Software Process Models

INTRODUCTION TO SOFTWARE ENGINEERING

ALNEELAIN UNIVERSITY SOFTWARE ENGINEERING

DEPT.

Prepared By: Ahmed Alageed

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2- SOFTWARE DEVELOPMENT PROCESS MODELS

2. SOFTWARE DEVELOPMENT PROCESS MODELS

Instructional Objectives Describe different process models

used for software development Teach to identify the most appropriate

software process model for a given problem

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2.1. THE GENERIC SOFTWARE LIFECYCLE [REF.1: PG. 30]

Generic activities in all software processes are: Specification - what the system should do

and its development constraints Development - production of the software

system Validation - checking that the software is

what the customer wants Evolution - changing the software in

response to changing demands

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2.2. WHAT IS A PROCESS MODEL? [REF.1: PG. 30-31, 87-88]

A structured set of activities required to develop a software system Specification; Design; Validation; Evolution.

A software process model is an abstract representation of a process. It presents a description of a process from some particular perspective.

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SOFTWARE SPECIFICATION

The process of establishing what services are required and the constraints on the system’s operation and development.

Requirements engineering process Feasibility study; Requirements elicitation and analysis; Requirements specification; Requirements validation.

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THE REQUIREMENTS ENGINEERING PROCESS

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Feasibilitystudy

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and systemrequirements

Requirementsdocument

SOFTWARE DESIGN AND IMPLEMENTATION The process of converting the system

specification into an executable system.

Software design Design a software structure that realises

the specification; Implementation

Translate this structure into an executable program;

The activities of design and implementation are closely related and may be inter-leaved. 7

DESIGN PROCESS ACTIVITIES

Architectural design Abstract specification Interface design Component design Data structure design Algorithm design

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THE SOFTWARE DESIGN PROCESS

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Architecturaldesign

Abstractspecification

Interfacedesign

Componentdesign

Datastructuredesign

Algorithmdesign

Systemarchitecture

Softwarespecification

Interfacespecification

Componentspecification

Datastructure

specification

Algorithmspecification

Requirementsspecification

Design activities

Design products

STRUCTURED METHODS Systematic approaches to developing a

software design. The design is usually documented as a

set of graphical models. Possible models

Object model; Sequence model; State transition model; Structural model; Data-flow model.

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PROGRAMMING AND DEBUGGING

Translating a design into a program and removing errors from that program.

Programming is a personal activity - there is no generic programming process.

Programmers carry out some program testing to discover faults in the program and remove these faults in the debugging process.

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THE DEBUGGING PROCESS

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Locateerror

Designerror repair

Repairerror

Re-testprogram

SOFTWARE VALIDATION Verification and validation (V & V) is

intended to show that a system conforms to its specification and meets the requirements of the system customer.

Involves checking and review processes and system testing.

System testing involves executing the system with test cases that are derived from the specification of the real data to be processed by the system.

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THE TESTING PROCESS

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Componenttesting

Systemtesting

Acceptancetesting

TESTING STAGES Component or unit testing

Individual components are tested independently;

Components may be functions or objects or coherent groupings of these entities.

System testing Testing of the system as a whole. Testing of

emergent properties is particularly important.

Acceptance testing (alpha testing) Testing with customer data to check that

the system meets the customer’s needs15

TESTING PHASES

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Requirementsspecification

Systemspecification

Systemdesign

Detaileddesign

Module andunit codeand test

Sub-systemintegrationtest plan

Systemintegrationtest plan

Acceptancetest plan

ServiceAcceptance

testSystem

integration testSub-system

integration test

SOFTWARE EVOLUTION Software is inherently flexible and can

change. As requirements change through

changing business circumstances, the software that supports the business must also evolve and change.

Although there has been a distinction between development and evolution (maintenance) this is increasingly irrelevant as fewer and fewer systems are completely new 17

SYSTEM EVOLUTION

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Assess existingsystems

Define systemrequirements

Propose systemchanges

Modifysystems

Newsystem

Existingsystems

GENERIC SOFTWARE PROCESS MODELS The waterfall model

Separate and distinct phases of specification and development.

Evolutionary development Specification, development and

validation are interleaved. Component-based software

engineering The system is assembled from

existing components.

