Transcript Overview of Software Engineering

Overview of Software
Engineering
CS 330
Spring 2007
Key Ingredients in successful
organizations
Process
People
Technology
A better view
Process and Technology supporting people
People
Processes
Technology
Pyramids are stable.
Wedges are not!
What is software?
 Computer programs and associated
documentation
 Software products may be developed for a
particular customer or may be developed for a
general market
 Software products may be
– Generic/COTS - developed to be sold to a range of
different customers
– Custom- developed for a customer according to their
specification
Engineering
 Engineering is …
– The application of scientific principles and methods to
the construction of useful structures & machines
 Examples
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Mechanical engineering
Computer engineering
Civil engineering
Chemical engineering
Electrical engineering
Nuclear engineering
Aeronautical engineering
Software Engineering
 The term is 35 years old: NATO Conferences
– Garmisch, Germany, October 7-11, 1968
– Rome, Italy, October 27-31, 1969
 The reality is it is finally beginning to arrive
– Computer science one the scientific basis
• Years of studies/experience/statistics provide basis too
– Many aspects have been made systematic
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Methods/methodologies/techniques
Languages
Tools
Processes
Why Engineer Software ?
 The problem is complexity
 Many sources, but size is a key:
– Mozilla contains 3 Million lines of code
– UNIX contains 4 million lines of code
– Windows 2000 contains 108 lines of code
 Second is role and combinatorics of “state”
 Third is uncertainty of “inputs” and their timing
 Fourth is the continuing changing “environment”
and demands.
Software engineering is about managing
all the sources of complexity to
produce effective software.
Software Engineering in a
Nutshell
 Development of software systems whose
size/complexity warrants team(s) of engineers
– multi-person construction of multi-version software
[Parnas 1987]
 Scope
– study of software process,
development/management principles, techniques,
tools and notations
 Goal
– production of quality software, delivered on time,
within budget, satisfying customers’ requirements
and users’ needs
What does a software
engineer do?
Software engineers should
– adopt a systematic and organised approach to all
aspects of software development.
– use appropriate tools and techniques depending on
• the problem to be solved,
• the development constraints and
• the resources available
– Understand and communicate processes for
improved software development within their
organization
– Be effective team members and/or leaders.
– Can be very technical or more managerial depending
on organizational need.
What is the difference between software
engineering and computer science?
Computer Science
Software Engineering
is concerned with
 theory
 fundamentals


the practicalities of developing
delivering useful software
Computer science theories are currently insufficient to
act as a complete underpinning for software
engineering, BUT it is a foundation for practical aspects
of software engineering
What is the difference between software
engineering and system engineering?
 Software engineering is part of System engineering
 System engineering is concerned with all aspects of
computer-based systems development including
– hardware,
– software and
– process engineering
 System engineers are involved in
system specification,
architectural design,
integration and deployment
Difficulties?
 SE is a unique brand of engineering
– Software is malleable
– Software construction is human-intensive
– Software is intangible and generally invisible
– Software problems are unprecedentedly complex
– Software directly depends upon the hardware
• It is at the top of the system engineering “food chain”
– Software solutions require unusual rigor
– Software “state” means behaviors can depend on history.
– Software has discontinuous operational nature
Software Engineering ≠ Software
Programming
 Software programming
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Single developer
“Toy” applications
Short lifespan
Single or few stakeholders
• Architect = Developer = Manager = Tester = Customer = User
– One-of-a-kind systems
– Built from scratch
– Minimal maintenance
Software Engineering ≠ Software
Programming
 Software engineering
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Teams of developers with multiple roles
Complex systems
Indefinite lifespan
Numerous stakeholders
• Architect ≠ Developer ≠ Manager ≠ Tester ≠ Customer ≠ User
– System families
– Reuse to amortize costs
– Maintenance accounts for 60%-80% of overall
development costs
Economic and Management
Aspects of SE
 Software Engineering is about improved ROI
(can be Capital and/or Social ROI)
 Software production =
development + maintenance
 Maintenance costs 60%-80% of all
(successful) development costs
– 20% corrective (12%-16% total costs)
– 30% adaptive (18%-24% total costs)
– 50% perfective (30-40% total costs)
 Quicker development is not always preferable
– higher up-front costs may defray downstream costs
– poorly designed/implemented software is a critical
cost factor in system cost and delays
Relative Costs of Fixing
Software Faults
200
30
10
1
Requirements
2
Specification
3
Planning
4
Design
Implementation
Integration
Maintenance
Mythical Man-Month
by Fred Brooks
 Published in 1975, republished in 1995
– Experience managing development of OS/360 in 1964-65
 Central argument
– Large projects suffer management problems different in kind than small
ones, due to division in labor
– Critical need is the preservation of the conceptual integrity of the
product itself
 Central conclusions
– Conceptual integrity achieved through chief architect
– Implementation achieved through well-managed effort
– “software developers” are not interchangeable work units.
