Some Software Engineering Principles by D. L. Parnas

Presented by Team 7:

Amitkumar Dhameja

Cincy Francis

Rong Gu

CS575 - Software Design, Team 7

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

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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

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

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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CS575 - Software Design, Team 7

Flowchart Information Flow

Modularization Information Locality/Hiding

Decomposition (Continued)

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

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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

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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

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

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

Principles discussed in this paper:

Some Software Engineering Principles

Finished… At Last!!!

Any Questions???

CS575 - Software Design, Team 7