耦合_wiki

https://en.wikipedia.org/wiki/Coupling_(computer_programming)

In software engineering, coupling is the degree of interdependence between software modules; a measure of how closely connected two routines or modules are;the strength of the relationships between modules.

注：在软件工程中，耦合指模块间的依赖程度，用于度量两个例程(routines)或模块间紧密程度。

Coupling is usually contrasted with cohesion. Low coupling often correlates with high cohesion, and vice versa. Low coupling is often a sign of a well-structured computer system and a good design, and when combined with high cohesion, supports the general goals of high readability and maintainability.

注：耦合和内聚相对，低耦合往往意味着高内聚，反之亦然。低耦合是良好的计算系统或设计的特征，同时，低耦合和高内聚可实现高可靠性和高可维护性的目标。

The software quality metrics of coupling and cohesion were invented by Larry Constantine in the late 1960s as part of Structured Design, 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.

注：用耦合和内聚来度量软件质量，最早在1960s有larry constantine在《结构化设计》中提及。

Coupling can be "low" (also "loose" 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.

1、Content coupling (high): Content coupling (also known as Pathological coupling) occurs when one module modifies or relies on the internal workings of another module (e.g., accessing local data of another module). In this situation, a change in the way the second module produces data (location, type, timing) might also require a change in the dependent module.; 依赖(操作&访问）其他模块的内部处理（含内部数据）
2、Common coupling: Common coupling (also known as Global coupling) occurs when two modules share the same global data (e.g., a global variable). Changing the shared resource might imply changing all the modules using it.; 共享全局数据（该数据不属于这两个模块）
3、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.; 共享外部数据格式/通信协议/设备接口。
4、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).; 控制另一个模块的处理流程。
5、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 only needs 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.; 共享数据结构但可能使用不同成员。
6、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).; 数据传递
7、Message coupling (low): This is the loosest type of coupling. It can be achieved by state decentralization (as in objects) and component communication is done via parameters or message passing.; 通过参数和消息来耦合。
8、No coupling: Modules do not communicate at all with one another.

以上分类不是很合理，个人理解的分类方式：

1、内容耦合：一个模块访问另一个模块的内部数据

2、公共耦合：两个模块共享全局数据

3、控制耦合：两个模块通过参数或消息交互，传递的数据可看到内部处理。控制耦合也意味着控制模块必须知道所控制模块内部的一些逻辑关系。

4、数据耦合：两个模块通过参数或消息交互，传递的数据经过抽象，和内部处理无关。

1和2一般程序可以避免，3和4可用于区分模块间耦合是否合理（3合理，4不合理），因此模块功能和模块间的接口设计成为关键。

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: 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.^[3]

Disadvantages

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 of changes in other modules.（一个模块的修改会产生涟漪效应，其他模块也需随之修改）
Assembly of modules might require more effort and/or time due to the increased inter-module dependency.（由于模块之间的相互依赖，模块的组合会需要更多的精力和时间）
A particular module might be harder to reuse and/or test because dependent modules must be included.（由于一个模块有许多依赖的模块，导致模块的可复用性低）

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 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 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, 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).

注：重点是模块功能和接口设计，《functional design》词条并没有提供更多信息。

Systems such as CORBA 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:

d_i: number of input data parameters
c_i: number of input control parameters
d_o: number of output data parameters
c_o: number of output control parameters

For global coupling:

g_d: number of global variables used as data
g_c: 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)

$mathrm{Coupling}(C) = 1 - frac{1}{d_{i} + 2 imes c_{i} + d_{o} + 2 imes c_{o} + g_{d} + 2 imes g_{c} + w + r}$

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

$C = 1 - frac{1}{1+0+1+0+0+0+1+0} = 1 - frac{1}{3} = 0.67$

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,

$C = 1 - frac{1}{5 + 2 imes 5 + 5 + 2 imes 5 + 10 + 0 + 3 + 4} = 0.98$