Software Engineering COMP 201

Software EngineeringCOMP 201Lecturer: Sebastian Coope

Ashton Building, Room G.18E-mail: [email protected]

COMP 201 web-page:http://www.csc.liv.ac.uk/~coopes/comp201

http://www.csc.liv.ac.uk/~pbell/comp201.html

Lecture 13 – Design Methodology

• 1• COMP201 - Software Engineering

http://www.csc.liv.ac.uk/~pbell/comp201.html

Software Design

Deriving a solution which satisfies software requirements


The Design ProblemYou have 4 developersYou have a 500 page requirements specificationHow to control break up the work?

• COMP201 - Software Engineering • 3

RequirementsClass1 Class2 Class3

Class4 Class5 Class6

Class7

Class11Class10 Class12

Class8 Class9Developer 1

Developer 2

Developer 3

Developer 4



Software DesignWhy

Software is produced by many peopleSoftware needs to be

Simple Understandable Flexible Portable Re-usable

HowComplex to simple Abstraction


Software Design in Reality

Design mixed with implementationDesign step 1

Implement and add more designDesign step 2

Implement and add more design

In effect much of software is designed while coded and the design document doesn’t reflect the final product


Stages of DesignProblem understanding

Look at the problem from different angles to discover the design requirements.

Identify one or more solutionsEvaluate possible solutions and choose the most appropriate

depending on the designer's experience and available resources.

Describe solution abstractionsUse graphical, formal or other descriptive notations to

describe the components of the design.Repeat process for each identified abstraction

until the design is expressed in primitive terms.• 6• COMP201 - Software Engineering

The Design Process

Any design may be modeled as a directed graph made up of entities with attributes which participate in relationships.

The system should be described at several different levels of abstraction.

Design takes place in overlapping stages. It is artificial to separate it into distinct phases but some separation is usually necessary.


Phases in the Design Process

Architecturaldesign

Abstractspecificatio

n

Interfacedesign

Componentdesign

Datastructuredesign

Algorithmdesign

Systemarchitecture

Softwarespecification

Interfacespecification

Componentspecification

Datastructure

specification

Algorithmspecification

Requirementsspecification

Design activities

Design products


Design Phases

• Architectural design: Identify sub-systems.• Abstract specification: Specify sub-systems.• Interface design: Describe sub-system interfaces.• Component design: Decompose sub-systems into

components.• Data structure design: Design data structures to hold

problem data.• Algorithm design: Design algorithms for problem functions.


DesignComputer systems are not monolithic: they are usually

composed of multiple, interacting modules.Modularity has long been seen as a key to cheap, high

quality software.The goal of system design is to decode:

What the modules are;What the modules should do;How the modules interact with one-another


Modular Programming

In the early days, modular programming was taken to mean constructing programs out of small pieces: “subroutines”

But modularity cannot bring benefits unless the modules are autonomous, coherent and robust


Procedural Abstraction

The most obvious design methods involve functional decomposition.

This leads to programs in which procedures represent distinct logical functions in a program.

Examples of such functions:“Display menu”“Get user option”

This is called procedural abstraction


Programs as FunctionsAnother view is programs as functions:

input output x f f (x)

the program is viewed as a function from a set I of legal inputs to a set O of outputs.

There are programming languages (ML, Miranda, LISP) that directly support this view of programming

Well-suited to certain application domains - e.g., compilers

Less well-suited to distributed, non-terminating systems - e.g., process control systems, operating systems like WinNT, ATM machines


Object-Oriented Design

• The system is viewed as a collection of interacting objects.

• The system state is decentralized and each object manages its own state.

• Note , use of internal state against functional programming

• Objects may be instances of an object class and communicate by exchanging messages.


Five Criteria for Design Methods

We can identify five criteria to help evaluate modular design methods:Modular decomposability;Modular composability;Modular understandability;Modular continuity;Modular protection.


Modular Decomposability

Modular decomposability - this criterion is met by a design method if the method supports the decomposition of a problem into smaller sub-problems, which can be solved independently.

In general, this method will be repetitive: sub-problems will be divided still further

Top-down design methods fulfil this criterion; stepwise refinement is an example of such a method


Hierarchical Design Structure

System level

Sub-systemlevel


Top-Down Design

• In principle, top-down design involves starting at the uppermost components in the hierarchy and working down the hierarchy level by level.

