Software Process Models

INTRODUCTION TO SOFTWARE ENGINEERING
2- SOFTWARE DEVELOPMENT PROCESS MODELS

Prepared By:
Ahmed Alageed

1

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

2

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

3

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.
4

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.

5

THE REQUIREMENTS ENGINEERING PROCESS

6

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

8

THE SOFTWARE DESIGN PROCESS

9

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.

10

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.

11

THE DEBUGGING PROCESS

12

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.

13

THE TESTING PROCESS

14

TESTING STAGES
 Component or unit testing
 Individualcomponents 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)
 Testingwith customer data to check that the
system meets the customer’s needs

15

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

18

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.

19

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.

20

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

21

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.

23

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

24

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
 Objectiveis to understand the system
requirements. Should start with poorly
understood requirements to clarify what is really
needed.
25

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

26

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.
27

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

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

29

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.

30

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
31

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

32

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.

33

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.
34

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

36

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.

37

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.

38

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.
39

AN ITERATIVE DEVELOPMENT PROCESS

40

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.

41

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
42
new requirements.

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

44

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.

46

PRINCIPLES OF AGILE METHODS
Principle Description
Customer involvement The customer should be closely involved throughout the
development process. Their role is provide and prioritise new
system requirements and to evaluate the iterations of the system.
Incremental delivery The software is developed in increments with the customer
specifying the requirements to be included in each increment.
People not process The skills of the development team should be recognised and
exploited. The team should be left to develop their own ways of
working without prescriptive processes.
Embrace change Expect the system requirements to change and design the system
so that it can accommodate these changes.
Maintain simplicity Focus on simplicity in both the software being developed and in
the development process used. Wherever possible, actively work
to eliminate complexity from the system.

47

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
48

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.

49

THE XP RELEASE CYCLE

50

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 ‘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.

51

EXTREME PROGRAMMING PRACTICES 2

Pair Programming Developers work in pairs, checking each otherÕ work and
s
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 qualit y 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.

52

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.

53

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

Downloading an d printing an article

First, you select the article that you want f
rom a displayed list.You
then have to tell the system how 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 f
orm from the system to 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 of the article is printed. You
tell the system if printing has been successful.
If the article is a print-only article, you canÕt keep the PDF version
so it is autom atically deleted from your com puter .

55

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.

57

TASK CARDS FOR DOCUMENT DOWNLOADING

Task 1: Imp lement p rincip al workflow

Task 2: Imp lement article catalog and selection

Task 3: Imp lement p ayment collection

Payment may be made in 3 different ways. The user
selects which way they wish to pay. If the user
has a library subscription, then they can input the
subscriber key which should be checked by the
system. Alternatively, they can input an or ganisational
account number. If this is valid, a debit of the cost
of the article is posted to this account. F inally 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.

58

TEST CASE DESCRIPTION

Test 4: Test credit card validity
Inpu t:
A string re e
prsenting thecred cardnumbera two intege r nting
it nd rs eprese
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 of the 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
Outpu t:
OK or error m essage indicating that the card is invalid

59

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.

60

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 61

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.

62

RAD ENVIRONMENT TOOLS
 Database programming language
 Interface generator

 Links to office applications

 Report generators

63

A RAD ENVIRONMENT

64

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

66

VISUAL PROGRAMMING WITH REUSE
Menu compon en t
Date co mpo nent

Fi l e Ed it Vi ews Layo ut Op ti on s Help

General
12th January 2 00 0 Ind ex
Rang e check in g
3.8 76
s crip t

Us er prompt
comp on en t +
Draw can vas s crip t
comp on en t

T di sp lay
ree
comp on en t

67

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.

68

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.

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.

THROW-AWAY PROTOTYPES
 Prototypes should be discarded after
development as they are not a good basis for
a production system:
 Itmay 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 it erat ion

Incept ion Elaborat ion Const ruct ion Transit ion

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
Work flow Description
Business modelli ng The business processes are modelled using business use cases.
Requirements Actors who interact with the system are identified and use cases are
developed to model the system requirements.
Analysis and design A design model is created and documented using architectural
models, component models, object models and sequence models.
Implementation The components in the system are implemented and structured into
implementation sub-systems. Automatic code generation from design
models helps accelerate this process.
Test Testing is an iterative process that is carried out in conjunction with
implementation. System testing follows the completion of the
implementation.
Deployment A product release is created, distributed to users and installed in their
workplace.
Configuration and This supporting workflow managed changes to the system (see
change management Chapter 29).
Project management This supporting workflow manages the system development (see
Chapter 5).
Environment This workflow is concerned with making appropriate software tools
available 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.
83

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
Tool type Examples
Planning tools PERT tools, estimation tools, spreadsheets
Editing tools T ext 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
T esting tools T est 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

85

ACTIVITY-BASED TOOL CLASSIFICATION
Re-en g i neeri ng t ool s

Test in g t oo ls

Debu ggi ng t oo ls

Prog ram analy si s t o ol s

Lang uage-p ro ces si ng
t oo ls

Meth od s up po r t t o ol s

Prot o ty pi ng t oo l s

Co nfi gurati on
management to ol s

Ch an ge man ag emen t t oo ls

Do cu men t at io n t oo ls

Ed it i ng t oo l s

Pl anni ng t o ol s

Sp eci f cat io n
i Desi gn Impl emen t at io n V ficat i on
eri
and
V dat io n
ali

86

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.

87

TOOLS, WORKBENCHES, ENVIRONMENTS
CASE
t echn olog y

T ls
oo Wor kb en ch es Envir ments
on

Fi l e Integ rat ed Process -cen tr ed
Ed it ors Co mpil ers
comp ar at ors en vir ments
on en vir ments
on

An alys i s an d
Pro gramming T in g
est
des ig n

Mu lt i-met ho d Si n gle-meth od General-pu rp os e Lang uage-sp ecifi c
workb en ch es workb en ch es workb en ch es workb en ch es

88

Software Process Models

More Related Content

What's hot (20)

Viewers also liked (20)

Similar to Software Process Models (20)

More from Ahmed Alageed (19)

Recently uploaded (20)

Software Process Models