On the Design and Development of Program Families

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On the Design and Development of Program Families

• David Parnas
• Presented by Gregory Brown
• IEEE Transactions on Software Engineering, VOL
  SE-2, NO. 1, March 1976

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Ouline

• Problem and Motivation
• Overview of Contribution
• Stepwise Refinement
• Module Specification
• Example Application
• Stepwise Refinement
• Module Specification
• Impact
• Open Questions

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Problem and Motivation

• Problem
• Investigated production of program families
• Classical Method sequential completion
• Each member was a full working program
• Each one built from an ancestor
• Each inherits the design decisions of ancestors
• Motivation
• Make good, common design decisions up front
• Parallel development of family members

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Overview of Contribution

• Stepwise Refinement
• Each member is a functioning program
• Successive steps add funcitonality
• Each step represents design decision(s)
• No unnecessary functionality
• No performance degradation
• Revert back to previous step

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Overview of Contribution

• Module Specification
• Intermediate steps not programs
• Steps are specifications of collections of
  functionality
• Modules made to hide design decisions
• Early work designed to postpone decisions
• Make a wide program family possible

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Overview of Contribution

• Module method avoids disadvantages of stepwise
  refinement
• Stepwise refinement does make design decisions -
  rollback can lose this
• The two are complementary
• Stepwise refinement lower complexity
• Module method requires more work

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Example application

• Stepwise refinement
• Dijkstras prime problem
• Step 1
• bestyet nullwhile not all.spaces considered
  do begin find next item from list of free
  spaces bestyet bestof(bestyet,candidate) end
  if bestyet null then erroraction allocate(be
  st yet)remove(best yet)

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Example application

• Stepwise refinement
• Step 2
• bestyet 0candidate 0while candidate ! N
  do begin candidate candidate1 bestyet
  bestof(bestyet,candidate) endif bestyet 0
  then erroractionallocate(best yet)remove(best
  yet)

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Example application

• Module Specification
• Dijkstras prime problem
• Split into information-hiding modules
• Modules
• Free space list
• Allocation
• Selection criterium

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Impact

• Stepwise refinement is often used
• Add functionality to existing software
• Using modules and patterns
• General strategy to developing systems
• Revision control systems
• Module specification part of OO design
• Encapsulation to hide implementation
• Commonly taught and used practice
• Module specification allowed for development of
  patterns

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Impact

• Stepwise refinement and module specification
  complementary
• Can create software in stages, but using modules
• Complementary approaches
• Use strategy pattern among others to ease work

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Open Questions

• How could the two best be combined to enhance
  software development?
• When is it a good idea to not use stepwise
  refinement or module specification?
• Does the classic approach Parnas described not
  have a place at all?