Software Design

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Software Design

• Dr Z He
• University of Ulster

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Lecture 4 Software Design

• Objectives
• What is Software Design?
• Data design, architectural design, interface
  design and component level design
• Translating the analysis model into a software
  design
• Software Design Concepts
• abstraction, modularity, information hiding
• Software Architectural Design
• what is software architecture?
• Mapping requirements into software architecture
• transform flow, transaction flow,
  transform/transaction mapping
• transform mapping
• JSD
• User Interface Design
• what is it?,steps, golden rules
• Component-level Design
• what is it?, notations (flowchart, decision
  table, PDL))

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What is Software Design?

• Software design
• sits at the technical kernel of software
  engineering
• begins once software requirements have been
  analysed and specified
• is the first of the three technical activities
  design, coding, testing

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Software Design

• Data design
• transforms the information domain model into data
  structure required to implement the software
• also performed in architectural design and
  component level design
• Architectural design
• defines the relationship between major structural
  elements of the software
• defines the design patterns that can be used to
  achieve the requirements
• defines the constraints that affect the way in
  which architectural design patterns can be
  applied
• Interface design
• describes how the software communicates within
  itself, to systems that interoperate with it, and
  with humans who use it
• Component-level design
• transforms structural elements of the software
  architecture into a procedural description of
  software components

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Software Design
State transition diagram
Component- level Design
Process Specification
Data Flow Diagram
Interface Design
Entity-relationship Diagram
Architectural Design
Data Design
Data Dictionary
Transforming the analysis model into a software
design
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Software Design Concepts Abstraction

• Design Considerations
• 1. Abstraction
• Ignore the details that are not relevant at this
  stage
• 2 types
• procedural abstraction
• data abstraction
• Procedural abstraction
• decompose the problem into sub-problems,
  decompose these into further sub-problems etc.
• This top down approach leads to modules which
  are easy to program
• sub-problems are ordered logically

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Software Design Concepts Abstraction

• Data Abstraction
• consider the data only to the level required
• e.g. paragraph, sentence, word ... in text
  processing.
• One example of data abstraction is data
  encapsulation.
• e.g. job queue in a compiler. At the top level we
  know there is a queue and that it has the
  functions initialise queue, add to the queue
  and delete from the queue. At a lower level we
  might consider how the queue is implemented, as
  records or arrays.
• (see later under object oriented design)

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Software Design Concepts Modularity

• 2. Modularity
• What is module? - a grouping of sections of code.
• Pascal - procedure
• C - function
• Fortran - subroutine
• BASIC - subroutine
• Cobol - subprogram
• Ada - procedure or package
• Advantages
• Easier to design small modules
• Easier to find bugs
• Easier to maintain
• facilitates independent development
• allows for reuse

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Software Design Concepts Modularity

• How big should a module be?
• How complex should a module be?
• How can we minimise the interactions between
  modules?

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