Dependency (computer science)
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In
software engineering Software engineering is the systematic application of engineering approaches to the software development, development of software. A software engineer is a person who applies the principles of software engineering to design, develop, maintain, tes ...
, coupling is the degree of interdependence between software
modules Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a syst ...
; a measure of how closely connected two routines or modules are; the strength of the relationships between modules. Coupling is usually contrasted with cohesion. Low coupling often correlates with high cohesion, and vice versa. Low coupling is often thought to be a sign of a well-structured
computer system A computer is a machine that can be programmed to carry out Sequence, sequences of arithmetic or logical operations automatically. Modern computers can perform generic sets of operations known as Computer program, programs. These programs enabl ...
and a good design, and when combined with high cohesion, supports the general goals of high
readability Readability is the ease with which a reader can understand a written text. In natural language, the readability of text depends on its content (the complexity of its vocabulary A vocabulary, also known as a wordstock or word-stock, is a ...
and
maintainability In engineering, maintainability is the ease with which a product can be maintained in order to: * correct defects or their cause, * Repair or replace faulty or worn-out components without having to replace still working parts, * prevent unexpected ...
.


History

The software quality metrics of coupling and cohesion were invented by
Larry Constantine Larry LeRoy Constantine (pronounced ''Constanteen''; born 1943) is an American software engineer, professor in the Center for Exact Sciences and Engineering at the University of Madeira Portugal Portugal (), officially the Portuguese Republ ...

Larry Constantine
in the late 1960s as part of a
structured design In software engineering, structured analysis (SA) and structured design (SD) are methods for analyzing business requirements and developing specifications for converting practices into computer programs, hardware configurations, and related manual p ...
, based on characteristics of “good” programming practices that reduced maintenance and modification costs. Structured design, including cohesion and coupling, were published in the article ''Stevens, Myers & Constantine'' (1974) and the book ''Yourdon & Constantine'' (1979), and the latter subsequently became standard terms.


Types of coupling

300px, upright=2.0, Conceptual model of coupling Coupling can be "low" (also "
loose Loose may refer to: Places * Loose, Germany *Loose, Kent Loose is a village some south of Maidstone, Kent, situated at the head of the Loose Valley, with which it forms the Loose Valley Conservation Area. The fast- flowing Loose Stream, River L ...
" and "weak") or "high" (also "tight" and "strong"). Some types of coupling, in order of highest to lowest coupling, are as follows:


Procedural programming

A module here refers to a subroutine of any kind, i.e. a set of one or more statements having a name and preferably its own set of variable names. ;Content coupling (high): Content coupling is said to occur when one module uses the code of another module, for instance a branch. This violates
information hiding In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algor ...
– a basic software design concept. ;Common coupling: Common coupling is said to occur when several modules have access to the same global data. But it can lead to uncontrolled error propagation and unforeseen side-effects when changes are made. ;External coupling: External coupling occurs when two modules share an externally imposed data format, communication protocol, or device interface. This is basically related to the communication to external tools and devices. ;Control coupling: Control coupling is one module controlling the flow of another, by passing it information on what to do (e.g., passing a what-to-do flag). ;Stamp coupling (data-structured coupling): Stamp coupling occurs when modules share a composite data structure and use only parts of it, possibly different parts (e.g., passing a whole record to a function that needs only one field of it). :In this situation, a modification in a field that a module does not need may lead to changing the way the module reads the record. ;Data coupling: Data coupling occurs when modules share data through, for example, parameters. Each datum is an elementary piece, and these are the only data shared (e.g., passing an integer to a function that computes a square root).


Object-oriented programming

;Subclass coupling: Describes the relationship between a child and its parent. The child is connected to its parent, but the parent is not connected to the child. ;Temporal coupling: It is when two actions are bundled together into one module just because they happen to occur at the same time. In recent work various other coupling concepts have been investigated and used as indicators for different modularization principles used in practice.


Dynamic coupling

The goal of this type of coupling is to provide a run-time evaluation of a software system. It has been argued that static coupling metrics lose precision when dealing with an intensive use of dynamic binding or inheritance. In the attempt to solve this issue, dynamic coupling measures have been taken into account.


Semantic coupling

This kind of coupling considers the conceptual similarities between software entities using, for example, comments and identifiers and relying on techniques such as
latent semantic indexing Latent semantic analysis (LSA) is a technique in natural language processing, in particular distributional semantics, of analyzing relationships between a set of documents and the terms they contain by producing a set of concepts related to the docu ...
(LSI).


Logical coupling

Logical coupling (or evolutionary coupling or change coupling) exploits the release history of a software system to find change patterns among modules or classes: e.g., entities that are likely to be changed together or sequences of changes (a change in a class A is always followed by a change in a class B).