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GENERIC SOFTWARE PROCESS MODELS

There are many variants of these models e.g. formal development where a waterfall-like process is used but the specification is a formal specification that is refined through several stages to an implementable design.

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2.3. THE WATERFALL MODEL [REF.1: PG. 88-90; REF. 2: PG. 79-80]

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Requirements

definition

System andsoftware design

Implementationand unit testing

Integration andsystem testing

Operation and

maintenance

WATERFALL MODEL (CLASSIC LIFECYCLE) Requirements analysis and definition System and software design Implementation and unit testing Integration and system testing Operation and maintenance The main drawback of the waterfall

model is the difficulty of accommodating change after the process is underway. One phase has to be complete before moving onto the next phase. 22

WATERFALL MODEL PROBLEMS

Inflexible partitioning of the project into distinct stages makes it difficult to respond to changing customer requirements.

Therefore, this model is only appropriate when the requirements are well-understood and changes will be fairly limited during the design process.

Few business systems have stable requirements.

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WATERFALL MODEL PROBLEMS

The waterfall model is mostly used for large systems engineering projects where a system is developed at several sites.

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2.4. PROTOTYPING MODEL [REF.1: PG. 90-91; REF.2: PG. 83-85]

Exploratory development Objective is to work with customers and to

evolve a final system from an initial outline specification. Should start with well-understood requirements and add new features as proposed by the customer.

Throw-away prototyping Objective is to understand the system

requirements. Should start with poorly understood requirements to clarify what is really needed.

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2.4. PROTOTYPING MODEL [REF.1: PG. 90-91; REF.2: PG. 83-85]

Concurrentactivities

ValidationFinal

version

DevelopmentIntermediate

versions

SpecificationInitial

version

Outlinedescription

2.4. PROTOTYPING MODEL

Problems Lack of process visibility; Systems are often poorly structured; Special skills (e.g. in languages for rapid

prototyping) may be required. Applicability

For small or medium-size interactive systems; For parts of large systems (e.g. the user

interface); For short-lifetime systems.

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2.5. COMPONENT-BASED SOFTWARE ENGINEERING (CBSE) [REF.1: PG. 91-93]

Based on systematic reuse where systems are integrated from existing components or COTS (Commercial-off-the-shelf) systems.

Process stages Component analysis; Requirements modification; System design with reuse; Development and integration.

This approach is becoming increasingly used as component standards have emerged 28

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Requirementsspecification

Componentanalysis

Developmentand integration

System designwith reuse

Requirementsmodification

Systemvalidation

2.5. COMPONENT-BASED SOFTWARE ENGINEERING (CBSE) [REF.1: PG. 91-93]

PROCESS ITERATION System requirements ALWAYS evolve in

the course of a project so process iteration where earlier stages are reworked is always part of the process for large systems.

Iteration can be applied to any of the generic process models.

Two (related) approaches Incremental delivery; Spiral development.

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2.6. INCREMENTAL DEVELOPMENT [REF.1: PG. 93-95; REF.2: PG. 80-81]

Rather than deliver the system as a single delivery, the development and delivery is broken down into increments with each increment delivering part of the required functionality.

User requirements are prioritised and the highest priority requirements are included in early increments.

Once the development of an increment is started, the requirements are frozen though requirements for later increments can continue to evolve

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Validateincrement

Develop systemincrement

Design systemarchitecture

Integrateincrement

Validatesystem

Define outline requirements

Assign requirements to increments

System incomplete

Finalsystem

2.6. INCREMENTAL DEVELOPMENT [REF.1: PG. 93-95; REF.2: PG. 80-81]

INCREMENTAL DEVELOPMENT ADVANTAGES

Customer value can be delivered with each increment so system functionality is available earlier.

Early increments act as a prototype to help elicit requirements for later increments.

Lower risk of overall project failure. The highest priority system services

tend to receive the most testing.

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SPIRAL DEVELOPMENT

Process is represented as a spiral rather than as a sequence of activities with backtracking.

Each loop in the spiral represents a phase in the process.

No fixed phases such as specification or design - loops in the spiral are chosen depending on what is required.

Risks are explicitly assessed and resolved throughout the process.

