SDE

Software Design Engineering

Coupling/Cohesion

 Software Design

• Software design is a creative process, just like designing anything else
  
• To see a wrong design, we can check with the requirements in the analysis model
  
• To see a bad design, we need to assess the design model and analyse the components, whether the performance can be improved by changing the modules or the interfaces

         In Analysing the software Design many factors are used, such as Coupling, Cohesion, Factoring, System Shape, etc.

 Coupling

• The degree of interdependence between two modules”
  
• We aim to minimise coupling - to make modules as independent as possible
  ​Low coupling can be achieve by:
• eliminating unnecessary relationships
• reducing the number of necessary relationships
• easeing the ‘tightness’ of necessary relationships

Types of Coupling

• Data coupling       (Most Required)   
                             
• Stamp coupling
   
• Control coupling
   
• Hybrid coupling
   
• Common coupling
   
• Content coupling  (Least Required)

Data Coupling 

• Modules communicate by parameters
   
• Each parameter is an elementary piece of data
   
• Each parameter is necessary to the communication
   
• Nothing extra is needed

Data coupling problems

• Too many parameters - makes the interface difficult to understand and possible error to occur
   
• Tramp data - data ‘traveling’ across modules before being used

Stamp coupling

• A composite data is passed between modules
   
• Internal structure contains data not used
   
• Bundling - grouping of unrelated data into an artificial structure

Control coupling

• A module controls the logic of another module through the parameter
   
• Controlling module needs to know how the other module works - not flexible!

Hybrid coupling

• A subset of data used as control
   
• Example: account numbers 00001 to 99999
  ​
• If 90000 - 90999, send mail to area code of last 3 digit (000 - 999)

Common coupling

• Use of global data as communication between modules
   
• Dangers of
• ripple effect
   
• inflexibility
   
• difficult to understand the use of data

Content coupling

• A module refers to the inside of another module
   
• Branch into another module
   
• Refers to data within another module
   
• Changes the internal workings of another module
   
• Mostly by low-level languages

 

Cohesion

• “The measure of the strength of functional relatedness of elements within a module”
   
• Elements: instructions, groups of instructions, data definition, call of another module
                                                                                                 ​
• We aim for strongly cohesive modules
  ​
• Everything in module should be related to one another - focus on the task
   
• Strong cohesion will reduce relations between modules - minimise coupling

      Types of Cohesion

• Functional cohesion (Most Required)
   
• Sequential cohesion
   
• Communicational cohesion
   
• Procedural cohesion
   
• Temporal cohesion
   
• Logical cohesion
   
• Coincidental cohesion (Least Required)

Functional cohesion

• All elements contribute to the execution of one and only one problem-related task
   
• Focussed - strong, single-minded purpose
   
• No elements doing unrelated activities
  ​
• Examples of functional cohesive modules:
• Compute cosine of angle
   
• Read transaction record
   
• Assign seat to airline passenger

Sequential cohesion

• Elements are involved in activities such that output data from one activity becomes input data to the next
   
• Usually has good coupling and is easily maintained
   
• Not so readily reusable because activities that will not in general be useful together
   
• Example of Sequential Cohesion
   
• module format and cross-validate record
   
•    use raw record
   
•     format raw record
   
•    cross-validate fields in raw record
   
•    return formatted cross-validated record

                 end module

Communicational Cohesion

• Elements contribute to activities that use the same input or output data
   
• Not flexible, for example, if we need to focus on some activities and not the others
   
• Possible links that cause activities to affect each other
   
• Better to split to functional cohesive ones
   
• Example of Communicational Cohesion
   
• module determine customer details
   
•    use customer account no
   
•    find customer name
   
•    find customer loan balance
   
•    return customer name, loan balance

end module

 

Procedural cohesion

• Elements are related only by sequence, otherwise the activities are unrelated
   
• Similar to sequential cohesion, except for the fact that elements are unrelated
   
• Commonly found at the top of hierarchy, such as the main program module
   
• Example of Procedural Cohesion
• module write read and edit something
•    use out record
   
•    write out record
   
•    read in record
   
•    pad numeric fields with zeros
   
•    return in record

         end module

Temporal cohesion

• Elements are involved in activities that are related in time
   
• Commonly found in initialisation and termination modules
   
• Elements are basically unrelated, so the module will be difficult to reuse
   
• Good practice is to initialise as late as possible and terminate as early as possible
   
• Example of Temporal Cohesion
• module initialise
•    set counter to 0
   
•    open student file 
   
•    clear error message variable
   
•    initialise array

       end module

 

Logical cohesion

• Elements contribute to activities of the same general category (type)
   
• For example, a report module, display module or I/O module
   
• Usually have control coupling, since one of the activities will be selected
   
• Example of Logical Cohesion
• module display record

      use record-type, record

      if record-type is student then

             display student record

      else if record-type is staff then

             display staff record

end module

 

Coincidental cohesion

• Elements contribute to activities with no meaningful relationship to one another
   
• Similar to logical cohesion, except the activities may not even be the same type
   
• Mixture of activities - like ‘rojak’!
   
• Difficult to understand and maintain, with strong possibilities of causing ‘side effects’ every time the module is modified
• Example of Coincidental Cohesion

module miscellaneous functions

   use customer record

   display customer record

   calculate total sales

   read transaction record

   return transaction record

end module

 

Determining Module Cohesion

 

 

Other Design Factors to Consider

• Factoring: reduce module size, clarifying system,minimise duplication of code, separating work from management, creating useful modules, simplifying
   
• System Shape (Structure)
   
• Redundancy
   
• Fan-in/Fan-out
   
• Restrictivity/Generality