 Brooks’ Law
– Adding personnel to a late project makes it later
Software Engineering:
From Principles to Tools
TOOLS
METHODOLOGIES
METHODS AND
TECHNIQUES
PRINCIPLES
Software Qualities
 Qualities are goals in the practice of
software engineering, and directly relate to
many of the guiding principles.
 External vs. Internal qualities
 Product vs. Process qualities
Software Qualities
 Critical Quality Attributes
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Correctness
Maintainability
Dependability
Usability
Reliability
 Other Attributes
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Completeness
Compatibility
Portability
Internationalization
Understandability
Scalability
Robustness
Testability
Reusability
Customizability
Efficiency
External vs. Internal Qualities
 External qualities are visible to the user
– reliability, usability, efficiency (maybe),
robustness, scalability
 Internal qualities are the concern of
developers
– they help developers achieve external qualities
– verifiability, maintainability, extensibility,
evolvability, adaptability, portability, testability,
reusability
Product vs. Process Qualities
 Product qualities concern the developed
artifacts
– maintainability, performance, understandability,
 Process qualities deal with the development
activity
– products are developed through process
– maintainability, productivity, predictability
Some Software Qualities
 Correctness
– ideal quality
– established w.r.t. the requirements/specification
– absolute
 Reliability
– statistical property
– probability that software will operate as expected
over a given period of time/inputs
– relative
Some Software Qualities (cont.)
 Robustness
– “reasonable” behavior in unforeseen
circumstances
– subjective
– a specified requirement is an issue of
correctness;
an unspecified requirement is an issue of
robustness
 Usability
– ability of end-users to easily use software
– extremely subjective
Some Software Qualities (cont.)
 Understandability
– ability of developers to easily understand
produced artifacts
– internal product quality
– subjective
 Verifiability
– ease of establishing desired properties
– performed by formal analysis or testing
– internal quality
Some Software Qualities (cont.)
 Performance
– equated with efficiency
– assessable by measurement, analysis, and
simulation
 Evolvability
– ability to add or modify functionality
– addresses adaptive and perfective maintenance
– problem: evolution of implementation is too easy
– evolution should start at requirements or design
Some Software Qualities (cont.)
 Reusability
– ability to construct new software from existing pieces
– must be planned for
– occurs at all levels: from people to process, from
requirements to code
 Interoperability
– ability of software (sub)systems to cooperate with
others
– easily integratable into larger systems
– common techniques include APIs, distributed
programming interfaces (CORBA, DCOM), plug-in
protocols, etc.
Some Software Qualities (cont.)
 Scalability
– ability of a software system to grow in size while
maintaining its properties and qualities
– assumes maintainability and evolvability
– goal of component-based development
Process Principles
 Prescribes all major activities
 Uses resources, within a set of constraints, to
produce intermediate and final products
 May be composed of sub-processes
 Each activity has entry and exit criteria
 Activities are organized in a sequence
 Has a set of guiding principles to explain goals
 Constraints may apply to activity, resource or
product
Software Development Stages
 Requirements Analysis & Specification
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Conceptual/System/Architectural Design
Detailed/Program Design
Implementation/Coding
Unit & Integration Testing
System Testing/Validation
System Delivery/Deployment
Maintenance
– Note there are many “variations” on the names. You are
responsible for the main categories above (an on the next
pages)..
Software Lifecycle Models
 Waterfall Model
 V Model
 Phased Development Model
– Incremental Model
 Prototyping Model
 Spiral Model
Software Development Lifecycle
Waterfall Model
Requirements
Plan/Schedule
Design
Replan/Reschedule
Implementation
Integration
Validation
Deployment
V Model
OPERATION
& MAINTENANCE
Validate requirements
REQUIREMENTS
ANALYSIS
ACCEPTANCE
TESTING
SYSTEM
DESIGN
Verify design
PROGRAM
DESIGN
[Pfleeger 98]
SYSTEM
TESTING
UNIT & INTEGRATION TESTING
CODING
DEVELOPERS
Phased Development Model
Development systems
Build Release 1
Build Release 2
Build Release 3
USERS
Time
Use Release 1
Use Release 2
Production systems
Use Release 3
[Pfleeger 98]
Software Development Lifecycle
Incremental Model
Requirements
Version 1:
Design
Complete General Design
Implementation
Integration
Validation
Deployment
Requirements
Design
Implementation
Version 2:
Integration
Design/Implement first set
Validation
of planned “new” features.