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SPIRAL MODEL SECTORS Objective setting

Specific objectives for the phase are identified.

Risk assessment and reduction Risks are assessed and activities put in

place to reduce the key risks. Development and validation

A development model for the system is chosen which can be any of the generic models.

Planning The project is reviewed and the next phase

of the spiral is planned. 35

SPIRAL MODEL OF THE SOFTWARE PROCESS

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RAPID SOFTWARE DEVELOPMENT

Because of rapidly changing business environments, businesses have to respond to new opportunities and competition.

Rapid software development and delivery is now often the most critical requirement for software systems.

Businesses may be willing to accept lower quality software if rapid delivery of essential functionality is possible.

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REQUIREMENTS Because of the changing environment,

it is often impossible to arrive at a stable, consistent set of system requirements.

Therefore a waterfall model of development is impractical and an approach to development based on iterative specification and delivery is the only way to deliver software quickly.

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CHARACTERISTICS OF RAPID SOFTWARE DEVELOPMENT PROCESS

The processes of specification, design and implementation are concurrent. There is no detailed specification, and design documentation is minimized.

The system is developed in a series of increments. End users evaluate each increment and make proposals for later increments.

System user interfaces are usually developed using an interactive development system.

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AN ITERATIVE DEVELOPMENT PROCESS

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Validateincrement

Build systemincrement

Specify systemincrement

Design systemarchitecture

Define systemdeliverables

Systemcomplete?

Integrateincrement

Validatesystem

Deliver finalsystem

YES

NO

ADVANTAGES OF INCREMENTAL DEVELOPMENT

Accelerated delivery of customer services. Each increment delivers the highest priority functionality to the customer.

User engagement with the system. Users have to be involved in the development which means the system is more likely to meet their requirements and the users are more committed to the system.

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PROBLEMS WITH INCREMENTAL DEVELOPMENT

Management problems Progress can be hard to judge and problems hard

to find because there is no documentation to demonstrate what has been done.

Contractual problems The normal contract may include a specification;

without a specification, different forms of contract have to be used.

Validation problems Without a specification, what is the system being

tested against? Maintenance problems

Continual change tends to corrupt software structure making it more expensive to change and evolve to meet new requirements. 42

PROTOTYPING For some large systems, incremental

iterative development and delivery may be impractical; this is especially true when multiple teams are working on different sites.

Prototyping, where an experimental system is developed as a basis for formulating the requirements may be used. This system is thrown away when the system specification has been agreed. 43

INCREMENTAL DEVELOPMENT AND PROTOTYPING

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Incrementaldevelopment

Throw-awayprototyping

Delivered system

Executable prototype +System specification

Outlinerequirements

CONFLICTING OBJECTIVES

The objective of incremental development is to deliver a working system to end-users. The development starts with those requirements which are best understood.

The objective of throw-away prototyping is to validate or derive the system requirements. The prototyping process starts with those requirements which are poorly understood. 45

2.9. AGILE METHODS [REF.1: PG. 418-420]

Dissatisfaction with the overheads involved in design methods led to the creation of agile methods. These methods: Focus on the code rather than the design; Are based on an iterative approach to software

development; Are intended to deliver working software quickly

and evolve this quickly to meet changing requirements.

Agile methods are probably best suited to small/medium-sized business systems or PC products.

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PRINCIPLES OF AGILE METHODS

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Principle Description

Customer involvement The customer should be closely involved throughout thedevelopment process. Their role is provide and prioritise newsystem requirements and to evaluate the iterations of the system.

Incremental delivery The software is developed in increments with the customerspecifying the requirements to be included in each increment.

People not process The skills of the development team should be recognised andexploited. The team should be left to develop their own ways ofworking without prescriptive processes.

Embrace change Expect the system requirements to change and design the systemso that it can accommodate these changes.

Maintain simplicity Focus on simplicity in both the software being developed and inthe development process used. Wherever possible, actively workto eliminate complexity from the system.

PROBLEMS WITH AGILE METHODS It can be difficult to keep the interest of

customers who are involved in the process. Team members may be unsuited to the

intense involvement that characterizes agile methods.

Prioritizing changes can be difficult where there are multiple stakeholders.

