Post on 30-Dec-2015
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 2
Objectives
To introduce software verification and validation and to discuss the distinction between them
To describe the program inspection process and its role in V & V
To explain static analysis as a verification technique To describe the Cleanroom software development
process
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 3
Topics covered
Introduction to testing Sources of errors Why do we need testing ? Verification and validation planning Software inspections Automated static analysis Cleanroom software development
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 4
Nasty question
Suppose you are being asked to lead the team to test the software that controls a new X-ray machine. Would you take that job?
Would you take it if you could name your own price?
What if the contract says you’ll be charged with murder in case a patient dies because of a mal-functioning of the software?
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 5
A few spectacular software failuresA few spectacular software failures
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 6
Failures in Production Software
NASA’s Mars lander, September 1999, crashed due to a units integration fault—over $50 million US !
Huge losses due to web application failures• Financial services : $6.5 million per hour• Credit card sales applications : $2.4 million per hour
In Dec 2006, amazon.com’s BOGO offer turned into a double discount 2007 : Symantec says that most security vulnerabilities are due to faulty
software Stronger testing could solve most of these problems
6
World-wide monetary loss due to poor software is World-wide monetary loss due to poor software is staggeringstaggering
Thanks to Dr. Sreedevi Sampath
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 7
Bypass Testing Results
v
— Vasileios Papadimitriou. Masters thesis, Automating Bypass Testing for Web Applications, GMU 2006
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 8
Why Does Testing Matter?
8
NIST report, “The Economic Impacts of Inadequate Infrastructure for Software Testing” (2002)
– Inadequate software testing costs the US alone between $22 and $59 billion annually
– Better approaches could cut this amount in half Major failures: Ariane 5 explosion, Mars Polar
Lander, Intel’s Pentium FDIV bug Insufficient testing of safety-critical software can cost
lives: THERAC-25 radiation machine: 3 dead
We want our programs to be reliable– Testing is how, in most cases, we find out if they
are
Mars PolarLander crashsite?
THERAC-25 design
Ariane 5:exception-handlingbug : forced selfdestruct on maidenflight (64-bit to 16-bitconversion: about370 million $ lost)
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 9
Software is a Skin that Surrounds Our Civilization
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 10
Airbus 319 Safety Critical Software Control
Loss of autopilot
Loss of both the commander’s and the co‑pilot’s primary flight and navigation displays
Loss of most flight deck lighting and intercom
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 11
Northeast Blackout of 2003
Affected 10 million people in Ontario,
Canada
Affected 40 million people in 8 US
states
Financial losses of
$6 Billion USD
508 generating units and 256
power plants shut down
The alarm system in the energy management system failed due to a software error and operators were not informed of
the power overload in the system
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 12
Software testing is getting more Software testing is getting more importantimportant
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 13
Testing in the 21st Century We are going through a time of change Software defines behavior
• network routers, finance, switching networks, other infrastructure Today’s software market :
• is much bigger
• is more competitive
• has more users
Agile processes put increased pressure on testers Embedded Control Applications
• airplanes, air traffic control
• spaceships
• watches
• ovens• remote controllers
– PDAs– memory seats – DVD players– garage door openers– cell phones
Industry is going through a revolution
in what testing means to the
success of software products
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 14
Testing in the 21st Century
More safety critical, real-time software Enterprise applications means bigger programs, more users Embedded software is ubiquitous … check your pockets Paradoxically, free software increases our expectations ! Security is now all about software faults
• Secure software is reliable software The web offers a new deployment platform
• Very competitive and very available to more users• Web apps are distributed• Web apps must be highly reliable
Industry desperately needs our inventions !Industry desperately needs our inventions !
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 15
Mismatch in Needs and Goals
Industry wants testing to be simple and easy• Testers with no background in computing or math
Universities are graduating scientists• Industry needs engineers
Testing needs to be done more rigorously Agile processes put lots of demands on testing
• Programmers must unit test – with no training, education or tools !
• Tests are key components of functional requirements – but who builds those tests ?
Bottom line—lots of crappy Bottom line—lots of crappy softwaresoftware
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 16
Here! Test This!
MicroSteff – bigsoftware systemfor the mac
V.1.5.1 Jan/2007
VerdatimDataLifeMF2-HD1.44 MBBig software
program
Jan/2007
My first “professional” job
A stack of computer printouts—and no documentation
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 1717
Cost of Testing
In the real-world, testing is the principle post-design activity
Restricting early testing usually increases cost
Extensive hardware-software integration requires more testing
You’re going to spend at least You’re going to spend at least half of your development half of your development budget on testing, whether you budget on testing, whether you want to or notwant to or not
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 18
State-of-the-Art
30-85 errors are made per 1000 lines of source code extensively tested software contains 0.5-3 errors per 1000
lines of source code testing is postponed, as a consequence: the later an error is
discovered, the more it costs to fix it. error distribution: 60% design, 40% implementation. 66% of
the design errors are not discovered until the software has become operational.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 19
Relative cost of error correction
1
2
5
10
20
50
100
RE design code test operation
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 20
Why Test?
