Skip to content
Software Architecture Guide

Modifiability

Explanation

Section titled “Explanation”

Modifiability is about change. How easily can people alter a system? ISO 25010, a standard quality model, defines the related quality maintainability. It is the degree to which the intended maintainers can modify a product effectively and efficiently. See the article Maintainability for that wider quality.

Architects need a way to reason about this. One well-known report from the Software Engineering Institute (SEI) uses three parameters:

  • Coupling: how strongly parts of the system depend on each other.
  • Cohesion: how closely the responsibilities inside one part belong together.
  • Cost: what a change takes to carry out.

The article Coupling and Cohesion explains the first two in more depth.

These three parameters govern how modifiable a system is. Architects can adjust them with tactics. A tactic is an architectural means of changing one of these parameters. The article Modifiability Tactic covers the specific tactics. The article Architectural Tactic explains the general idea.

The same report also describes standard architectural patterns and their variants. It does so in terms of modifiability tactics. This links tactics to the larger structures that teams often use. The article Relationship between Tactics and Architectural Styles/Patterns goes further.

Example

Section titled “Example”

A team builds an order system. They notice that many parts depend directly on each other, so a small change touches many places. They treat this as high coupling. They pick a tactic that lowers it. They also ask what the change will cost and weigh that against the benefit. Coupling, cohesion, and cost give the team shared words for the discussion.

Trade-offs

Section titled “Trade-offs”

Cost is one of the three parameters, so applying a tactic is not free. A team should weigh the effort of a tactic against the modifiability it brings.

Checks

Section titled “Checks”
  • The document names coupling, cohesion, and cost as the parameters it uses to reason about modifiability.
  • Each modifiability tactic in the document states which parameter it adjusts.
  • Patterns used for modifiability are described in terms of the tactics they apply.
Section titled “Related”

Sources

Section titled “Sources”