What is high cohesion and how to use it / make it?

Question

I\'m learning computer programming and at several places I\'ve stumbled upon the concept of cohesion and I understand that it is desirable for a software to have \"high cohe

Answer 1

The term cohesion was originally used to describe modules of source code as a qualitative measure of how well the source code of the module was related to each other. The idea of cohesion is used in a variety of fields. For instance a group of people such as a military unit may be cohesive, meaning the people in the unit work together towards a common goal.

The essence of source code cohesion is that the source code in a module work together towards a common, well defined goal. The minimum amount of source code needed to create the module outputs is in the module and no more. The interface is well defined and the inputs flow in over through the interface and the outputs flow back out through the interface. There are no side effects and the emphasis is on minimalism.

A benefit of functionally cohesive modules is that developing and automating unit tests is straightforward. In fact a good measure of the cohesion of a module is how easy it is to create a full set of exhaustive unit tests for the module.

A module may be a class in an object oriented language or a function in a functional language or non-object oriented language such as C. Much of the original work in this area of measuring cohesion mostly involved work with COBOL programs at IBM back in the 1970s so cohesion is definitely not just an object oriented concept.

The original intent of the research from which the concept of cohesion and the associated concept of coupling came from was research into what where the characteristics of programs that were easy to understand, maintain, and extend. The goal was to be able to learn best practices of programming, codify those best practices, and then teach the practices to other programmers.

The goal of good programmers is to write source code whose cohesion is as high as possible given the environment and the problem being solved. This implies that in a large application some parts of the source code body will vary from other parts as to the level of cohesion of the source code in that module or class. Some times about the best you can get is temporal or sequential cohesion due to the problem you are trying to solve.

The best level of cohesion is functional cohesion. A module with functional cohesion is similar to a mathematical function in that you provide a set of inputs and you get a specific output. A truly functional module will not have side effects in addition to the output nor will it maintain any kind of state. It will instead have a well defined interface which encapsulates the functionality of the module without exposing any of the internals of the module and the person using the module will provide a particular set of inputs and get a particular output in return. A truly functional module should be thread safe as well.

Many programming language libraries contain a number of examples of functional modules whether classes, templates, or functions. The most functional cohesive examples would be mathematical functions such as sin, cosine, square root, etc.

Other functions may have side effects or maintain state of some kind resulting in making the use of those functions more complicated.

For instance a function which throws an exception or sets a global error variable (errno in C) or must be used in a sequence (strtok() function is an example from the Standard C library as it maintains an internal state) or which provides a pointer which must then be managed or issues a log to some log utility are all examples of a function that is no longer functional cohesion.

I have read both Yourdon and Constantine's original book, Structured Programming, where I first came across the idea of cohesion in the 1980s and Meilir Page-Jones' book Practical Guide to Structured Systems Design, and Page-Jones did a much better job of describing both coupling and cohesion. The Yourdon and Constantine book seems a bit more academic. Steve McConnell's book Code Complete is quite good and practical and the revised edition has quite a bit to say about good programming practice.