Coupling and Cohesion: Types, Comparison and Target Combination

Coupling = interdependence between modules (want it low).
Cohesion = relatedness of elements within a module (want it high).

Types of coupling (best to worst)

Type	Meaning	Quality
Data	Modules share only simple parameters	Best
Stamp	Share a composite structure (record) but use only part of it	Good
Control	One module passes a flag that controls the other's logic	Moderate
External	Share an external device, format or protocol	Poor
Common	Share global data	Poor
Content	One module modifies or jumps into another's internals	Worst

Types of cohesion (worst to best)

Type	Meaning	Quality
Coincidental	Unrelated tasks grouped randomly	Worst
Logical	Tasks of the same category, selected by a flag (e.g. all input routines)	Poor
Temporal	Tasks executed at the same time (e.g. initialization)	Poor
Procedural	Tasks follow a fixed order of execution	Moderate
Communicational	Tasks work on the same data	Good
Sequential	Output of one task is input to the next	Better
Functional	All elements contribute to one single function	Best

Comparison

Basis	Coupling	Cohesion
Concerns	Between modules	Within a module
Goal	Minimize	Maximize
Measures	Dependence	Relatedness
Effect when poor	Changes ripple across modules	Module is hard to understand

Target combination for a maintainable modular system

Low coupling (data coupling) with high cohesion (functional cohesion).

• A change stays inside one module, so maintenance is cheaper.
• Modules can be tested, reused and developed independently.
• Errors do not spread to other modules.

Avoid content/common coupling and coincidental/logical cohesion.