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Cohesion (Component Modeling)

4 min readStatus:Seedling

Core Idea

Cohesion measures how well the elements within a module belong together. High cohesion means module elements are strongly related and work toward a unified purpose; low cohesion indicates loosely related or unrelated elements grouped arbitrarily.

What Is Cohesion?

Core Definition: Cohesion is a qualitative measure of how focused and unified a module’s internal elements are:

  • High cohesion: Module’s classes, functions, and data work together to achieve a single, well-defined purpose
  • Low cohesion: Module contains elements that don’t naturally belong together—perhaps grouped by convenience, historical accident, or organizational structure rather than logical necessity

Origins and Key Question: The concept of cohesion emerged from structured programming research:

  • Remains fundamental to modern software architecture
  • Asks a deceptively simple question: “Do these things actually belong together?”

Examples:

  • High cohesion: A module handling all aspects of user authentication—validating credentials, managing sessions, enforcing password policies
  • Low cohesion: Arbitrarily combining authentication logic, email formatting, and database connection pooling simply because “that’s where the code ended up”

Levels of Cohesion (strongest to weakest): Computer scientists have identified different levels of cohesion:

  1. Functional cohesion — The ideal; all elements contribute to a single, well-defined task
  2. Sequential cohesion — Elements form a processing pipeline where one element’s output feeds into the next
  3. Communicational cohesion — Groups elements that operate on the same data
  4. Procedural cohesion — Elements execute in sequence but serve different purposes
  5. Temporal cohesion — Elements execute at the same time but are otherwise unrelated
  6. Logical cohesion — Elements perform similar types of operations but on different data
  7. Coincidental cohesion — No meaningful relationship between elements at all

Relationship to Coupling: The relationship between cohesion and Coupling creates the foundation for evaluating modular design:

  • Architectural ideal: High cohesion within modules and low coupling between them
  • Result: Modules that are internally focused yet externally independent
  • This combination maximizes:
    • Maintainability
    • Testability
    • Ability to reason about system behavior

Why This Matters

Code Comprehension:

  • When developers open a module, they should encounter a focused set of related concerns
  • Not a scattered collection of unrelated functionality
  • This focus reduces cognitive load
  • Developers can understand what a module does without holding the entire system in their heads

Maintainability and Change Management:

  • When functionality is cohesively organized, changes tend to localize within single modules
  • Example: If authentication requirements change, developers should only need to modify the authentication module, not hunt through disparate parts of the system
  • Low cohesion consequence: Scatters related functionality across multiple modules, turning simple changes into archaeological expeditions

Testing:

  • Highly cohesive modules are easier to test in isolation
  • The focused scope means:
    • Fewer test cases are needed to achieve coverage
    • Tests are more meaningful because they exercise logically related functionality rather than arbitrary combinations

Architectural Scope: Cohesion isn’t just a code-level metric—it applies to architectural components and even entire services:

  • In microservices architectures, service cohesion determines whether:
    • Services have clear boundaries and purposes
    • Or services become tangled with multiple responsibilities
  • Poor service cohesion consequence: Forces teams to coordinate across service boundaries even for logically related changes, negating many benefits of service-oriented design

Sources

  • Richards, Mark and Neal Ford (2020). Fundamentals of Software Architecture: An Engineering Approach. O’Reilly Media. ISBN: 978-1-492-04345-4.

  • Yourdon, Edward and Larry L. Constantine (1979). Structured Design: Fundamentals of a Discipline of Computer Program and Systems Design. Prentice Hall. ISBN: 978-0138544713.

    • Original formalization of cohesion types and measurement

Note

This content was drafted with assistance from AI tools for research, organization, and initial content generation. All final content has been reviewed, fact-checked, and edited by the author to ensure accuracy and alignment with the author’s intentions and perspective.

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