Tuesday, July 25, 2017

Component-Level Design Guidelines | Cohesion | Coupling

Component-Level Design Guidelines

  • Introduction : 
  • In addition to the principles , a set of pragmatic (Practical) design guidelines can be applied as component-level design proceeds. 
  • These guidelines apply to 
    • Components, 
    • Interfaces 
    • Dependencies and inheritance 
  • Components 
    • Naming conventions should be established for components that are specified as part of the architectural model and then refined and elaborated as part of the component-level model
  • Interfaces 
    • Interfaces provide important information about communication and collaboration.
  • Dependencies and Inheritance 
    • it is a good idea to model dependencies from left to right and inheritance from bottom (derived classes) to top (base classes).

Cohesion

  • Introduction : As per context of component-level design for 
  • Conventional view:
      • the “single-mindedness” of a module
    • OO view (Object Oriented View):
      • cohesion implies that a component or class encapsulates only attributes and operations that are closely related to one another and to the class or component itself
    • Levels of cohesion OR different types of cohesion
      • Functional
      • Layer
      • Communicational
    (1) Functional
    • Exhibited primarily by operations, this level of cohesion occurs when a component performs a targeted computation and then returns a result.
    (2) Layer
    • Exhibited by packages, components, and classes, this type of cohesion occurs when a higher layer accesses the services of a lower layer, but lower layers do not access higher layers.
    • For example, the SafeHome security function requirement to make an outgoing phone call if an alarm is sensed. It might be possible to define a set of layered packages as shown in Figure.
    Cohesion Example

    (3) Communicational
    • All operations that access the same data are defined within one class. In general, such classes focus only on the data for accessing and storing it.
    • Classes and components that exhibit functional, layer, and communicational cohesion are relatively easy to implement, test, and maintain.
    • It is important to note, however, that pragmatic (Practical) design and implementation issues sometimes force you to opt for lower levels of cohesion.

    Coupling

    • As per 
    • Conventional view coupling is : 
      • The degree to which a component is connected to other components and to the external world 
    • OO view coupling is :
      • A qualitative measure of the degree to which classes are connected to one another 
    • As classes (and components) become more interdependent, coupling increases. An important objective in component-level design is to keep coupling as low as is possible. 
    • Coupling categories  OR Level of coupling 
      • Content Coupling 
      • Common coupling 
      • Control coupling 
      • Stamp coupling 
      • Data coupling 
      • Routine call coupling 
      • Type use coupling 
      • Inclusion or import coupling 
      • External coupling
    • Content Coupling 
      • It occurs when one component “secretly modifies data that is internal to another component” 
      • This violates information hiding—a basic design concept.
    • Common coupling 
      • It Occurs when a number of components all make use of a global variable. 
      • Although this is sometimes necessary (e.g., for establishing default values that are applicable throughout an application)
    • Control coupling 
      • It Occurs when operation A() invokes operation B() and passes a control flag to B. The control flag then “directs” logical flow within B. 
    • Stamp coupling
      • It Occurs when ClassB is declared as a type for an argument of an operation of ClassA. Because ClassB is now a part of the definition of ClassA, modifying the system becomes more complex.
    • Data coupling
      • It Occurs when operations pass long strings of data arguments.
      • The “bandwidth” of communication between classes and components grows and the complexity of the interface increases. 
      • Testing and maintenance are more difficult. 
    • Routine call coupling 
      • It Occurs when one operation invokes another.
      • This level of coupling is common and is often quite necessary. 
      • However, it does increase the connectedness of a system.
    • Type use coupling 
      • It Occurs, when component A uses a data type defined in component B (e.g., this occurs whenever “a class declares an instance variable or a local variable as having another class for its type”.
      • If the type definition changes, every component that uses the definition must also change.
    • Inclusion or import coupling
      • It  Occurs when component A imports or includes a package or the content of component B. 
    • External coupling 
      • It Occurs when a component communicates or collaborates with infrastructure components (e.g., operating system functions, database capability, telecommunication functions).
      • Although this type of coupling is necessary, it should be limited to a small number of components or classes within a system.

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