Deployment
Note overlap with V1 schedule
Version 3:
Design/Implement second set
of planned “new” features
Requirements
Design
Implementation
Integration
Validation
Deployment
Prototyping Model
Listen to
Customer
Build/Revise
Mock-Up
Customer
Test-drives
Mock-up
[Pressman 97]
Prototyping Model
LIST OF
REVISIONS
revise
prototype
LIST OF
REVISIONS
user/
customer
review
PROTOTYPE
REQUIREMENTS
SYSTEM
REQUIREMENTS
(sometimes informal
or incomplete)
[Pfleeger 98]
LIST OF
REVISIONS
PROTOTYPE
DESIGN
PROTOTYPE
SYSTEM
TEST
DELIVERED
SYSTEM
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
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.
Evolutionary development
 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
– Objective is to understand the system requirements.
Should start with poorly understood requirements to
clarify what is really needed.
Evolutionary development
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.
Component-based software
engineering
 Based on systematic reuse where systems are
integrated from existing components or COTS
(Commercial-off-the-shelf) systems.
 Process stages
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Component analysis;
Requirements modification;
System design with reuse;
Development and integration.
 This approach is becoming increasingly used as
component standards have emerged.
Reuse-oriented development
Component-Based Development
 Develop generally applicable components of a
reasonable size and reuse them across systems
 Make sure they are adaptable to varying contexts
 Extend the idea beyond code to other
development artifacts
 Question: what comes first?
– Integration, then deployment
– Deployment, then integration
Different Flavors of Components
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Third-party software “pieces”
Plug-ins / add-ins
Applets
Frameworks
Open Systems
Distributed object infrastructures
Compound documents
Legacy systems
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.
Incremental delivery
 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 development
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.
Extreme programming
 An approach to development based on the
development and delivery of very small
increments of functionality.
 Relies on constant code improvement, user
involvement in the development team and
pairwise programming.
 Covered in Chapter 17
Software Development Lifecycle
Waterfall Model
Requirements
Plan/Schedule
Design
Replan/Reschedule
Implementation
Integration
Validation
Deployment
Software specification
 The process of establishing what services are
required and the constraints on the system’s
operation and development.
 Requirements engineering process
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Feasibility study;
Requirements elicitation and analysis;
Requirements specification;
Requirements validation.
Requirements
 Problem Definition → Requirements/Specification
– determine exactly what the customer and user need (maybe want)
– Requirements develop a contract with the customer
– Specification say what the software product is to do
 Difficulties
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client is computer/software illiterate (no idea what is doable)
client asks for wrong product (want vs need)
client is computer/software literate (specifies solution not need)
specifications are ambiguous, inconsistent, incomplete
 Studies have shown that the percentage of defects
originating during requirements engineering is estimated at
more than 50 percent. The total percentage of project
budget due to requirements defects is 25 to 40 percent.
The requirements engineering process
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.
Design process activities
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Architectural design
Abstract specification
Interface design
Component design
Data structure design
Algorithm design
The software design process
Structured methods
 Systematic approaches to developing a software
design.
 The design is usually documented as a set of
graphical models.
 Possible models
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Object model;
Sequence model;
State transition model;
Structural model;
Data-flow model.
Architecture vs. Design
[Perry & Wolf 1992]
 Architecture is concerned with the selection of
architectural elements, their interactions, and the
constraints on those elements and their interactions
necessary to provide a framework in which to satisfy
the requirements and serve as a basis for the design.
 Design is concerned with the modularization and
detailed interfaces of the design elements, their
algorithms and procedures, and the data types needed
to support the architecture and to satisfy the
requirements.
Architecture/Design
 Requirements/Specification → Architecture/Design
– architecture: decompose software into
modules/objects/components with interfaces
– design: develop module/object/component specifications
(algorithms, data types) and communication details
– maintain a record of design decisions and traceability
– specifies how the software product is to do its tasks
 Difficulties
– miscommunication between module designers
– design may be inconsistent, incomplete, ambiguous
– “How” to achieve a requirement may be unknown
Planning/Scheduling
 Before undertaking cost of development, need to
estimate the costs/sizes of various steps
– Estimate Code size
– Estimate tools needed
– Estimate personnel
 Often Done after Architecture and before rest of
design, but revised again after full design.
 Develop schedule for aspects of project lifecycle
 If doing predictive/quantitative SE, build on past
experience, considering how to improve process.
Implementation & Integration
 Design → Implementation
– implement modules; verify that they meet their
specifications
– combine modules according to the design
– specifies how the software design is realized
 Difficulties
– module interaction errors
– order of integration may influence quality and
productivity
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.
