Chapter 10

1. 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. 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. Global data structure Module A Module B Module coupling • 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 A Module B 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 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.