principles:low_coupling
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principles:low_coupling [2013-04-03 14:21] – [Evidence] christian | 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//. | ||
+ | |||
+ | 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. | ||
+ | |||
+ | There are different forms of couplings which can be rated according to their strength((G. J. Myers: //Reliable Software through Composite Design//)): | ||
+ | |||
+ | * //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. | ||
+ | |||
+ | The forms ranging from no coupling to stamp coupling can be considered " | ||
+ | |||
+ | There are also some additional forms of undesirable couplings: | ||
+ | |||
+ | * //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 //A// makes many and detailed assumptions about //B//, there is also a high probability that //A// has to change despite only relying | + | Furthermore |
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 | + | * Hide information: Information which is hidden cannot be depended upon. |
===== Caveats ===== | ===== Caveats ===== | ||
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===== 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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===== 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: //A Handbook of Software and Systems Engineering// | + | * Albert Endres and Dieter Rombach: //[[resources: |
* [[wp> | * [[wp> | ||
* [[wiki> | * [[wiki> | ||
* Martin Fowler: // | * Martin Fowler: // | ||
- | * Craig Larman: //Applying UML and Patterns | + | * {{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