Disadvantages of tight coupling

Tightly coupled systems tend to exhibit the following developmental characteristics, which are often seen as disadvantages: # A change in one module usually forces a
ripple effect A ripple effect occurs when an initial disturbance to a system propagates outward to disturb an increasingly larger portion of the system, like ripples expanding across the water when an object is dropped into it. The ripple effect is often used ...

ripple effect
of changes in other modules. # Assembly of modules might require more effort and/or time due to the increased inter-module dependency. # A particular module might be harder to
reuse Reuse is the action or practice of using an item, whether for its original purpose (conventional reuse) or to fulfill a different function (creative reuse Upcycling, also known as creative reuse, is the process of transforming by-products, waste m ...
and/or test because dependent modules must be included.


Performance issues

Whether loosely or tightly coupled, a system's performance is often reduced by message and parameter creation, transmission, translation (e.g. marshaling) and message interpretation (which might be a reference to a string, array or data structure), which require less overhead than creating a complicated message such as a
SOAP 300px, The chemical structure of sodium laureth sulfate, a typical ingredient found in liquid soaps. Soap is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Sa ...

SOAP
message. Longer messages require more CPU and memory to produce. To optimize runtime performance, message length must be minimized and message meaning must be maximized. ;Message Transmission Overhead and Performance: Since a message must be transmitted in full to retain its complete meaning, message transmission must be optimized. Longer messages require more CPU and memory to transmit and receive. Also, when necessary, receivers must reassemble a message into its original state to completely receive it. Hence, to optimize runtime performance, message length must be minimized and message meaning must be maximized. ;Message Translation Overhead and Performance: Message protocols and messages themselves often contain extra information (i.e., packet, structure, definition and language information). Hence, the receiver often needs to translate a message into a more refined form by removing extra characters and structure information and/or by converting values from one type to another. Any sort of translation increases CPU and/or memory overhead. To optimize runtime performance, message form and content must be reduced and refined to maximize its meaning and reduce translation. ;Message Interpretation Overhead and Performance: All messages must be interpreted by the receiver. Simple messages such as integers might not require additional processing to be interpreted. However, complex messages such as
SOAP 300px, The chemical structure of sodium laureth sulfate, a typical ingredient found in liquid soaps. Soap is a salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of Sa ...

SOAP
messages require a parser and a string transformer for them to exhibit intended meanings. To optimize runtime performance, messages must be refined and reduced to minimize interpretation overhead.


Solutions

One approach to decreasing coupling is
functional design Functional Design is a paradigm used to simplify the design of hardware and software devices such as computer software Software is a collection of instructions that tell a computer A computer is a machine that can be programmed to carry o ...
, which seeks to limit the responsibilities of modules along functionality. Coupling increases between two classes ''A'' and ''B'' if: *''A'' has an attribute that refers to (is of type) ''B''. *''A'' calls on services of an object ''B''. *''A'' has a method that references ''B'' (via return type or parameter). *''A'' is a subclass of (or implements) class ''B''. Low coupling refers to a relationship in which one module interacts with another module through a simple and stable interface and does not need to be concerned with the other module's internal implementation (see
Information Hiding In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algor ...
). Systems such as
CORBA The Common Object Request Broker Architecture (CORBA) is a Standardization, standard defined by the Object Management Group (OMG) designed to facilitate the communication of systems that are deployed on diverse platforms. CORBA enables collabora ...
or COM allow objects to communicate with each other without having to know anything about the other object's implementation. Both of these systems even allow for objects to communicate with objects written in other languages.


Coupling versus cohesion

Coupling and cohesion are terms which occur together very frequently. Coupling refers to the interdependencies between modules, while cohesion describes how related the functions within a single module are. Low cohesion implies that a given module performs tasks which are not very related to each other and hence can create problems as the module becomes large.


Module coupling

Coupling in Software Engineering describes a version of metrics associated with this concept. For data and control flow coupling: * di: number of input data parameters * ci: number of input control parameters * do: number of output data parameters * co: number of output control parameters For global coupling: * gd: number of global variables used as data * gc: number of global variables used as control For environmental coupling: * w: number of modules called (fan-out) * r: number of modules calling the module under consideration (fan-in) \mathrm(C) = 1 - \frac Coupling(C) makes the value larger the more coupled the module is. This number ranges from approximately 0.67 (low coupling) to 1.0 (highly coupled) For example, if a module has only a single input and output data parameter C = 1 - \frac = 1 - \frac = 0.67 If a module has 5 input and output data parameters, an equal number of control parameters, and accesses 10 items of global data, with a fan-in of 3 and a fan-out of 4, C = 1 - \frac = 0.98


See also

*
Cohesion (computer science)In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a specific task. Programming involves tasks such as: analysis, generatin ...
*
Connascence (computer science) Connascence () is a Software metric, software quality metric invented by Meilir Page-Jones to allow reasoning about the complexity caused by dependency relationships in object-oriented design much like Coupling (computer programming), coupling did f ...
*
Coupling (physics) In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Ph ...
* Dead code elimination * Dependency hell * Efferent coupling * Inversion of control * List of object-oriented programming terms * Loose coupling * Make (software) * Static code analysis


References


Further reading

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{{DEFAULTSORT:Coupling (Computer Science) Programming principles Object-oriented programming Software architecture Software metrics