility” packages. Propose one boundary change and predict its impact on build/test/release coupling.

Suggested reading

• Robert C. Martin, component cohesion and coupling principles.