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Table of Contents
Low Coupling (LC)
Variants and Alternative Names
- Loose Coupling
Context
Principle Statement
Description
A module should not interact with too many other modules. Furthermore if a module A interacts with another module B, this interaction should be loose, which means that A should not make too many assumptions about B.
Rationale
If a module A interacts with a module B, there is a certain dependency between these modules. When for example A uses a certain functionality of B, then A depends on B. A makes the assumption that B provides a certain service, and moreover it makes assumptions on how this service can be used (by which mechanism, which parameters, etc.). If one of these assumptions is not true anymore because B has changed for some reason, A also has to change. So the fewer dependencies there are, the less likely it is that A stops working and has to be changed.
Furthermore A makes many and detailed assumptions about B, there is also a high probability that A has to change despite only relying one one other module. This is because in such a case A also needs to change when only a certain detail of B changes.
But if coupling is low, there are only few assumptions between the modules which can be violated. This reduces the chance of ripple effects.
Strategies
- Indirection:
- Dependency Inversion/Abstract Couplings:
- Use lower form of coupling:
- Merge modules:
- Hide information
Caveats
Coupling can be reduced by several technical measures (see strategies). But while these measures reduce the coupling technically, they do not necessarily reduce the logical coupling. In such a case two modules A and B may seem decoupled, but ripple effects may occur anyway because of the logical coupling. In such a case it is better to make the coupling explicit by not applying a decoupling strategy. It may also be possible to find a better suitable strategy or a better way of applying the strategy to also get rid of the logical coupling.
Furthermore note that coupling to a stable module is often no problem. The problematic cases are couplings to unstable modules. This means that applying decoupling strategies is beneficial when a coupling to an unstable module is reduced. But it may not be beneficial in the other cases.
See also section contrary principles.
Origin
Evidence
- Accepted: The concept of low coupling is widely known and described in several well-known books for example in Craig Larman's Applying UML and Patterns
Relations to Other Principles
Generalizations
Specializations
- Constantine's Law: Constantine's Law is just the combination of the two principles LC and HC.
- Dependency Inversion Principle (DIP): LC aims at reducing the dependencies to other modules. One way to do so is to only depend on abstractions. DIP is about this aspect.
Contrary Principles
- Keep It Simple Stupid (KISS): Reducing the coupling often involves the use of complicated interaction patterns, indirections, etc.
- High Cohesion (HC): A system consisting of one single module has a very low coupling as there are no dependencies on other modules. But such a system also has low cohesion. The other extreme, very many highly cohesive modules, naturally has a higher coupling between the modules. So here a compromise has to be found.
- Rule of Explicitness (RoE): Direct communication typically has the disadvantage of a higher coupling. Indirection reduces coupling but creates implicit/indirect communication paths.
Complementary Principles
- Tell, don't Ask/Information Expert (TdA/IE): IE may help to reduce coupling. Although there are also contrary cases (see caveats).
- Model Principle (MP): LC aims at reducing the dependencies to other modules. So a module shall depend on only a few others. MP now tells which dependencies are allowed and which aren't.
- Information Hiding/Encapsulation (IH/E): Higher forms of couplings (especially content couplings) break encapsulation.
Principle Collections
Examples
Description Status
Further Reading
- Albert Endres and Dieter Rombach: A Handbook of Software and Systems Engineering. p. 43pp.
- Martin Fowler: Reducing Coupling
- Craig Larman: Applying UML and Patterns – An Introduction to Object-Oriented Analysis and Design and Iterative Development