Coupling and Cohesion in Module Design

Both are design-quality measures for the modules that make up a system.

Cohesion

Cohesion measures the degree to which the elements inside a single module belong together — how strongly the module's internal parts are related to a single purpose.

• High cohesion (good): the module performs one well-defined logical task; all its elements contribute to that task. Easy to understand, test, and reuse.
• Low cohesion (bad): a module does many unrelated things (e.g., a "utility" module mixing file I/O, math, and GUI code); hard to understand, test, and maintain.

Cohesion levels (best → worst):

1. Functional cohesion — all elements contribute to one function (best).
2. Sequential cohesion — output of one part feeds the next.
3. Communicational cohesion — parts operate on the same data.
4. Procedural cohesion — parts follow a process/sequence.
5. Temporal cohesion — parts executed at the same time.
6. Logical cohesion — parts are logically similar but unrelated (e.g., all error messages).
7. Coincidental cohesion — parts have no meaningful relationship (worst).

Coupling

Coupling measures the degree of interdependence between different modules — how much one module depends on another.

• Low coupling (good): modules are independent; changes in one module have little impact on others.
• High coupling (bad): modules are tightly interwoven; a change in one ripples through many, making maintenance and testing hard.

Coupling levels (best → worst):

1. Data coupling — modules share data via parameters (best).
2. Stamp coupling — share a data structure (record) but only use part of it.
3. Control coupling — one module passes control information (flags) to another.
4. External coupling — share an externally imposed format/protocol.
5. Common coupling — share global data.
6. Content coupling — one module directly accesses/alters the internals of another (worst).

Why low coupling + high cohesion is preferred

1. Maintainability — a change stays local: with high cohesion, the change belongs to one module; with low coupling, other modules are unaffected. This directly reduces maintenance cost and effort.
2. Understandability — each module is a self-contained, single-purpose unit, easy to read.
3. Reusability — cohesive, loosely coupled modules can be lifted out and reused in other systems.
4. Testability — modules can be unit-tested in isolation; fewer inter-module interactions to cover.
5. Reliability & reduced defect spread — errors are contained within a module instead of propagating.
6. Parallel development — teams can develop modules concurrently with minimal coordination conflicts.

Idea	Good	Bad
Cohesion	High (functional)	Low (coincidental)
Coupling	Low (data)	High (content)

Summary: "high cohesion, low coupling" is a cornerstone design objective — it yields modules that are simple to build, test, maintain, and reuse.