Maintaining simplicity requires extra work. Contracts may be a problem as with other

approaches to iterative development

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2.10. EXTREME PROGRAMMING [REF.1: PG. 420-427] Perhaps the best-known and most

widely used agile method. Extreme Programming (XP) takes an

‘extreme’ approach to iterative development. New versions may be built several times

per day; Increments are delivered to customers

every 2 weeks; All tests must be run for every build and

the build is only accepted if tests run successfully.

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THE XP RELEASE CYCLE

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Break downstories to tasks

Select userstories for this

releasePlan release

Releasesoftware

Evaluatesystem

Develop/integrate/test software

EXTREME PROGRAMMING PRACTICES 1

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Incremental planning Requirements are recorded on Story Cards and the Stories to be included in a release are determined by the time available and their relative priority. The developers break these Stories into development ‘Tasks’.

Small Releases The minimal useful set of functionality that provides business value is developed first. Releases of the system are frequent and incrementally add functionality to the first release.

Simple Design Enough design is carried out to meet the current requirements and no more.

Test first development An automated unit test framework is used to write tests for a new piece of functionality before that functionality itself is implemented.

Refactoring All developers are expected to refactor the code continuously as soon as possible code improvements are found. This keeps the code simple and maintainable.

EXTREME PROGRAMMING PRACTICES 2

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Pair Programming Developers work in pairs, checking each otherÕs work andproviding the support to always do a good job.

Collective Ownership The pairs of developers work on all areas of the system, so thatno islands of expertise develop and all the developers own all thecode. Anyone can change anything.

Continuous Integration As soon as work on a task is complete it is integrated into thewhole system. After any such integration, all the unit tests in thesystem must pass.

Sustainable pace Large amounts of over-time are not considered acceptable as thenet effect is often to reduce code quality and medium termproductivity

On-site Customer A representative of the end-user of the system (the Customer)should be available full time for the use of the XP team. In anextreme programming process, the customer is a member of thedevelopment team and is responsible for bringing systemrequirements to the team for implementation.

XP AND AGILE PRINCIPLES Incremental development is supported through

small, frequent system releases. Customer involvement means full-time

customer engagement with the team. People not process through pair programming,

collective ownership and a process that avoids long working hours.

Change supported through regular system releases.

Maintaining simplicity through constant refactoring of code.

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REQUIREMENTS SCENARIOS

In XP, user requirements are expressed as scenarios or user stories.

These are written on cards and the development team break them down into implementation tasks. These tasks are the basis of schedule and cost estimates.

The customer chooses the stories for inclusion in the next release based on their priorities and the schedule estimates. 54

STORY CARD FOR DOCUMENT DOWNLOADING

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Downloading and printing an article

First, you select the article that you want from a displayed list. Youthen have to tell the system how you will pay for it - this can eitherbe through a subscription, through a company account or by creditcard.

After this, you get a copyright form from the system to fill in and,when you have submitted this, the article you want is downloadedonto your computer.

You then choose a printer and a copy of the article is printed. Youtell the system if printing has been successful.

If the article is a print-only article, you canÕ t keep the PDF versionso it is automatically deleted from your computer .

XP AND CHANGE Conventional wisdom in software

engineering is to design for change. It is worth spending time and effort anticipating changes as this reduces costs later in the life cycle.

XP, however, maintains that this is not worthwhile as changes cannot be reliably anticipated.

Rather, it proposes constant code improvement (refactoring) to make changes easier when they have to be implemented. 56

TESTING IN XP Test-first development. Incremental test development from

scenarios. User involvement in test development

and validation. Automated test harnesses are used to

run all component tests each time that a new release is built.

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TASK CARDS FOR DOCUMENT DOWNLOADING

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Task 1: Implement principal workflow

Task 2: Implement article catalog and selection

Task 3: Implement payment collection

Payment may be made in 3 dif ferent ways. The userselects which way they wish to pay. If the userhas a library subscription, then they can input thesubscriber key which should be checked by thesystem. Alternatively , they can input an or ganisationalaccount number. If this is valid, a debit of the costof the article is posted to this account. Finally , theymay input a 16 digit credit card number and expirydate. This should be checked for validity and, ifvalid a debit is posted to that credit card account.

