1
Chapter 2 SW Process Models
2
Objectives Understand various process models Understand the pros and cons of each model Evaluate the applicability of each model for a
certain type of software development
Overview Why SW process models? What are SW process models? The waterfall model Evolutionary models The Spiral CBSE Which is best?
3
4
Why SW process models? Some problems with ad-hoc development
Difficult to distinguish between tasks and therefore some important tasks may be ignored
Problem if multiple people are involved, as it is not easy to review their work.
If problems are found late, the more costly it is to fix.
Division of work Helps in managing the process
5
What is it? A SW process is a structured set of activities
required to develop a software system Specification; Design and Implementation; Validation; Evolution.
A software process model is an abstract representation of a process. It presents a description of a process from some particular perspective.
May take months or years to complete.
6
SW Process Models Waterfall model
Separate and distinct phases of specification and development. Evolutionary models
Specification, development and validation are interleaved. Exploratory development The incremental model The prototyping model The spiral model
Component-based development model The system is assembled from existing components
7
The Waterfall Model Oldest model Called “linear sequential model” Most widely used model for SW engineering Documentation is produced at each stage.
8
The Waterfall model phases Requirements analysis and definition System and software design Implementation and unit testing Integration and system testing Operation and maintenance
9
The Waterfall model
Requirements
definition
System andsoftware design
Implementationand unit testing
Integration andsystem testing
Operation and
maintenance
10
The Waterfall model Disadvantages Inflexible partitioning of the project into
distinct stages makes it difficult to respond to changing customer requirements.
Only appropriate when the requirements are well-understood and changes will be fairly limited during the design process.
The waterfall model is mostly used for large systems engineering projects.
11
Evolutionary Models The Exploratory Model
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.
The Incremental 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.
The Prototyping Model Objective is to understand the system requirements. Should start with
poorly understood requirements to clarify what is really needed. The Spiral Model
Process is represented as a spiral rather than as a sequence of activities with backtracking
12
The Exploratory ModelConcurrentactivities
ValidationFinal
version
DevelopmentIntermediate
versions
SpecificationInitial
version
Outlinedescription
13
The Exploratory Model Problems
Lack of process visibility; Systems are often poorly structured;
Applicability For small or medium-size interactive systems; For parts of large systems (e.g. the user
interface); For short-lifetime systems.
14
The Incremental Model 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.
15
The Incremental Model
Validateincrement
Develop systemincrement
Design systemarchitecture
Integrateincrement
Validatesystem
Define outline requirements
Assign requirements to increments
System incomplete
Finalsystem
16
The Incremental Model- 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.
17
The Incremental Model- Disadvantages Increments should be relatively small (20,000
lines of code) Can be difficult to map the customer’s
requirements onto increments of the right size Hard to identify common functions
18
The Prototyping Model When a customer defines a set of general
objectives for a software but does not identify detailed input, processing, or output requirement.
It consists of the iterating phases: Requirements gathering Design and build SW prototype Evaluate prototype with customer Refine requirements
19
The Prototyping Model
Build/ revise prototype
Customer test drives prototype
Listen to customer
20
The Prototyping Model Advantages
Users get a feel for the actual system Developers get to build something immediately Specifications can be developed incrementally
Disadvantages The developer may make implementation compromises in
order to get a prototype working quickly. The process in not visible (few documents that reflect
every version of the system) Systems poorly structured
21
The Spiral Model Defined by Barry Boehm in his 1988 article A Spiral
Model of Software Development and Enhancement. 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. Suitable for large, expensive and complicated
projects
22
The Spiral Model
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
23
The Spiral Model 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.
The Spiral Model Risk driven process model
Different risk patterns can lead to choosing different process models
What is a risk? Situations or possible events that may cause a project to fail to
meet its goal. Example risks:
Experienced staff leave the project Hardware which is essential for the system will not be delivered on
schedule
(more about risks in Chapter 3)24
The Spiral Model- Advantages Risks are explicitly assessed and resolved
throughout the process. Software engineers can start working on the
project earlier rather than wading through a lengthy early design process.
25
The Spiral Model- Disadvantages Requires highly skilled people in risk analysis
and planning Requires more time, and is more expensive Estimates of budget and time are harder to
judge at the beginning of the project since the requirements evolve through the process
26
27
Component Based Software Engineering (CBSE) 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
Component Based Software Engineering (CBSE)
Requirementsspecification
Componentanalysis
Developmentand integration
System designwith reuse
Requirementsmodification
Systemvalidation
29
Component Based Software Engineering (CBSE) Advantages:
Reduce amount of software to be developed Reduce costs and risks Faster delivery
Disadvantages: Requirements compromises, system does not
meet real needs of users Control over system evolution is lost
Which model is best? The choice of a model depends on the project
circumstances and requirements. A combination of models is used sometimes get the
benefits of more than one model. Criteria for evaluating models:
Risk management Quality / cost control Visibility of progress Early system functionality Customer involvement and feedback
30
31
Model/Criteria Matrix Rate each model 1-5 in each of the categories shown:
WaterfallIncrementalPrototypingSpiralCBSE
Risk Management
QualityCost Control
System Functionality
Visibility of Progress
Customer Involvement
32
Model/Criteria Matrix Rate each model 1-5 in each of the categories shown:
WaterfallIncrementalPrototypingSpiralCBSE
Risk Management22152QualityCost Control
System Functionality
Visibility of Progress
Customer Involvement
33
Model/Criteria Matrix Rate each model 1-5 in each of the categories shown:
WaterfallIncrementalPrototypingSpiralCBSE
Risk Management22152QualityCost Control43253System Functionality
Visibility of Progress
Customer Involvement
34
Model/Criteria Matrix Rate each model 1-5 in each of the categories shown:
WaterfallIncrementalPrototypingSpiralCBSE
Risk Management22152QualityCost Control43253System Functionality15443Visibility of Progress
Customer Involvement
35
Model/Criteria Matrix Rate each model 1-5 in each of the categories shown:
WaterfallIncrementalPrototypingSpiralCBSE
Risk Management22152QualityCost Control43253System Functionality15443Visibility of Progress54243Customer Involvement
36
Model/Criteria Matrix Rate each model 1-5 in each of the categories shown:
WaterfallIncrementalPrototypingSpiralCBSE
Risk Management22152QualityCost Control43253System Functionality15443Visibility of Progress54243Customer Involvement24532
Assignment 1 Please check the weblog and download
Assignment 1.
37
Top Related