• In practice, large systems design is never truly top-down. Some branches are designed before others. Designers reuse experience (and sometimes components) during the design process.


An Example of Top-Down Design


• Imagine designing a word processor program from scratch. What subsystems could be initially found at the top level?

• File I/O, printing, graphical user interface, text processing etc.

• For each of these components we can then decompose further, i.e., File I/O comprises of saving documents and opening documents..

Modular ComposabilityModular composability - a method satisfies this criterion if

it leads to the production of modules that may be freely combined to produce new systems.

Composability is directly related to the issue of reusability Note that composability is often at odds with

decomposability; top-down design, for example, it tends to produce modules that may not

be composed in the way desiredThis is because top-down design leads to modules which

fulfil a specific function, rather than a general one


ExamplesThe Numerical Algorithm Group (NAG) libraries contain a

wide range of routines for solving problems in linear algebra, differential equations, etc.

The Unix shell provides a facility called a pipe, written “|”, whereby the standard output of one program may be redirected

to the standard input of another; this convention favours composability.


Modular Understandability Abstraction

int A=21int age_in_years = 21; print_out_document(Document d)

Modular Understandability - a design method satisfies this criterion if it encourages the development of modules which are easily understandable.

COUNTER EXAMPLE 1. Take a thousand lines program, containing no procedures; it’s just a long list of sequential statements. Divide it into twenty blocks, each fifty statements long; make each block a method.

COUNTER EXAMPLE 2. “Go to” statements.


Modular Understandability

• Related to several component characteristics– Can the component be understood on its own?– Are meaningful names used?– Is the design well-documented?– Are complex algorithms used?

• Informally, high complexity means many relationships between different parts of the design.


Modular Continuity

Modular continuity - a method satisfies this criterion if it leads to the production of software such that a small change in the problem specification leads to a change in just one (or a small number of ) modules.

EXAMPLE. Some projects enforce the rule that no numerical or textual literal should be used in programs: only symbolic constants should be used

COUNTER EXAMPLE. Static arrays (as opposed to dynamic arrays) make this criterion harder to satisfy.


Modular ProtectionModular Protection - a method satisfied this criterion if it

yields architectures in which the effect of an abnormal condition at run-time only effects one (or very few) modules

EXAMPLE. Validating input at source prevents errors from propagating throughout the program (design by contract)

COUNTER EXAMPLE. Using int types where subrange or short types are appropriate.


A Real Life Example – Ariane 5 Flight 501

The failure of an Ariane 5 space launcher is possibly the most expensive software bug in history at around $370 million.

Other bugs have been even worse by causing loss of life, for example Therac-25 radiation machines.


The Ariane 5 Space LauncherWhile developing the Ariane 5 space launcher, the designers

reused a component (the inertial reference software) which was successfully used in the Ariane 4 launcher

This component failed 37 seconds into the flight and the ground crew had to instruct the launcher to self-destruct.

The error was caused by an unhandled numerical conversion exception causing a numeric overflow.

Component reuse is usually a good thing but care must be taken that assumptions made when the component was developed are still valid!


Repository ModelsSub systems making up a system must exchange and share data

so they can work together effectively.There are two main approaches to this:

The repository model – All shared data is held in a central database which may be accessed by all sub-systems

Each sub-system or component maintains its own database. Data is then exchanged between sub-systems via message passing.

There are advantages and disadvantages to each approach as we shall now see.


Repository ModelThe advantages include:

Databases are an efficient way to share large amounts of data and data does not have to be transformed between different sub-systems (they agree on a single data representation).

Sub-systems producing data need not be concerned with how that data is used by other sub-systems.

Many standard operations such as backup, security, access control, recovery and data integrity are centralised and can be controlled by a single repository manager.

The data model is visible through the repository schema.


Repository ModelThe disadvantages include:

Sub-systems must agree on the data model which means compromises must be made, for example with performance.

Evolution may be difficult since a large amount of data is generated and translation may be difficult and expensive.

Different systems have different requirements for security, recovery and backup policies which may be difficult to enforce in a single database.

It may be difficult to distribute the repository over a number of different machines.


Lecture Key Points

We have studied five criteria to help evaluate modular design methods and their advantages:Modular decomposability; modular composability; modular

understandability; modular continuity; modular protection.We have seen the advantages and the disadvantages of

repository models


Documents

Software Engineering COMP 201