Transcript On the Design and Development of Program Families David Parnas

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
Ouline
• Problem and Motivation
• Overview of Contribution
– Stepwise Refinement
– Module Specification
• Example Application
– Stepwise Refinement
– Module Specification
• Impact
• Open Questions
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
Overview of Contribution
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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
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
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
Example application
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Stepwise refinement
Dijkstra’s prime problem
Step 1:
bestyet := null
while 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(best yet):remove(best yet)
Example application
• Stepwise refinement
• Step 2:
• bestyet := 0
candidate := 0
while candidate != N do
begin
candidate := candidate+1
bestyet := bestof(bestyet,candidate)
end
if bestyet = 0 then erroraction
allocate(best yet)
remove(best yet)
Example application
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Module Specification
Dijkstra’s prime problem
Split into information-hiding modules
Modules:
– Free space list
– Allocation
– Selection criterium
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
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
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?