l2 l3 l4 software process models
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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.
2Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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
3Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
Waterfall model
Requirements
definition
System andsoftware design
Implementationand unit testing
Integration andsystem testing
Operation and
maintenance
4Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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
5Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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
Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh 7
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.
8Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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
9Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
10Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
11Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
Evolutionary development Throw-away prototyping
Objective is to understand the system requirements.
Should start with poorly understood requirements to clarify what is really needed.
Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh 12
Evolutionary development
Concurrentactivities
ValidationFinal
version
DevelopmentIntermediate
versions
SpecificationInitial
version
Outlinedescription
13Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
14Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
15Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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
Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh 16
Benefits of CBSE Quick development Low cost
Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh 17
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
18Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
19Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
Incremental delivery
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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
21Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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
22Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
23Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
24Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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
25Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
26Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
27Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
Principles of agile methods
28Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
29Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
30Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
The XP release cycle
Break downstories to tasks
Select userstories for this
releasePlan release
Releasesoftware
Evaluatesystem
Develop/integrate/test software
31Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
32Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
33Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
34Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
35Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
36Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
37Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
38Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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
39Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
40Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
41Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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.
42Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
RAD environment tools Database programming language Interface generator Links to office applications Report generators
43Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
A RAD environment
DBprogramming
language
Interfacegenerator
Officesystems
Reportgenerator
Database management system
Rapid applicationdevelopment environment
44Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
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
45Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh
ReferenceReference Software Engineering by Ian Sommerville,
Chapter 4 and 17, 7th Edition.
Rushdi Shams, Lecturer, Dept of CSE, KUET, Bangladesh 46
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