Verification and Validation
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Assuring that a software system
meets a user's needs
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 1
Verification vs validation
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Verification:
"Are we building the product right"
Validation:
"Are we building the right product"
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 4
Static and dynamic verification
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Software inspections (static verification and
validation)
Software testing (dynamic verification)
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 6
Static and dynamic V&V
Static
verification
Requirements
specification
High-level
design
Formal
specification
Detailed
design
Dynamic
validation
Prototype
©Ian Sommerville 2000
Program
Software Engineering, 6th edition. Chapter 19
Slide 7
Program testing
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Can reveal the presence of errors, NOT their
absence
A successful test is a test which discovers one
or more errors
The only validation technique for non-functional
requirements
Should be used in conjunction with static
verification and validation to provide full V&V
coverage
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 8
Types of testing
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Defect testing
Statistical testing
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 9
V& V goals
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Verification and validation should
establish confidence that the software is fit
for its purpose
This does NOT mean completely free of
defects
Must be good enough for its intended use
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 10
Testing and debugging
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Defect testing and debugging are distinct
processes
Verification and validation is concerned with
establishing the existence of defects in a program
Debugging is concerned with locating and
repairing these errors
Debugging involves formulating a hypothesis
about program behaviour then testing these
hypotheses to find the system error
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 12
The debugging process
Test
results
Locate
error
©Ian Sommerville 2000
Test
cases
Specification
Design
error repair
Repair
error
Software Engineering, 6th edition. Chapter 19
Re-test
program
Slide 13
19.1 V & V planning
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Careful planning is required to get the most out of
testing and inspection processes
Planning should start early in the development
process
The plan should identify the balance between
static verification and testing
Planning should define standards for the testing
process
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 14
The V-model of development
Requir ements
specification
System
specification
System
integration
test plan
Acceptance
test plan
Service
©Ian Sommerville 2000
System
design
Acceptance
test
Detailed
design
Sub-system
integration
test plan
System
integration test
Module and
unit code
and tess
Sub-system
integration test
Software Engineering, 6th edition. Chapter 19
Slide 15
The structure of a software test plan
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The testing process
Requirements traceability
Tested items
Testing schedule
Test recording procedures
Hardware and software requirements
Constraints
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 16
19.2 Software inspections
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Involve people examining the source
representation with the aim of discovering
anomalies and defects
Do not require execution of a system so may be
used before implementation
May be applied to any representation of the
system (requirements, design, test data, etc.)
Very effective technique for discovering errors
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 17
Inspection success
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Many different defects may be discovered in a
single inspection. In testing, one defect ,may
mask another so several executions are required
They reuse domain and programming knowledge
- reviewers are likely to have seen the types of
error that commonly arise and look out for them
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 18
Inspections and testing
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Inspections and testing are complementary and not
opposing verification techniques
Inspections can check conformance with a
specification but not conformance with the customer’s
real requirements
Inspections cannot test integration of subsystems
Inspections cannot check non-functional
characteristics such as performance, usability, etc.
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 19
Program inspections
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Group meeting to carefully, line-by-line review code
Intended explicitly for defect DETECTION (not
correction)
Defects may be logical errors, or non-compliance
with standards
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 20
Inspection teams
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Made up of at least 4 members
Author of the code being inspected
Inspector who finds errors, omissions and
inconsistencies
Reader who reads the code to the team
Moderator who chairs the meeting and notes
discovered errors
Other roles are Scribe and Chief moderator
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 21
Inspection pre-conditions
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A precise specification must be available
Team members must be familiar with the
organisation standards
Syntactically correct code must be available
An error checklist should be prepared
Management must accept that inspection will
increase costs early in the software process
Management must not use inspections for staff
appraisal
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 22
The inspection process
Planning
Overview
Follow-up
Individual
preparation
©Ian Sommerville 2000
Rework
Inspection
meeting
Software Engineering, 6th edition. Chapter 19
Slide 23
Inspection checklists
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Checklist of common errors should be used to
drive the inspection
Error checklist is programming language
dependent
The 'weaker' the type checking, the larger the
checklist
Examples: Initialization, Constant naming, loop
termination, array bounds, etc.
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 25
Fault clas s
Data faults
Ins pection check
Are all program variab les in itialis ed befo re th eir values
are u sed?
Hav e all con stants been n amed ?
Sh ould the lower bou nd o f arrays be 0, 1, or something
else?
Sh ould the upp er b ound of array s b eequal to the size of
the array or Size -1?
If character s tring s are us ed , is a delimiter explicitly
ass igned?
