Understanding Stages, Processes, and Criteria for Effective Software Design, Slides of Software Engineering

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Software Design

Deriving a solution which satisfies

software requirements

Stages of Design

 Problem understanding  Look at the problem from different angles to discover the design requirements.  Identify one or more solutions  Evaluate possible solutions and choose the most appropriate depending on the designer's experience and available resources.  Describe solution abstractions  Use graphical, formal or other descriptive notations to describe the components of the design.  Repeat process for each identified abstraction  until the design is expressed in primitive terms.

Phases in the Design Process

Architectu ral design Abstract specificatio n Interface design C ompon ent design Data stru ctu re design Alg orithm design Sy stem architectu re So ftware specificatio n Interface specificatio n C ompon ent specificatio n Data stru ctu re specificatio n Alg orithm specificatio n Requ irements specificatio n Desig n activities Desig n prod ucts

Design Phases

• Architectural design: Identify sub-systems.
• Abstract specification: Specify sub-systems.
• Interface design: Describe sub-system interfaces.
• Component design: Decompose sub-systems into components.
• Data structure design: Design data structures to hold problem data.
• Algorithm design: Design algorithms for problem functions.

Modular Programming

 In the early days, modular programming was taken to mean constructing programs out of small pieces: “subroutines”  But modularity cannot bring benefits unless the modules are  autonomous,  coherent and  robust

Procedural Abstraction

 The most obvious design methods involve functional decomposition.  This leads to programs in which procedures represent distinct logical functions in a program.  Examples of such functions:  “Display menu”  “Get user option”  This is called procedural abstraction

Object-Oriented Design

• The system is viewed as a collection of interacting objects.
• The system state is decentralized and each object manages its own state.
• Objects may be instances of an object class and communicate by exchanging messages.

Five Criteria for Design Methods

 We can identify five criteria to help evaluate modular design methods:  Modular decomposability;  Modular composability;  Modular understandability;  Modular continuity;  Modular protection.

Hierarchical Design Structure

System level Sub-system level

Top-Down Design

• In principle , top-down design involves starting at the uppermost components in the hierarchy and working down the hierarchy level by level.
• In practice , large systems design is never truly top-down. Some branches are designed before others. Designers reuse experience (and sometimes components) during the design process.

Exercise

Modular Composability

 Modular composability - a method satisfies this criterion if it leads to the production of modules that may be freely combined to produce new systems.  Composability is directly related to the issue of reusability  Note that composability is often at odds with decomposability; top-down design,  for example, it tends to produce modules that may not be composed in the way desired  This is because top-down design leads to modules which fulfil a specific function, rather than a general one

Modular Understandability

 Modular Understandability - a design method satisfies this criterion if it encourages the development of modules which are easily understandable.  COUNTER EXAMPLE 1. Take a thousand lines program, containing no procedures; it’s just a long list of sequential statements. Divide it into twenty blocks, each fifty statements long; make each block a method.  COUNTER EXAMPLE 2. “Go to” statements.

Modular Understandability

• Related to several component characteristics
  • Can the component be understood on its own?
  • Are meaningful names used?
  • Is the design well-documented?
  • Are complex algorithms used?
• Informally, high complexity means many relationships between different parts of the design.