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 notEinstein 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 Softwarecomplex 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 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.

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.
 They are not mutually exclusive- used together, often

Waterfall model
Requir ements
definition
System and
software design
Implementa tion
and unit testing
Integration and
system testing
Oper ation and
maintenance

Waterfall model phases
Requirements analysis and definition
Consultation with system user
System and software design
System design partitions the requirements to hardware or
software systems
software design involves indentifying and distributing software
abstractions.
Implementation and unit testing
Unit testing involves verifying that each unit meets its
specification

Waterfall model phases
Integration and system testing
Testing as a complete system. Software is delivered to customer
Operation and maintenance
Longest 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. 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

Evolutionary development
 Based on the idea of
1. developing an initial implementation
2. Exposing this to the user for comments
3. Refining and retuning through many
versions until an adequate system is
developed.

 Exploratory development
Objective is to work with customers
to evolve a final system from an initial outline
specification.
Should start with well-understood requirements
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.

Concurr ent
activities
Valida tion
Final
version
Development
Intermedia te
versions
Specification
Initial
version
Outline
description

 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 system
Development 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
Requirements
specification
Component
analysis
Development
and integ ration
System design
with reuse
Requirements
modification
System
validation

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
Valida te
increment
Develop system
increment
Design system
architectur e
Integrate
increment
Validate
system
Define outline
requirements
Assign requirements
to increments
System incomplete
Final
system

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
Risk
anal ysis
Risk
anal ysis
Risk
anal ysis
Risk
anal ysis Proto-
type 1
Prototype 2
Prototype 3
Oper a-
tional
protoype
Concept of
Oper ation
Simulations , models , benchmar ks
S/W
requir ements
Requir ement
validation
Design
V&V
Product
design Detailed
design
Code
Unit test
Integ ration
test
Acceptance
testService Develop , verify
next-level pr oduct
Evalua te alterna tives,
identify, resolv e risks
Deter mine objecti ves,
alterna tives and
constr aints
Plan ne xt phase
Integ ration
and test plan
Development
plan
Requir ements plan
Life-cy cle 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 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.

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
Breakdown
stories to tasks
Select user
stories for this
release
Plan release
Release
software
Evaluate
system
Develop/integ rate/
test software

Extreme programming practices
1
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 ŌTasksÕ.
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
Pair Programming Developers work in pairs, checking each otherÕs work and
providing the support to always do a good job.
Collective Ownership The pairs of developers work on all areas of the system, so that
no islands of expertise develop and all the developers own all the
code. Anyone can change anything.
Continuous Integration As soon as work on a task is complete it is integrated into the
whole system. After any such integration, all the unit tests in the
system must pass.
Sustainable pace Large amounts of over-time are not considered acceptable as the
net effect is often to reduce code quality and medium term
productivity
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 an
extreme programming process, the customer is a member of the
development team and is responsible for bringing system
requirements 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 froma displayed list.You
then have to tell the systemhow you will pay for it - this can either
be through a subscription, through a company account or by credit
card.
After this, you get a copyright formfromthe systemto fill in and,
when you have submitted this, the article you want is downloaded
onto your computer.
You then choose a printer and a copy ofthe article is printed.You
tell the systemifprinting has been successful.
Ifthe article is a print-only article, you canÕt keep the PDF version
so it is automatically deleted fromyour 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 user
selects which way they wish to pay. Ifthe user
has a library subscription, then they can input the
subscriber key which should be checked by the
system.Alternatively, they can input an organisational
account number. Ifthis is valid, a debit ofthe cost
ofthe article is posted to this account. Finally, they
may input a 16 digit credit card number and expiry
date.This should be checked for validity and, if
valid a debit is posted to that credit card account.

Test case description
Test 4:Test credit card validity
Input:
Astring representing the credit card number and two integers representing
the month and year when the card expires
Tests:
Check that all bytes in the string are digits
Check that the month lies between 1 and 12 and the
year is greater than or equal to the current year.
Using the first 4 digits ofthe credit card number,
check that the card issuer is valid by looking up the
card issuer table. Check credit card validity by submitting the card
number and expiry date information to the card
issuer
Output:
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
DB
prog ramming
langua ge
Interface
gener ator
Office
systems
Repor t
gener ator
Database mana gement system
Rapid application
development environment

Visual programming with
reuse
File Edit Views Layout Options Help
General
Index
Menu component
Date component
Range checking
script
Tree display
component
Draw canvas
component
User prompt
component +
script
12th January 2 000
3.876

ReferenceReference
 Software Engineering by Ian Sommerville,
Chapter 4 and 17, 7th
Edition.

L2 l3 l4 software process models

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