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Chapter 3Prescriptive Process
ModelsSoftware Engineering: A Practitioner’s Approach, 6th editionby Roger S. Pressman
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Software process model Attempt to organize the software life cycle by
defining activities involved in software production order of activities and their relationships
Goals of a software process standardization, predictability, productivity, high
product quality, ability to plan time and budget requirements
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Code&FixThe earliest approach Write code Fix it to eliminate any errors that have been
detected, to enhance existing functionality, or to add new features
Source of difficulties and deficiencies impossible to predict impossible to manage
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Models are needed Symptoms of inadequacy: the software
crisis scheduled time and cost exceeded user expectations not met poor quality
The size and economic value of software applications required appropriate "process models"
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Process model goals (B. Boehm 1988)
"determine the order of stages involved in software development and evolution, and to establish the transition criteria for progressing from one stage to the next. These include completion criteria for the current stage plus choice criteria and entrance criteria for the next stage. Thus a process model addresses the following software project questions:
What shall we do next?How long shall we continue to do it?"
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Process as a "black box"
Product
Process
Informal Requirements
Quality?Uncertain /Incomplete requirementIn the beginning
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Problems The assumption is that requirements can be
fully understood prior to development Interaction with the customer occurs only at
the beginning (requirements) and end (after delivery)
Unfortunately the assumption almost never holds
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Process as a "white box"
Product
Process
Informal Requirements
feedback
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Advantages Reduce risks by improving visibility Allow project changes as the project
progresses based on feedback from the customer
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The main activities of software production They must be performed independently
of the model The model simply affects the flow
among activities
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Prescriptive Models
That leads to a few questions … If prescriptive process models strive for structure and order, are
they inappropriate for a software world that thrives on change? Yet, if we reject traditional process models (and the order they
imply) and replace them with something less structured, do we make it impossible to achieve coordination and coherence in software work?
Prescriptive process models advocate an Prescriptive process models advocate an orderly approach to software engineeringorderly approach to software engineering
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The Waterfall Model
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Waterfall Model Assumptions1. The requirements are knowable in advance of implementation.
2. The requirements have no unresolved, high-risk implications e.g., risks due to COTS choices, cost, schedule, performance,
safety, security, user interfaces, organizational impacts3. The nature of the requirements will not change very much
During development; during evolution4. The requirements are compatible with all the key system
stakeholders’ expectations e.g., users, customer, developers, maintainers, investors
5. The right architecture for implementing the requirements is well understood.
6. There is enough calendar time to proceed sequentially.
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Process for Offshore?
Deploy
Analysis
Design
Accept. test
Construct
System test
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The V ModelIf we rely on testing alone, defects created first are detected last
SystemRequirements
SoftwareRequirements
SoftwareDesign
SoftwareImplementation
UnitTesting
IntegrationTesting
SoftwareTesting
SystemTesting
system test plan
software test plan
integration plan
unit plan
ProductRelease
time
UserNeed
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Incremental Models: Incremental
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Incremental Models: RAD Model
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Evolutionary Models: Prototyping
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Risk Exposure
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Unified Process ModelA software process that is:A software process that is:
use-case drivenuse-case driven architecture-centricarchitecture-centric iterative and incrementaliterative and incremental
Closely aligned with theClosely aligned with theUnified Modeling Language Unified Modeling Language (UML)(UML)
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inception
The Unified Process (UP)
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UP Work Products
inception
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Lifecycle for Enterprise Unified Process
inception
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Synchronize-and Stabilize Model Microsoft’s life-cycle model Requirements analysis—interview potential
customers Draw up specifications Divide project into 3 or 4 builds Each build is carried out by small teams
working in parallel
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Synchronize-and Stabilize Model (contd) At the end of the day—synchronize (test and
debug) At the end of the build—stabilize (freeze build) Components always work together
Get early insights into operation of product
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Evolutionary Models: The Spiral
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Spiral Model Simplified form
Waterfall model plus risk analysis
Precede each phase by Alternatives Risk analysis
Follow each phase by Evaluation Planning of next
phase
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Simplified Spiral Model If risks
cannot be resolved, project is immediately terminated
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Full Spiral Model Radial dimension: cumulative cost to date Angular dimension: progress through the
spiral
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Full Spiral Model (contd)
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Analysis of Spiral Model Strengths
Easy to judge how much to test No distinction between development, maintenance
Weaknesses For large-scale software only For internal (in-house) software only
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Object-Oriented Life-Cycle Models Need for iteration within and between phases
Fountain model Recursive/parallel life cycle Round-trip gestalt Unified software development process
All incorporate some form of Iteration Parallelism Incremental development
Danger CABTAB
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Fountain Model Features Overlap
(parallelism) Arrows
(iteration) Smaller
maintenance circle
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Software Process Spectrum
lightweight heavyweightmiddleweight
XPSCRUM
DSDMFDD RUPdX
ICONIXCrystal Clear Crystal Violet
EUPOPEN
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Conclusions Different life-cycle models Each with own strengths Each with own weaknesses Criteria for deciding on a model include
The organization Its management Skills of the employees The nature of the product
Best suggestion “Mix-and-match” life-cycle model
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Model Driven Architecture
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What is MDA in essence? Automated approach to translate high level
design to low level implementation by means of separation of concerns
From high-level model to running application
Risk proportional to expectations!
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Finding the “right” language
Assembler
3GL
IDEs & 4GL
Model Driven Architecture
Computer Hardware
Developer Autom
ation
Abstraction
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“ You should use iterative development only on projects you want to
succeed”
Martin Fowler
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Model Driven Architecture Can you actually have incremental MDA?
Or is it automated waterfall?
Need rigorous models Need high quality requirements
Capture requirements Understand requirements
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MDA or Offshore? Automation versus reduce cost of labor
Objectives Reduce required intelligence Increase repetition
Goal Reduce costs of development