1. Communication between modules, cohesion and coupling
Chapter 10
Communication between modules, cohesion and coupling

2. Objectives To introduce communication between modules
To develop solution algorithms that pass parameters between modules
To introduce cohesion as a measure of the internal strength of a module
To introduce coupling as a measure of the extent of information interchange between modules

3. Communication between modules
10.1
Communication between modules

4. Communication between modules
Necessary to consider flow of information between modules
This flow of information is called ‘intermodule communication’ and can be accomplished by the scope of the variable

5. Communication between modules
Scope of a variable
The portion of a program in which that variable has been defined and to which it can be referenced
Variables can be global where the scope of the variable is the whole program
Scope of the variable is simple the module which it is defined

6. Communication between modules
Global data
Date that can be used by all the modules in a program
Every module in the program can access and change data
Lifetime of a global variable spans the execution of the whole program

7. Communication between modules
Local data
Variable are defined within the submodule are called local variables
The scope of a local variable is simply the module in which it is defined
The lifetime of a local variable is limited to the execution of the single submodule in which it is defined

8. Communication between modules
Side effects
Side effect is a form of a cross-communication of a module with other parts of a program,
Occurs when a subordinate module alters the value of a global variable inside a module

9. Communication between modules
Passing parameters
Parameters are simply data items transferred from a calling module to its subordinate module at the time of calling
To pass parameters between modules, two things can happen:
The calling module must name the parameters that it wants to pass to the submodule
The submodule must be able to receive those parameters and return them to the calling module if required

10. Communication between modules
Formal and actual parameters
Parameters names that appear when a submodule is defined are known as formal parameters
Variables and expressions that are passed to a submodule in a particular call are called actual parameters

11. Communication between modules
Value and reference parameters
Parameters may have one of three function:
To pass information from a calling module to a subordinate module
To pass information from a subordinate module to its calling module
To fulfil a two-way communication role

12. Communication between modules
Value and reference parameters
Value parameters
Value parameters pass a copy of the value of a parameter from one module to another
Reference parameters
Reference parameter pass the memory address of a parameter from one module to another

13. Communication between modules
Hierarchy charts and parameters
Data parameters contain actual variables or data items that will be passed between modules
Status parameters act as a program flag and should contain just one of two values; true or false
Data parameters
Status parameters

14. 10.3
Module cohesion

15. Module cohesion
Cohesion is a measure of the internal strength of a module
It indicates how closely the elements or the statements of a module are associated with each other
The more closely the elements of a module are associated with each other, the higher the cohesion of the module

16. Module cohesion Coincidental cohesion
Occurs when elements are collected into a module simply because they happen to fall together
Occur as a result of one of the following conditions:
Existing program may have been arbitrarily segmented into small modules
Existing program may have been arbitrarily subdivided to conform to a badly considered programming standard
A number of existing modules have been combined into one module

17. Module cohesion Logical cohesion
Logical cohesion occurs when the element of a module are grouped together according to a certain class of activity
The element falls into some general category because they all do the same kind of thing

18. Module cohesion Temporal cohesion
Occurs when the elements of a module are grouped together because they are related by time
Typical examples are initialisation and finalisation modules in which elements are placed together because they perform certain housekeeping functions at the beginning or end of a program

19. Module cohesion Procedural cohesion
Occurs when the elements of a module are related because they operate according to a particular procedure
The elements are executed in a particular sequence so that the objectives of the program are achieved

20. Module cohesion Communicational cohesion
Occurs when the element of a module are grouped together because they all operate on the same (central) piece of data
Are commonly found in business application because of the close relationship of a business program to the data it is processing

21. Module cohesion Sequential cohesion
Occurs when a module contains elements that depend on the processing of previous elements
Contain elements in which the output data from one element serves as input data to the next

22. 10.4
Module coupling

23. Module coupling
Coupling is a measure of the extent of information interchange between modules
Tight coupling implies large dependence on the structure of one module by another
Loose coupling is the opposite of tight coupling. Modules with loose coupling are more independent and easier to maintain

24. Module coupling Common coupling
Global data structure
Module A
Module B
Common coupling
Occurs when modules reference the same global data structure
External coupling
Occurs when two or more modules access the same global data variable (similar to common coupling except that the global data is an elementary data item, rather than a data structure)
Global data variable
Module A
Module B

25. Module coupling Control coupling
Occurs when a module passes another module a control variable that is intended to control the other module’s logic
Stamp coupling
Occurs when one module passes a non-global data structure to another module in the form of a parameter
Module A
Module B
Module A
Data structure
Module B

26. Module coupling Data coupling
Occurs when a module passes a non-global data variable to another module (similar to stamp coupling except that the non-global data variable is an elementary data item, nota data structure)
Module A
Elementary data item
Module B

27. Module coupling Summary of coupling levels
If the programming language allows it, try to uncouple each module from its surroundings by
Passing data to a subordinate module in the form of parameters, rather than using global data
Writing each subordinate module as a self-contained unit

28. Summary Introduced communication between modules and parameters.
Intermodule communication is the flow of information or data between modules.
Passing of parameters was introduced as a form of intermodule communication.

29. Summary
The differences between formal and actual parameters and value and reference parameters was explained.
Module cohesion and module coupling must be considered when designing modular programs.
Cohesion is a measure of the internal strength of a module.
Seven levels of cohesion were discussed.

30. Summary
Coupling is a measure of the extent of information interchange between modules.
Five levels of coupling were discussed.