Some Software Engineering Principles by D. L. Parnas

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Some Software Engineering Principles by D. L.
Parnas

• Presented by Team 7
• Amitkumar Dhameja
• Cincy Francis
• Rong Gu

CS575 - Software Design, Team 7
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Topics Covered

• Introduction to Software Engineering
• A Well-Structured Program?
• Modules Modularization
• Program Structure Control Techniques
• Error Handling
• Hierarchical Structure Subsetable Systems
• Designing Abstract Interfaces
• Summary

CS575 - Software Design, Team 7
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What is Software Engineering

• Multi-Person construction of Multi-Version
  programs
• Excludes solo programming
• Some Problems

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Challenges due to Multi-Person Programming

• How to divide the project into work assignments
  for programmers?
• How to specify the behavior of each component?
• How to communicate to all people about the run
  time errors?

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Challenges due to Multi-Version Programming

• How to write programs that are easy to modify?
• How to write program with useful subsets?
• How to write program that are easy to expand?

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Well-Structured Software

• Structure refers to the partial description of
  the system
• Describes a system as a set of parts and
  specifies some connections between the parts
• Systems in which connections between parts
  contain little information are termed as
  well-structured systems

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About Module

• Module has several definitions
• A job assignment
• A subroutine
• A memory load
• A functional component

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Decomposition

• Approaches
• Old - Divide project according to flow chart
• Correct - Design each module to hide or
  contain an assumption that is likely to change

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Decomposition (Continued)
Flowchart ? Information Flow Modularization ?
Information Locality/Hiding
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Specification

• Shortcomings of Common Approaches
• Approach 1 - reveals a rough description of the
  internal structure of the module
• Approach 2 - reveals a description of a
  hypothetic implementation of the module

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Specification (Continued)

• New approach proposed
• Precise specification of externally visible
  aspects without suggesting internal construction
• Relate externally visible value functions to
  each other rather than to a (real or imagined)
  lower level machine

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Error handling

• Treatment of run-time errors becomes difficult
  due to information hiding approach
• Error information likely to be in other modules
• Proper UE handling requires systematic approach
  in every part of software

CS575 - Software Design, Team 7
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Hierarchical Structure and Subsetable Systems

• It is easy to find useful subsets if the uses
  relation defines a hierarchy among programs
• Uses hierarchy should be designed before the
  coding begins
• Rule of thumb for designing uses hierarchy
  lower level programs vs. upper level programs

CS575 - Software Design, Team 7
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Designing Abstract Interface

• Modules have abstract interfaces that are
  relatively unlikely to change than the designing
  assumptions they hide
• The development of a list of assumptions
  unlikely to change during the product life cycle
• The specification of a set of interface
  functions around these assumptions

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Summary
Principles discussed in this paper

• Decomposing system
• Specification
• Error handling
• Use hierarchy structure
• Design abstract interface

These Principles are intended to be applied in
the early phases of software product design
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Some Software Engineering Principles

• Finished At Last!!!
• Any Questions???

CS575 - Software Design, Team 7