TEST CASE DESCRIPTION

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Test 4: Test credit card validity

Input:A string representing the credit card number and two integers representingthe month and year when the card expiresTests:Check that all bytes in the string are digitsCheck that the month lies between 1 and 12 and theyear is greater than or equal to the current year .Using the first 4 digits of the credit card number ,check that the card issuer is valid by looking up thecard issuer table. Check credit card validity by submitting the cardnumber and expiry date information to the cardissuerOutput:OK or error message indicating that the card is invalid

TEST-FIRST DEVELOPMENT Writing tests before code clarifies the

requirements to be implemented. Tests are written as programs rather

than data so that they can be executed automatically. The test includes a check that it has executed correctly.

All previous and new tests are automatically run when new functionality is added. Thus checking that the new functionality has not introduced errors.

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PAIR PROGRAMMING In XP, programmers work in pairs, sitting

together to develop code. This helps develop common ownership of

code and spreads knowledge across the team. It serves as an informal review process as

each line of code is looked at by more than 1 person.

It encourages refactoring as the whole team can benefit from this.

Measurements suggest that development productivity with pair programming is similar to that of two people working independently

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2.11. RAPID APPLICATION DEVELOPMENT (RAD) [REF.1: PG. 427-431; REF.2: PG. 81-83]

Agile methods have received a lot of attention but other approaches to rapid application development have been used for many years.

These are designed to develop data-intensive business applications and rely on programming and presenting information from a database.

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RAD ENVIRONMENT TOOLS Database programming language Interface generator Links to office applications Report generators

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A RAD ENVIRONMENT

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DBprogramming

language

Interfacegenerator

Officesystems

Reportgenerator

Database management system

Rapid applicationdevelopment environment

INTERFACE GENERATION Many applications are based around

complex forms and developing these forms manually is a time-consuming activity.

RAD environments include support for screen generation including: Interactive form definition using drag and

drop techniques; Form linking where the sequence of forms

to be presented is specified; Form verification where allowed ranges in

form fields is defined.65

VISUAL PROGRAMMING Scripting languages such as Visual

Basic support visual programming where the prototype is developed by creating a user interface from standard items and associating components with these items

A large library of components exists to support this type of development

These may be tailored to suit the specific application requirements

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VISUAL PROGRAMMING WITH REUSE

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File Edit Views Layout Options Help

GeneralIndex

Menu componentDate component

Range checkingscript

Tree displaycomponent

Draw canvascomponent

User promptcomponent +

script

12th January 2000

3.876

PROBLEMS WITH VISUAL DEVELOPMENT Difficult to coordinate team-based

development. No explicit system architecture. Complex dependencies between parts

of the program can cause maintainability problems.

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COTS REUSE

An effective approach to rapid development is to configure and link existing off the shelf systems.

For example, a requirements management system could be built by using: A database to store requirements; A word processor to capture requirements

and format reports; A spreadsheet for traceability

management;69

COMPOUND DOCUMENTS For some applications, a prototype can

be created by developing a compound document.

This is a document with active elements (such as a spread sheet) that allow user computations.

Each active element has an associated application which is invoked when that element is selected.

The document itself is the integrator for the different applications.

APPLICATION LINKING

Word processor Spreadsheet Audio player

Text 1 Text 2 Text 3

Text 5

Table 1 Sound 1

Text 4Table 2 Sound 2

Compound document

SOFTWARE PROTOTYPING A prototype is an initial version of a

system used to demonstrate concepts and try out design options.

A prototype can be used in: The requirements engineering process to help

with requirements elicitation and validation; In design processes to explore options and

develop a UI design; In the testing process to run back-to-back

tests.

BENEFITS OF PROTOTYPING

Improved system usability. A closer match to users’ real needs. Improved design quality. Improved maintainability. Reduced development effort.

BACK TO BACK TESTING

Test data

Resultscomparator

Systemprototype

Applicationsystem

Differencereport

THE PROTOTYPING PROCESS

Establishprototypeobjectives

Defineprototype

functionality

Developprototype

Evaluateprototype

Prototypingplan

Outlinedefinition

Executableprototype

Evaluationreport

THROW-AWAY PROTOTYPES Prototypes should be discarded after

development as they are not a good basis for a production system: It may be impossible to tune the system to

meet non-functional requirements; Prototypes are normally undocumented; The prototype structure is usually

degraded through rapid change; The prototype probably will not meet

normal organizational quality standards.

