Cohesion
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This is page is a about the computer concept "Cohesion", for the concept in chemistry see Cohesion (chemistry).
In computer programming, cohesion is the degree to which each part of a module is associated with each other part, in terms of functional relation. Parts of a module are functionally related if each part is essential to the functionality and the interface of a well-defined module (a well-defined module is one that has a single task, or models a single object). Cohesion can be considered "high" or "low". High cohesion means each part of a module is functionally related, and low cohesion means each part of a module is not. High cohesion of a module is considered better design in a computer system.
The types of cohesion, in order of lowest to highest, are as follows:
- Coincidental cohesion - Coincidental cohesion is when parts of a module are grouped arbitrarily; the parts have no significant relationship (e.g. a module of frequently used functions).
- Logical cohesion - Logical cohesion is when parts of a module are grouped because of a slight relation (e.g. using control coupling to decide which part of a module to use, such as how to operate on a bank account).
- Temporal cohesion - Temporal cohesion is when parts of a module are grouped by when they are processed - the parts are processed at a particular time in program execution (e.g. a function which is called after catching an exception which closes open files, creates an error log, and notifies the user).
- Procedural cohesion - Procedural cohesion is when parts of a module are grouped because they always follow a certain sequence of execution (e.g. a function which checks file permissions and then opens the file).
- Communicational cohesion - Communicational cohesion is when parts of a module are grouped because they operate on the same data (e.g. a method updateStudentRecord which operates on a student record, but the actions which method performs are not clear).
- Sequential cohesion - Sequential cohesion is when parts of a module are grouped because the output from one part is the input to another part (e.g. a function which reads data from a file and processes the data).
- Functional cohesion - Functional cohesion is when parts of a module are grouped because they all contribute to a single well-defined task of the module (a perfect module).
Cohesion is usually contrasted with coupling. High cohesion often correlates with low coupling, and vice versa. The software quality metrics of coupling and cohesion were invented by Larry Constantine, original developer of structured design.