Cohesion and Coupling

When the function of one module cooperates with another to perform a single objective then the module is said to be good cohesion. The primary characteristics of neat module decomposition are low coupling and high cohesion. Cohesion is a measure of the functional strength of a module. A module having high cohesion and low coupling is said to be functionally independent of other modules. By the term functional independence, we mean that a cohesive module performs a single task or function. A functionally independent module has minimal interaction with other modules. High cohesion has features of reliability, robustness, and reusability whereas low cohesion has difficulty to test, reuse even difficult to understand, and maintain.

Classification of cohesion

The different classes of cohesion that a module may possess are described as follows from lower cohesion to higher where coincidental cohesion worst type and functional is the best type.

Functional cohesion: Functional cohesion is said to exist if different elements/parts/functions of a module cooperate to achieve a single task. For example, a module containing all the functions required to manage employees’ pay-roll exhibits functional cohesion such as compute-overtime(), compute-work(), compute-deduction() etc . It is considered one of the most superior cohesion in the module but in certain cases not achievable and instead communicational cohesion overcome the solution.

Sequential cohesion: A module is said to possess sequential cohesion if the elements of a module form the parts of the sequence, where the output of one function is input to the next. For example, in a Transaction Processing System, the get-input (), validate-input (), sort-input () functions are grouped into one module.

Another example is an online store system when customers request some items then functions involved are following as the sequence of create-order(), check-avail(), and then order ().

Communicational cohesion: Communicational cohesion is also called informational cohesion. Parts/functions of the modules grouped together to operate on the same data. A module is said to have communicational cohesion, if all functions of the module refer to or update the same data structure, e.g. the set of functions defined on an array or a stack.

another example of informational cohesion is to let us consider a module name student which contains various functions like admit student, enter marks, print grade sheet etc, and all the data stored in an array name student record that is defined within a module.

Procedural cohesion: A module is said to possess procedural cohesion, if the set of functions of the module are all part of a procedure (algorithm)in which a certain sequence of steps have to be carried out for achieving an objective, e.g. the algorithm for decoding a message. As in the above, the parts/functions of the modules always follow a certain sequence of execution. Another example is function check file permission and then opened. Plus we can consider the activities included in the trading house can contain functions login (), place-order (), print-bill (),…log-out() etc which operates on different data but flow sequentially.

Temporal cohesion: When a module contains functions that are related to each other and all the functions must be executed in the same time span, then the module is said to exhibit temporal cohesion. For example when the computer booted several functions need to be performed at the same time one by one such as initialization of memory, devices, loading OS etc. and The set of functions in a single module responsible for initialization, start-up, the shutdown of some process, etc. exhibit temporal cohesion.

Another example we can consider as functions which called after catching an exception which closes opened files, give notification to the user, create error log etc.

Logical cohesion: A module is said to be logically cohesive, if all elements of the module perform similar operations, e.g. error handling, data input, data output, etc. An example of logical cohesion is the case where a set of print functions generating different output reports are arranged into a single module, such as modules contain a set of print functions to generate various types of output reports such as salary, grade sheet, manual report etc. Here parts of the modules are logically categorized to do the same thing even if they are different by nature (e.g grouping all mouse and keyboards input handling routines).

Coincidental cohesion: A module is said to have coincidental cohesion if it performs a set of tasks that relate to each other very loosely. In this case, the module contains a random collection of functions. It is likely that the functions have been put in the module out of pure coincidence without any thought or design or parts of the module grouped arbitrarily. Different functions in the module carry out different activities. For example, in a transaction processing system (TPS), the get-input, print-error, and summarize-members functions are grouped into one module. The grouping does not have any relevance to the structure of the problem.

Coupling

Coupling between two modules/classes/components is a measure of the degree of interdependence or interaction between the two modules or mutual interdependence between modules. A module having high cohesion and low coupling is said to be functionally independent of other modules.

Two modules are said to be highly coupled when function calls between two modules involve & shared a large chunk of data. -interaction occurs through shared data.

If two modules interchange large amounts of data, then they are highly interdependent. The degree of coupling between two modules depends on their interface complexity. The interface complexity is determined based on the parameter, no. of parameters etc. Module with low coupling is better.

Classification of Coupling

Classification of the different types of coupling will help to quantitatively estimate the degree of coupling between two modules. Five types of coupling can occur between any two modules. These are shown below according to the flow of lower coupling to higher, indicated by an arrow

Data                          Low --- (Best)

Stamp

Control

Common

Content              High --- ( Worst)

Data coupling: Two modules are data coupled if they communicate by using an elementary data item e.g. an integer, afloat, a character, etc such as passing any float value through a function that computes square root. This data item should be problem-related and not used for the control purpose.

Stamp coupling: Two modules are stamp coupled if they communicate using a composite data item such as a record in PASCAL or a structure in C and used part of these data.

Control coupling: Control coupling exists between two modules if data from one module is used in another module to direct the order of instructions execution. An example of control coupling is a flag set in one module and tested in another module.

Common coupling: Two modules are commonly coupled if they share data through some global data items. It is also known as global coupling. For example, sharing global variables. Changing global variable leads to changes in all modules.

Content coupling: Also known as pathological coupling. Content coupling exists between two modules, if they share code, e.g. a branch from one module into another module or if one module changes on the internal working of another module (e.g accessing local data of another module).

No coupling exists when modules do not communicate with each other. The above degree of coupling increases from the data coupling (that has less interdependency, coordination, and data flow) to content coupling(that has high interdependency, coordination, and information flow). Another coupling may be there but not given in the book.

High coupling among modules makes design solution difficult to understand and maintenance, increase development effort, harder to reuse and test particular module, assembling of the module may require more effort, change in one module usually leads a ripple effect and difficult to develop modules in an independent way, these are the disadvantages. High cohesion is contrasted with high coupling and somehow it correlates with loose coupling and vice versa.