THE RATIONAL UNIFIED PROCESS

A modern process model derived from the work on the UML and associated process.

Normally described from 3 perspectives A dynamic perspective that shows phases

over time; A static perspective that shows process

activities; A practice perspective that suggests good

practice.

RUP PHASE MODEL

Phase iteration

Inception Elaboration Construction Transition

RUP PHASES

Inception Establish the business case for the system.

Elaboration Develop an understanding of the problem

domain and the system architecture. Construction

System design, programming and testing. Transition

Deploy the system in its operating environment.

RUP GOOD PRACTICE

Develop software iteratively Manage requirements Use component-based architectures Visually model software Verify software quality Control changes to software

STATIC WORKFLOWS

Workflow Description

Business modelling The business processes are modelled using business use cases.

Requirements Actors who interact with the system are identified and use cases aredeveloped to model the system requirements.

Analysis and design A design model is created and documented using architecturalmodels, component models, object models and sequence models.

Implementation The components in the system are implemented and structured intoimplementation sub-systems. Automatic code generation from designmodels helps accelerate this process.

Test Testing is an iterative process that is carried out in conjunction withimplementation. System testing follows the completion of theimplementation.

Deployment A product release is created, distributed to users and installed in theirworkplace.

Configuration andchange management

This supporting workflow managed changes to the system (seeChapter 29).

Project management This supporting workflow manages the system development (seeChapter 5).

Environment This workflow is concerned with making appropriate software toolsavailable to the software development team.

COMPUTER-AIDED SOFTWARE ENGINEERING

Computer-aided software engineering (CASE) is software to support software development and evolution processes.

Activity automation Graphical editors for system model

development; Data dictionary to manage design entities; Graphical UI builder for user interface

construction; Debuggers to support program fault

finding; Automated translators to generate new

versions of a program.82

CASE TECHNOLOGY

Case technology has led to significant improvements in the software process. However, these are not the order of magnitude improvements that were once predicted Software engineering requires creative

thought - this is not readily automated; Software engineering is a team activity

and, for large projects, much time is spent in team interactions. CASE technology does not support these much.

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CASE CLASSIFICATION Classification helps us understand the

different types of CASE tools and their support for process activities.

Functional perspective Tools are classified according to their

specific function. Process perspective

Tools are classified according to process activities that are supported.

Integration perspective Tools are classified according to their

organisation into integrated units. 84

FUNCTIONAL TOOL CLASSIFICATION

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Tool type Examples

Planning tools PERT tools, estimation tools, spreadsheets

Editing tools Text editors, diagram editors, word processors

Change management tools Requirements traceability tools, change control systems

Configuration management tools Version management systems, system building tools

Prototyping tools Very high-level languages, user interface generators

Method-support tools Design editors, data dictionaries, code generators

Language-processing tools Compilers, interpreters

Program analysis tools Cross reference generators, static analysers, dynamic analysers

Testing tools Test data generators, file comparators

Debugging tools Interactive debugging systems

Documentation tools Page layout programs, image editors

Re-engineering tools Cross-reference systems, program re-structuring systems

ACTIVITY-BASED TOOL CLASSIFICATION

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Specification Design Implementation Verificationand

Validation

Re-engineering tools

Testing tools

Debugging tools

Program analysis tools

Language-processingtools

Method support tools

Prototyping tools

Configurationmanagement tools

Change management tools

Documentation tools

Editing tools

Planning tools

CASE INTEGRATION Tools

Support individual process tasks such as design consistency checking, text editing, etc.

Workbenches Support a process phase such as

specification or design, Normally include a number of integrated tools.

Environments Support all or a substantial part of an entire

software process. Normally include several integrated workbenches.

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TOOLS, WORKBENCHES, ENVIRONMENTS

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Single-methodworkbenches

General-purposeworkbenches

Multi-methodworkbenches

Language-specificworkbenches

Programming TestingAnalysis and

design

Integratedenvironments

Process-centredenvironments

Filecomparators

CompilersEditors

EnvironmentsWorkbenchesTools

CASEtechnology