Written test objectives and requirements are rare
What are your planned coverage levels?
How much testing is enough?
Common objective – spend the budget …
If you don’t know If you don’t know whywhy you’re you’re conducting a test, it won’t be very conducting a test, it won’t be very helpfulhelpful
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 21
Lessons
Many errors are made in the early phases
These errors are discovered late
Repairing those errors is costly
It pays off to start testing real early
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 2222
Why Test?
1980: “The software shall be easily maintainable”
Threshold reliability requirements?
What fact is each test trying to verify?
Requirements definition teams should include testers!
If you don’t start planning for each test If you don’t start planning for each test when the functional requirements are when the functional requirements are formed, you’ll never know why you’re formed, you’ll never know why you’re conducting the testconducting the test
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 23
Cost of Not Testing
Not testing is even more expensive
Planning for testing after development is prohibitively expensive
A test station for circuit boards costs half a million dollars …
Software test tools cost less than $10,000 !!!
Program Managers often Program Managers often say: “Testing is too say: “Testing is too
expensive.”expensive.”
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 24
How then to proceed?
Exhaustive testing most often is not feasible
Random statistical testing does not work either if you want to find errors
Therefore, we look for systematic ways to proceed during testing
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 25
Some preliminary questions
What exactly is an error?
How does the testing process look like?
When is test technique A superior to test technique B?
What do we want to achieve during testing?
When to stop testing?
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 26
Error, fault, failure
an error is a human activity resulting in software containing a fault
a fault is the manifestation of an error
a fault may result in a failure
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 27
When exactly is a failure a failure?
Failure is a relative notion: e.g. a failure w.r.t. the specification document
Verification: evaluate a product to see whether it satisfies the conditions specified at the start:
Have we built the system right?
Validation: evaluate a product to see whether it does what we think it should do:
Have we built the right system?
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 28
What is our goal during testing?
Objective 1: find as many faults as possible
Objective 2: make you feel confident that the software works OK
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 29
Verification: "Are we building the product right”.
The software should conform to its specification.
Validation: "Are we building the right product”.
The software should do what the user really requires.
Verification vs validation
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 30
Is a whole life-cycle process - V & V must be applied at each stage in the software process.
Has two principal objectives• The discovery of defects in a system;• The assessment of whether or not the system is
useful and useable in an operational situation.
The V & V process
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 31
V& V goals
Verification and validation should establish confidence that the software is fit for purpose.
This does NOT mean completely free of defects.
Rather, it must be good enough for its intended use and the type of use will determine the degree of confidence that is needed.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 32
V & V confidence
Depends on system’s purpose, user expectations and marketing environment• Software function
• The level of confidence depends on how critical the software is to an organisation.
• User expectations• Users may have low expectations of certain kinds of
software.
• Marketing environment• Getting a product to market early may be more
important than finding defects in the program.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 33
Software inspections. Concerned with analysis of the static system representation to discover problems (static verification)• May be supplement by tool-based document and code
analysis Software testing. Concerned with exercising and
observing product behaviour (dynamic verification)• The system is executed with test data and its operational
behaviour is observed
Static and dynamic verification
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 34
Static and dynamic V&V
Formalspecification
High-leveldesign
Requirementsspecification
Detaileddesign
Program
Prototype Programtesting
Softwareinspections
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 35
Can reveal the presence of errors NOT their absence.
The only validation technique for non-functional requirements as the software has to be executed to see how it behaves.
Should be used in conjunction with static verification to provide full V&V coverage.
Program testing
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 36
Defect testing• Tests designed to discover system defects.• A successful defect test is one which reveals the
presence of defects in a system.• Covered in Chapter 23
Validation testing• Intended to show that the software meets its
requirements.• A successful test is one that shows that a requirements
has been properly implemented.
Types of testing
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 37
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.
Testing and debugging
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 38
The debugging process
Locateerror
Designerror repair
Repairerror
Retestprogram
Testresults
Specification Testcases
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 39
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.
Test planning is about defining standards for the testing process rather than describing product tests.
V & V planning
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 40
The V-model of development
Systemspecification
Systemdesign
Detaileddesign
Module andunit codeand test
Sub-systemintegrationtest plan
Systemintegrationtest plan
Acceptancetest plan
ServiceAcceptance
testSystem
integration testSub-system
integration test
Requirementsspecification
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 41
The structure of a software test plan
The testing process. Requirements traceability. Tested items. Testing schedule. Test recording procedures. Hardware and software requirements. Constraints.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 43
Software inspections
These involve people examining the source representation with the aim of discovering anomalies and defects.
