Coupling: Two modules are considered independent if one can function completely without the presence of other. Obviously, if two modules are independent, they are solvable and modifiable separately. However, all the modules in a system cannot be independent of each other, as they must interact so that together they produce the desired external behavior of the system.
The more connections between modules, the more dependent they are in the sense that more knowledge about one module is required to understand or solve the other module. Hence, the fewer and simpler the connections between modules, the easier it is to understand one without understanding the other. Coupling between modules is the strength of interconnection between modules or a measure of independence among modules.
To solve and modify a module separately, we would like the module to be loosely coupled with other modules. The choice of modules decides the coupling between modules. Coupling is an abstract concept and is not easily quantifiable. So, no formulas can be given to determine the coupling between two modules. However, some major factors can be identified as influencing coupling between modules.
Among them the most important are the type of connection between modules, the complexity of the interface, and the type of information flow between modules. Coupling increase with the complexity and obscurity of the interface between modules. To keep coupling low we would like to minimize the number of interfaces per module and the complexity of each interface. An interface of a module is used to pass information to and from other modules. Complexity of the interface is another factor affecting coupling.
The more complex each interface is, higher will be the degree of coupling. The type of information flow along the interfaces is the third major factor-affecting coupling. There are two kinds of information that can flow along an interface: data or control, Passing or receiving control information means that the action of the module will depend on this control information, which makes it more difficult to understand the module and provide its abstraction. Transfer of data information means that a module passes as input some data to another module and gets in return some data as output.
Cohesion: Cohesion is the concept that tries to capture this intra-module. With cohesion we are interested in determining how closely the elements of a module are related to each other. Cohesion of a module represents how tightly bound the internal elements of the module are to one another. Cohesion of a module gives the designer an idea about whether the different elements of a module belong together in the same module. Cohesion and coupling are clearly related. Usually the greater the cohesion of each module in the system, the lower the coupling between modules is. There are several levels of Cohesion:
Coincidental is the lowest level, and functional is the highest. Coincidental Cohesion occurs when there is no meaningful relationship among the elements of a module. Coincidental Cohesion can occur if an existing program is modularized by chopping it into pieces and making different pieces modules.
A module has logical cohesion if there is some logical relationship between the elements of a module, and the elements perform functions that fill in the same logical class. A typical example of this kind of cohesion is a module that performs all the inputs or all the outputs. Temporal cohesion is the same as logical cohesion, except that the elements are also related in time and are executed together. Modules that perform activities like “initialization”, “clean-up” and “termination” are usually temporally bound.
A procedurally cohesive module contains elements that belong to a common procedural unit. For example, a loop or a sequence of decision statements in a module may be combined to form a separate module. A module with communicational cohesion has elements that are related by a reference to the same input or output data. That is, in a communicationally bound module, the elements are together because they operate on the same input or output data.
When the elements are together in a module because the output of one forms the input to another, we get sequential cohesion. Functional cohesion is the strongest cohesion. In a functionally bound module, all the elements of the module are related to performing a single function. By function, we do not mean simply mathematical functions; modules accomplishing a single goal are also included.