The debugging process
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.
Verification and Validation
 Analysis
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Static
“Science”
Formal verification
Informal reviews and walkthroughs
 Testing
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Dynamic
“Engineering”
White box vs. black box
Structural vs. behavioral
Issues of test adequacy
The testing process
Testing stages
 Component or unit testing
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Individual components are tested independently;
Components may be functions or objects or coherent
groupings of these entities.
 System testing
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Testing of the system as a whole. Testing of emergent
properties is particularly important.
 Acceptance testing
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Testing with customer data to check that the system
meets the customer’s needs.
Testing phases
Quality Assurance
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Done as part of each step
Reduce costs by catching errors early.
Help determine ambiguities/inconsistencies
Help ensure quality product.
200
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Requirements
2
Specification
3
Planning
4
Design
10
Implementation Integration
Maintenance
Deployment
 Completed End-User Documentation
– Separate from Developer documentation
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Installation Process(es)
Customer test procedures
Support Processes (help desk, etc…)
Trouble Tracking
Repair/rework to address bugs
Regression testing (as bugs are fixed)
Maintenance & Evolution
 Operation → Change
– maintain software during/after user operation
– determine whether the product still functions correctly
 Difficulties
– Rigid or fragile designs
– lack of documentation
– personnel turnover
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 demarcation between
development and evolution (maintenance) this is
increasingly irrelevant as fewer and fewer systems
are completely new.
System evolution
Why I include CASE Tools
 Computer Aides Software
Engineering tools support
good SE processes (e.g. UML)
 Some tools absolute
requirement for scaling e.g.
build and configuration
management.
 Integrated CASE (ICASE)
tools embody good processes
and improve productivity (E.g.
Rational tool set)
 Some tools (e.g. debuggers,
Purify) do almost impossible
for humans.
 But.. Tools change
– No SE tools from my first
3 jobs exist (except
Fortran/C languages)
– I use regularly use 3 SE
tools from my next set of
jobs.
– Other tools I learned have
been replaced with similar
but expanded concepts..
Understanding today;s
tools gives a basis for
learning future ones.
ICASE Design Tools
 Rational Rose and
Rational Unified
Development.
 From UML drawing to
code and back.
 Generates stubs and
eventually testing code.
 Supports multiple
languages
Car
public class Car
{
public Driver theDriver;
/**
* @roseuid 3EAFF17E035B
*/
public Car()
{

Associations are
}
}

implemented as reference
attributes.
No explicit role name
defined so, Rose adds
automatically a role name
to the code: theDriver
Driver
public class Driver
{
/**
* @roseuid 3EAFF53F02FD
*/
public Driver()
{

}
}

Templates for the default
constructors are provided.
(Similar for
methods/members when
given in the class
diagram.)
Configuration Management
 CM is a discipline whose goal is to control
changes to large software through the
functions of
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Component identification
Change tracking
Version selection and baselining
Managing simultaneous updates (team work)
Build processes with automated regression
testing
– Software manufacture
CM in Action
1.0
1.1
1.2
2.0
1.3
2.1
1.4
2.2
1.5
4.0
3.0
3.1
Build Tools
 Necessary for large projects. Keep track of what depends
upon on what, and what needs recompiled or regenerated
when things change.
 Important even for small 1-person projects as soon as you
have multiple files.
 Can do much more than just “compile”, can generate
document (if using code-based docs), generate
manufactured code (e.g. SOAP interfaces), even send
emails or suggest alternatives.
 E.g. in our “IUE” project, edit some files compile was one in
seconds, edit another and a rebuild taking days would be needed. If
more than 30 files impacted, our make process recommend a new
“branch” to avoid conflicts!
Debugging Tools
 How do you see what the code is really doing (not
what it seems it should do)?
 How to you see what happened to code during
compiler optimization?
 How do you find/track down the cause of
Segfault/GFP in code you’ve never seen before?
 How can you “test” various possibilities without
generating special code or recompiling.
 How do you track down a memory leak?
Tools, workbenches, environments
CASE
tech no lo g y
Wo rk ben ch es
To ols
Editors
Compilers
File
co mpar ato rs
Analy sis an d
d esign
Multi-metho d
wo rk ben ch es
In teg rated
en v iro nmen ts
Pro grammin g
Sing le-meth od
wo rk ben ch es
Env iro nmen ts
Pro ces s-cen tr ed
en v iro nmen ts
Tes tin g
Gen er al-pu rp os e
wo rk ben ch es
Lang u ag e-s pecific
wo rk ben ch es
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 practive perspective that suggests good practice.