Inspections not require execution of a system so may be used before implementation.
They may be applied to any representation of the system (requirements, design,configuration data, test data, etc.).
They have been shown to be an effective technique for discovering program errors.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 44
Inspection success
Many different defects may be discovered in a single inspection. In testing, one defect ,may mask another so several executions are required.
The reuse domain and programming knowledge so reviewers are likely to have seen the types of error that commonly arise.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 45
Inspections and testing
Inspections and testing are complementary and not opposing verification techniques.
Both should be used during the V & V process. Inspections can check conformance with a
specification but not conformance with the customer’s real requirements.
Inspections cannot check non-functional characteristics such as performance, usability, etc.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 46
Program inspections
Formalised approach to document reviews Intended explicitly for defect detection (not
correction). Defects may be logical errors, anomalies in
the code that might indicate an erroneous condition (e.g. an uninitialised variable) or non-compliance with standards.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 47
Inspection pre-conditions
A precise specification must be available. Team members must be familiar with the
organisation standards. Syntactically correct code or other system
representations must be available. An error checklist should be prepared. Management must accept that inspection will
increase costs early in the software process. Management should not use inspections for staff
appraisal ie finding out who makes mistakes.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 48
The inspection process
Inspectionmeeting
Individualpreparation
Overview
Planning
Rework
Follow-up
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 49
Inspection procedure
System overview presented to inspection team.
Code and associated documents are distributed to inspection team in advance.
Inspection takes place and discovered errors
are noted. Modifications are made to repair discovered
errors. Re-inspection may or may not be required.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 51
Inspection checklists
Checklist of common errors should be used to drive the inspection.
Error checklists are programming language dependent and reflect the characteristic errors that are likely to arise in the language.
In general, the 'weaker' the type checking, the larger the checklist.
Examples: Initialisation, Constant naming, loop termination, array bounds, etc.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 54
Inspection rate
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
man/hours effort - about £2800 at UK rates.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 55
Automated static analysis
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.
They are very effective as an aid to inspections - they are a supplement to but not a replacement for inspections.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 57
Stages of static analysis
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
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 58
Stages of static analysis
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. They must be used with care.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 60
Use of static analysis
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.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 61
Verification and formal methods
Formal methods can be used when a mathematical specification of the system is produced.
They are the ultimate static verification technique.
They involve detailed mathematical analysis of the specification and may develop formal arguments that a program conforms to its mathematical specification.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 62
Arguments for formal methods
Producing a mathematical specification requires a detailed analysis of the requirements and this is likely to uncover errors.
They can detect implementation errors before testing when the program is analysed alongside the specification.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 63
Arguments against formal methods
Require specialised notations that cannot be understood by domain experts.
Very expensive to develop a specification and even more expensive to show that a program meets that specification.
It may be possible to reach the same level of confidence in a program more cheaply using other V & V techniques.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 64
The name is derived from the 'Cleanroom' process in semiconductor fabrication. The philosophy is defect avoidance rather than defect removal.
This software development process is based on:• Incremental development;• Formal specification;• Static verification using correctness arguments;• Statistical testing to determine program reliability.
Cleanroom software development
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 65
The Cleanroom process
Constructstructuredprogram
Definesoftware
increments
Formallyverifycode
Integrateincrement
Formallyspecifysystem
Developoperational
profileDesign
statisticaltests
Testintegratedsystem
Error rework
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 66
Cleanroom process characteristics
Formal specification using a state transition model.
Incremental development where the customer prioritises increments.
Structured programming - limited control and abstraction constructs are used in the program.
Static verification using rigorous inspections. Statistical testing of the system (covered in
Ch. 24).
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 67
Formal specification and inspections
The state based model is a system specification and the inspection process checks the program against this mode.l
The programming approach is defined so that the correspondence between the model and the system is clear.
Mathematical arguments (not proofs) are used to increase confidence in the inspection process.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 68
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.
Cleanroom process teams
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 69
The results of using the Cleanroom process have been very impressive with few discovered faults in delivered systems.
Independent assessment shows that the process is no more expensive than other approaches.
There were fewer errors than in a 'traditional' development process.
However, the process is not widely used. It is not clear how this approach can be transferred to an environment with less skilled or less motivated software engineers.
Cleanroom process evaluation
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 70
Key points
Verification and validation are not the same thing. Verification shows conformance with specification; validation shows that the program meets the customer’s needs.
Test plans should be drawn up to guide the testing process.
Static verification techniques involve examination and analysis of the program for error detection.
Sommerville, Ammann-Offutt, Mejia, 2009 Software Engineering Slide 71
Key points
Program inspections are very effective in discovering errors.
Program code in inspections is systematically checked by a small team to locate software faults.
Static analysis tools can discover program anomalies which may be an indication of faults in the code.
The Cleanroom development process depends on incremental development, static verification and statistical testing.