Con trol faults
Fo r each co nditional s tatement, is the con ditio n correct?
Is each loo p certain to termin ate?
Are compou nd s tatements correctly bracketed ?
In case statements, are all p oss ible cas es accounted fo r?
Inp ut/outp ut faults
Are all inpu t variab les us ed ?
Are all outpu t variab les ass igned a value before they are
ou tput?
Interface faults
Do all fu nction and procedu re calls hav e th e correct
nu mb er o f parameters?
Do fo rmal and actual parameter typ es match?
Are th e p arameters in the righ t order?
If compon ents acces s s hared memory, do th ey hav e the
s ame mo del of th e s hared memory s tructure?
Sto rage man agement If a linked s tructure is mo dified,
have all links been
faults
correctly reass igned?
If d ynamic s torag e is u sed, h as sp ace b een allocated
correctly?
Is sp ace explicitly de-allocated after it
is no lon ger
required?
Exception
Hav e all pos sib le error con ditio ns been taken
in to
man agement faults
account?
Inspection checks
Inspection rate
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500 statements/hour during overview
125 source statement/hour during individual
preparation
90-125 statements/hour can be inspected
Inspection is therefore an expensive process
Inspecting 500 lines costs about 40 person-hours
effort
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 27
19.3 Automated static analysis
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Static analysers are software tools for source text
processing
They parse the program text and try to discover
potentially erroneous conditions and bring these
to the attention of the V & V team
Very effective as an aid to inspections. A
supplement to but not a replacement for
inspections
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 28
Static analysis checks
Fault clas s
Data faults
Static analys is check
Variables us ed before initialisation
Variables declared but never used
Variables ass igned twice but never us ed
between ass ignments
Poss ible array bound violations
Undeclared variables
Control faults
Unreachable code
Unconditional branches into loops
Input/output faults
Variables output twice with no intervening
ass ignment
Interface faults
Parameter type mismatches
Parameter number mismatches
Non-usage of the results of functions
Uncalled functions and procedures
Storage
management Unass igned pointers
faults
Pointer arithmetic
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 29
Stages of static analysis
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Control flow analysis. Checks for loops with
multiple exit or entry points, finds unreachable
code, etc.
Data use analysis. Detects uninitialised
variables, variables written twice without an
intervening assignment, variables which are
declared but never used, etc
Interface analysis. Checks the consistency of
routine and procedure declarations and their
use. Functions never used, results of functions not used
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 30
Stages of static analysis
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Information flow analysis. Identifies the
dependencies of output variables. Does not
detect anomalies itself but highlights
information for code inspection or review
Path analysis. Identifies paths through the
program and sets out the statements executed in
that path. Again, potentially useful in the review
process
Both these stages generate vast amounts of
information. Must be used with care.
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 31
Use of static analysis
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Particularly valuable when a language such as C
is used which has weak typing and hence many
errors are undetected by the compiler
Less cost-effective for languages like Java that
have strong type checking and can therefore
detect many errors during compilation
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 33
19.4 Cleanroom software development
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The name is derived from the 'Cleanroom'
process in semiconductor fabrication. The
philosophy is defect avoidance rather than
defect removal
Software development process based on:
• Formal specification.
• Incremental development
• Structured programming
• Static verification using correctness arguments
• Statistical testing to determine program reliability.
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 34
The Cleanroom process
Formally
specify
system
Error rework
Define
software
increments
Develop
operational
profile
©Ian Sommerville 2000
Construct
structured
program
Formally
verify
code
Design
statistical
tests
Software Engineering, 6th edition. Chapter 19
Integrate
increment
Test
integrated
system
Slide 35
Incremental development
Frozen
specification
Establish
rerquirements
Formal
specification
Develop s/w
increment
Deliver
software
Requir ements change request
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 37
Cleanroom process teams
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Specification team. Responsible for developing
and maintaining the system specification
Development team. Responsible for
developing and verifying the software. The
software is NOT executed or even compiled
during this process
Certification team. Responsible for developing
a set of statistical tests to exercise the software
after development. Reliability growth models
used to determine when reliability is acceptable
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 39
Cleanroom process evaluation
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Results in IBM have been very impressive with
few discovered faults in delivered systems
Independent assessment shows that the
process is no more expensive than other
approaches
Fewer errors than in a 'traditional' development
process
Not clear how this approach can be transferred
to an environment with less skilled or less
highly motivated engineers
©Ian Sommerville 2000
Software Engineering, 6th edition. Chapter 19
Slide 40