RUP phase model
P has e i terati on
Incepti on
Elaborati on
Cons tructi on
Transi tion
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
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Develop software iteratively
Manage requirements
Use component-based architectures
Visually model software
Verify software quality
Control changes to software
Static workflows
W ork flow
Descri ption
Business modelling
The business processes are modelled using business use cases.
Requirement s
Actors who interact with the system are ident ified and use cases are
developed to model the system requirement s.
Analysis and design
A design model is created and documented using architectural
models, component models, object models and sequ ence models.
Implementat ion
The components in the system are implemented and structured into
implementat ion sub-systems. Automat ic code generat ion from design
models helps accelerate this process.
Test
Test ing is an iterat ive process that is carried out in conjunct ion with
implementat ion. System test ing follows the completion of the
implementat ion.
Deployment
A product release is created, distributed to users and installed in their
workplace.
Configurat ion and
change management
This supporting workflow managed changes to t he system (see
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
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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.
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 really support these.
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.
Functional tool classification
Tool type
Examples
Planning tools
PERT tools, estimation tools, spreadsheets
Editing tools
Text editors, diagram editors, word processors
Change ma nagement tools
Requirements traceability tools, change control systems
Configuration management tools
Version management systems, system b uilding 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
Testing tools
Test data generators, file comp arators
Debugging tools
Interactive debugging systems
Documentation tools
Page layout programs , ima ge editors
Re-engineering tools
Cross-reference systems , program re-structuring systems
Activity-based tool classification
Re-eng in eerin g to ols
Tes tin g to ols
Deb ug g in g too ls
Prog ram an aly sis to ols
Lang u ag e-p ro ces sin g
to ols
Meth o d s up po r t too ls
Prototy p ing to ols
Con fig uration
man ag emen t to ols
Chang e man ag emen t too ls
Documen tatio n too ls
Editing too ls
Plan ning to o ls
Specificatio n
Design
Implemen tatio n
Verification
an d
Validatio n
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.
Boult’s view of SE
 SE must balance risks in software development process:
– Risks of error in
•
•
•
•
•
requirements
specification,
design,
implementation,
and integration
– Risks of exceeding available resources
– Risks of being late on delivery or missing the market
 Don’t let push for formality dominate your process.
 Don’t let push for expedience destroy your process.
Software Process Qualities
 Process is reliable if it consistently leads to highquality products
 Process is robust if it can accommodate
unanticipated changes in tools and
environments
 Process performance is productivity
 Process is evolvable if it can accommodate new
management and organizational techniques
 Process is reusable if it can be applied across
projects and organizations
Assessing Software Qualities
 Qualities must be measurable/quantifiable
 Measurement requires that qualities be
precisely defined
 Improvement requires accurate and
consistent measurements
 For most SD groups, qualities are informally
defined and are difficult to assess
Software Engineering “Axioms”
 Adding developers to a project will likely result in further
delays and accumulated costs
 The longer a fault exists in software
– the more costly it is to detect and correct
– the less likely it is to be properly corrected
 Up to 70% of all faults detected in large-scale software
projects are introduced in requirements and design
– detecting the causes of those faults early may reduce their
resulting costs by a factor of 200 or more
 Basic tension of software engineering
– better, cheaper, faster — pick any two!
– functionality, scalability, performance — pick any two!
 Want/Need Management’s buy in to formal SE process.
 If you don’t document your process, you don’t have one!
Boehm’s Spiral Model
EVALUATE ALTERNATIVES
AND RISKS
DETERMINE GOALS,
ALTERNATIVES,
CONSTRAINTS
Risk analysis
Risk analysis
3
Risk analysis
2
Risk analysis
Budget
4
Budget
3
Budget
2
start
Budget
1
Prototype
1
Requirements,
life-cycle plan
1
4
Proto type 2
Proto type 3
Proto type 4
Concept of
operation
Detailed
design
Code
Unit test
Implementation
plan
PLAN
Acceptance
test
System
test
DEVELOP AND TEST
Key points
 Software processes are the activities involved in
producing and evolving a software system.
 Software process models are abstract representations
of these processes.
 General activities are specification, design and
implementation, validation and evolution.
 Generic process models describe the organisation of
software processes. Examples include the waterfall
model, evolutionary development and componentbased software engineering.
 Iterative process models describe the software process
as a cycle of activities.
Key points
 Requirements engineering is the process of developing
a software specification.
 Design and implementation processes transform the
specification to an executable program.
 Validation involves checking that the system meets to
its specification and user needs.
 Evolution is concerned with modifying the system
after it is in use.
 The Rational Unified Process is a generic process
model that separates activities from phases.
 CASE technology supports software process activities.