principles:low_coupling
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principles:low_coupling [2014-07-14 13:19] – [Rationale] 2001:8d8:1fe:100:fc69:ab47:c44c:75d1 | principles:low_coupling [2021-09-02 12:46] – old revision restored (2021-05-11 22:05) 65.21.179.175 | ||
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===== Context ===== | ===== Context ===== | ||
/* fill in contexts here: */ | /* fill in contexts here: */ | ||
- | * [[contexts: | + | * [[contexts: |
- | * [[contexts: | + | |
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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//. | 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//. | ||
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- | Coupling is a measure of dependency between modules. The more dependencies there are, the stronger the dependencies are, and the more assumptions are made upon other modules, the higher is the coupling. | ||
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- | There are different forms of couplings which can be rated according to their strength((G. J. Myers: //Reliable Software through Composite Design//)): | ||
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- | * //No coupling//: The modules do not know each other. | ||
- | * //Call coupling//: A module calls another one. | ||
- | * //Data coupling//: A module calls another one passing parameters to it. | ||
- | * //Stamp coupling//: A module calls another one passing complex parameters to it. | ||
- | * //Control coupling//: A module influences the control flow of another module. | ||
- | * //External coupling//: The modules communicate using a simple global variable. | ||
- | * //Common coupling//: The modules communicate using a common global data structure. | ||
- | * //Content coupling//: A modules depends on the inner working of another module. This is the strongest form of coupling. | ||
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- | The forms ranging from no coupling to stamp coupling can be considered " | ||
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- | There are also some additional forms of undesirable couplings: | ||
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- | * //Tramp coupling//: A module is only coupled to a data structure because some other module needs the data. The module gets the data and passes it to the other module without touching the "tramp data" ((M. Page-Jones: //The Practical Guide to Structured Systems Design//)). | ||
- | * //Logical coupling//: A module makes some assumptions about another module without referencing it. For example a module //A// only sorts a list because some other module //B// which //A// technically does not know about needs it sorted. | ||
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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. | 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 | + | Furthermore //A// makes many and detailed assumptions about //B//, there is also a high probability that //A// has to change despite only relying |
But if coupling is low, there are only few assumptions between the modules which can be violated. This reduces the chance of [[glossary: | But if coupling is low, there are only few assumptions between the modules which can be violated. This reduces the chance of [[glossary: | ||
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===== Strategies ===== | ===== Strategies ===== | ||
- | * Indirection: | + | * Indirection: |
* Dependency Inversion/ | * Dependency Inversion/ | ||
- | * Use lower form of coupling | + | * Use lower form of coupling: |
- | * Merge modules: | + | * Merge modules: |
- | * Hide information: Information which is hidden cannot be depended upon. | + | * Hide information |
- | + | ||
- | ===== Caveats ===== | + | |
- | + | ||
- | Coupling can be reduced by several technical measures (see [[# | + | |
- | + | ||
- | 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 ===== | ===== Origin ===== | ||
/* the *primary* source */ | /* the *primary* source */ | ||
- | * W. P. Stevens, | + | |
===== Evidence ===== | ===== Evidence ===== | ||
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/* * [[wiki: | /* * [[wiki: | ||
- | * [[wiki: | + | * [[wiki: |
- | * [[wiki: | + | * [[wiki: |
/* * [[wiki: | /* * [[wiki: | ||
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==== Specializations ==== | ==== Specializations ==== | ||
+ | * [[Tell, don't Ask/ | ||
* [[Constantine' | * [[Constantine' | ||
* [[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. | * [[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. | ||
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* [[Keep It Simple Stupid]] (KISS): Reducing the coupling often involves the use of complicated interaction patterns, indirections, | * [[Keep It Simple Stupid]] (KISS): Reducing the coupling often involves the use of complicated interaction patterns, indirections, | ||
* [[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. | * [[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/ | + | |
==== Complementary Principles ==== | ==== Complementary Principles ==== | ||
- | * [[Tell, don't Ask/ | ||
* [[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' | * [[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' | ||
* [[Information Hiding/ | * [[Information Hiding/ | ||
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{{page> | {{page> | ||
- | ===== Examples | + | ===== Example |
===== Description Status ===== | ===== Description Status ===== | ||
/* Choose one of the following and comment out the rest: */ | /* Choose one of the following and comment out the rest: */ | ||
- | /*[[wiki: | + | [[wiki: |
- | + | / | |
- | [[wiki: | + | |
/ | / | ||
===== Further Reading ===== | ===== Further Reading ===== | ||
- | * Albert Endres and Dieter Rombach: //[[resources: | + | * Albert Endres and Dieter Rombach: //A Handbook of Software and Systems Engineering// |
* [[wp> | * [[wp> | ||
* [[wiki> | * [[wiki> | ||
* Martin Fowler: // | * Martin Fowler: // | ||
- | * {{page> | ||
- | |||
- | ===== Discussion ===== | ||
- | Discuss this wiki article and the principle on the corresponding [[talk: |
principles/low_coupling.txt · Last modified: 2021-10-18 21:49 by christian