Software Engineering | Coupling and Cohesion

Cohesion:

Cohesion is the degree to which the elements inside a module belong together. It is a measure of the functional strength of a module. Cohesion is defined as the degree to which the elements of a particular module are functionally related. Cohesion of a module represents how tightly bound the internal elements of the module are to one another.

The measure of how strongly the elements are related functionally inside a module is called cohesion. It is a measure that defines the degree of intra-dependability within an element of a module. A good software design will have high cohesion. A module with high cohesion contains elements that are tightly related to each other and united in their purpose. A module is said to have low cohesion if it contains unrelated elements.

Types of Cohesion: 

Functional Cohesion: Functional Cohesion is said to exist if the different elements of a module, cooperate to achieve a single function. This is the best cohesion and the ideal one.  In functional cohesion, the elements of module are grouped together because they all contribute to a single function. This is the most desirable type of cohesion. It  is  considered  to  be  the  highest  degree  of  cohesion.

Sequential Cohesion: Sequential cohesion occurs when the output of one element is the input for the next element. In this cohesion technique, output from one part of the module is an input to some other element of the same module.

Communicational Cohesion: A module is said to have communicational cohesion, if all tasks of the module refer to or update the same data structure.

Procedural Cohesion: A module is said to be procedural cohesion if the set of purpose of the module are all parts of a procedure in which particular sequence of steps has to be carried out for achieving a goal.

 

Temporal Cohesion: The elements are related by their timing involved. A module connected with temporal cohesion all the tasks must be executed in the same time span. This cohesion contains the code for initializing all the parts of the system. Lots of different activities occur, all at unit time.

 

Logical Cohesion: A Module has Logical Cohesion only if there is some Logical relationship between the elements of the module. The elements are logically related and not functionally.

 

Coincidental Cohesion: The elements are not related (unrelated). The elements have no conceptual relationship other than location in source code. It is accidental and the worst form of cohesion.

 

Coupling:

Coupling is the degree of interdependence between software modules. Coupling between modules is the strength of interconnections between modules and a measure of interdependence among modules. Two modules with high coupling are strongly interconnected and thus dependent on each other. Modules should have low coupling. Two modules with low coupling are not dependent on one another.

Coupling is defined as the degree to which the two modules are dependent on each other. It measures the strength of relationships between modules. Coupling between any two modules is identified from the number and types of resources that the modules share with other modules. 

Highly coupled modules are joined by strong interconnections, while loosely coupled modules have weak interconnections. Independent modules have no interconnections. Coupling is measured by the number of relations between the modules. A good software design will have low coupling.

 

Types of Coupling: 

 

Data Coupling: If the dependency between the modules is based on the fact that they communicate by passing only data, then the modules are said to be data coupled. In data coupling, the components are independent of each other and communicate through data. This is the desirable type of coupling.

 

Stamp Coupling In stamp coupling, the complete data structure is passed from one module to another module.

 

Control Coupling: Control coupling is one module controlling the flow of another, by passing it information on what to do. If the modules communicate by passing control information, then they are said to be control coupled.

External Coupling: In external coupling, the modules depend on other modules, external to the software being developed or to a particular type of hardware. Ex- protocol, external file, device format, etc.

 

Common Coupling: Common coupling is said to occur when several modules have access to the same global data. The modules have shared data such as global data structures.

 

Content Coupling: Content coupling is said to occur when one module uses the data of other module. In a content coupling, one module can modify the data of another module, or control flow is passed from one module to the other module. This is the worst form of coupling and should be avoided.

Difference between Coupling and Cohesion:

Cohesion	Coupling
Cohesion is the degree to which the elements inside a module belong together	Coupling is the degree of interdependence between the modules.
A module with high cohesion contains elements that are tightly related to each other and united in their purpose.	Two modules with high coupling are strongly interconnected and thus dependent on each other.
A module is said to have low cohesion if it contains unrelated elements.	Two modules with low coupling are not dependent on one another.
Highly cohesive module reflects higher quality of software design.	Loose coupling reflects the higher quality of software design
Cohesion is Intra – Module Concept	Coupling is Inter -Module Concept