L2 l3 l4 software process models

1Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh

““Einstein looked for a simplified definition of nature as he believed that God is not Einstein looked for a simplified definition of nature as he believed that God is not complex or arbitrary. No such belief will work for Software Engineers as Software complex or arbitrary. No such belief will work for Software Engineers as Software

Engineering is complex and arbitrary” – Fred BrooksEngineering is complex and arbitrary” – Fred Brooks

Lecture 2, 3 and 4Lecture 2, 3 and 4

Software ProcessSoftware Process

The software process A structured set of activities required to develop a

software systemSpecification;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.


Generic software process models The waterfall model

Separate and distinct phases of specification and development.

Evolutionary developmentSpecification, development and

validation are interleaved. Component-based software engineering

The system is assembled from existing components.

They are not mutually exclusive- used together, often


Waterfall model

Requirements

definition

System andsoftware design

Implementationand unit testing

Integration andsystem testing

Operation and

maintenance


Waterfall model phases

Requirements analysis and definitionConsultation with system user

System and software designSystem design partitions the requirements to hardware or software systemssoftware design involves indentifying and distributing software abstractions.

Implementation and unit testingUnit testing involves verifying that each unit meets its specification


Waterfall model phases

Integration and system testingTesting as a complete system. Software is delivered to customer

Operation and maintenanceLongest software life cycle phase. Involves correcting errors that were not discovered early, improvement of system units and evolution

Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh 6

Waterfall model benefits The result of each phase is one or more

approved documents Fits with other process models


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. The waterfall model is mostly used for large systems

engineering projects where a system is developed at several sites.

One phase has to be complete before moving onto the next phase- that is absent in reality where overlapping is common.


Waterfall model problems Prone to Software freeze, problems are left for later

resolution, ignored or programmed around. May result inefficient software and badly structured

design


Evolutionary development Based on the idea of 1. developing an initial implementation2. Exposing this to the user for comments3. Refining and retuning through many

versions until an adequate system is developed.


Evolutionary development Exploratory development

Objective is to work with customersto evolve a final system from an initial outline

specification. Should start with well-understood requirements add new features as proposed by the customer.


Evolutionary development Throw-away prototyping

Objective is to understand the system requirements.

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


Evolutionary development

Concurrentactivities

ValidationFinal

version

DevelopmentIntermediate

versions

SpecificationInitial

version

Outlinedescription


Evolutionary development 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.


Component-based software engineering

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

Happens when people working on project know of designs or code which is similar to that required

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


Component-based software engineering Process stages

Component analysis○ What components are required, what are available

Requirements modification○ Not exactly what you need is found, so would you

use that or modify your requirements?System design with reuse○ Design the system in a way so that others are able

to reuse your systemDevelopment and integration○ Combining all of the reusable components


Benefits of CBSE Quick development Low cost


Drawbacks of CBSE Mostly never meets requirements Needs expert knowledge on component

analysis, reusability and integration Less innovation More or less depends on the pros and cons

of the components.

Reuse-oriented development

Requirementsspecification

Componentanalysis

Developmentand integration

System designwith reuse

Requirementsmodification

Systemvalidation


Incremental delivery An in-between approach that combines waterfall and

evolutionary model 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.


Incremental delivery


Validateincrement

Develop systemincrement

Design systemarchitecture

Integrateincrement

Validatesystem

Define outline requirements

Assign requirements to increments

System incomplete

Finalsystem

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. User engagement with the system Accelerated delivery of customer services


Incremental development disadvantages

Increments should be relatively small (20,000 LoCs)

Each increment should provide system functionality

Difficult to map customer’s requirements onto increments of right size

Hard to identify common facilities that are needed for all increments


Incremental development disadvantages

Progress can be hard to judge and problems hard to find because there is no documentation to demonstrate what has been done.

The normal contract may include a specification; without a specification, different forms of contract have to be used.

Without a specification, what is the system being tested against?

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


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.


Spiral model of the software process

Riskanalysis

Riskanalysis

Riskanalysis

Riskanalysis Proto-

type 1

Prototype 2

Prototype 3Opera-tionalprotoype

Concept ofOperation

Simulations, models, benchmarks

S/Wrequirements

Requirementvalidation

DesignV&V

Productdesign Detailed

design

Code

Unit test

IntegrationtestAcceptance

testService Develop, verifynext-level product

Evaluate alternatives,identify, resolve risks

Determine objectives,alternatives and

constraints

Plan next phase

Integrationand test plan

Developmentplan

Requirements planLife-cycle plan

REVIEW


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 validationA 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.


Agile methods 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.


Principles of agile methods


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 characterises agile methods. Prioritising 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.


Extreme programming 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.


The XP release cycle

Break downstories to tasks

Select userstories for this

releasePlan release

Releasesoftware

Evaluatesystem

Develop/integrate/test software


Extreme programming practices 1

Incremental planning Requirements are recorded on Story Cards and the Stories to beincluded in a release are determined by the time available andtheir relative priority. The developers break these Stories intodevelopment ŌTasksÕ.

Small Releases The minimal useful set of functionality that provides businessvalue is developed first. Releases of the system are frequent andincrementally add functionality to the first release.

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

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

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


Extreme programming practices 2

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.


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.


Story card for document downloading

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 canÕt keep the PDF versionso it is automatically deleted from your computer.


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.


Task cards for document downloading

Task 1: Implement principal workflow

Task 2: Implement article catalog and selection

Task 3: Implement payment collection

Payment may be made in 3 different 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 organisationalaccount 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

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.


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.


Rapid application development 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.


RAD environment tools Database programming language Interface generator Links to office applications Report generators


A RAD environment

DBprogramming

language

Interfacegenerator

Officesystems

Reportgenerator

Database management system

Rapid applicationdevelopment environment


Visual programming with reuse

File Edit Views Layout Options Help

GeneralIndex

Menu componentDate component

Range checkingscript

Tree displaycomponent

Draw canvascomponent

User promptcomponent +

script

12th January 2000

3.876


ReferenceReference Software Engineering by Ian Sommerville,

Chapter 4 and 17, 7th Edition.


L2 l3 l4 software process models

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