Introduction to Software
Engineering |
Lecture 8 – Software
Testing |
Topics
covered
² Development
testing
² Test-driven
development
² Release
testing
² User
testing
Program
testing
² Testing
is intended to show that a program does what it is intended to do and to
discover program defects before it is put into use.
² When
you test software, you execute a program using artificial data.
² You
check the results of the test run for errors, anomalies or information about
the program’s non-functional attributes.
² Can reveal the presence of errors NOT their absence.
² Testing is part of a more general verification and
validation process, which also includes static validation techniques.
Program
testing goals
² To
demonstrate to the developer and the customer that the software meets its
requirements.
§
For custom software, this means that there should
be at least one test for every requirement in the requirements document. For
generic software products, it means that there should be tests for all of the
system features, plus combinations of these features, that will be incorporated
in the product release.
² To
discover situations in which the behavior of the software is incorrect,
undesirable or does not conform to its specification.
§
Defect testing is concerned with rooting out
undesirable system behavior such as system crashes, unwanted interactions with
other systems, incorrect computations and data corruption.
Validation
and defect testing
² The
first goal leads to validation testing
§
You expect the system to perform correctly using a
given set of test cases that reflect the system’s
expected use.
² The
second goal leads to defect testing
§
The test cases are designed to expose defects. The
test cases in defect testing can be deliberately obscure and need not reflect
how the system is normally used.
Testing
process goals
² Validation testing
§
To demonstrate to the developer and the system
customer that the software meets its requirements
§
A successful test shows that the system operates as
intended.
² Defect
testing
§
To discover faults or defects in the software where
its behaviour is incorrect or not in conformance with
its specification
§
A successful test is a test that makes the system
perform incorrectly and so exposes a defect in the system.
An
input-output model of program testing
Verification vs validation
² Verification: "Are we building the
product right”.
²
The software should conform to its specification.
² Validation: "Are we building the right product”.
²
The software should do what the user really requires.
V & V confidence
² Aim of V & V is to
establish confidence that the system is ‘fit for purpose’.
² Depends on system’s
purpose, user expectations and marketing environment
§ Software purpose
• 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.
Inspections and testing
² 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.
§
Discussed in Chapter 15.
² Software testing Concerned with exercising and
observing product behaviour (dynamic verification)
§
The system is executed with test data and its operational behaviour is
observed.
Software inspections
² These involve people examining the source
representation with the aim of discovering anomalies and defects.
² Inspections not require execution of a system so
may be used before implementation.
² They may be applied to any representation of the
system (requirements, design,configuration
data, test data, etc.).
² They have been shown to be an effective technique
for discovering program errors.
Advantages
of inspections
² During
testing, errors can mask (hide) other errors. Because inspection is a static
process, you don’t have to be concerned with interactions between errors.
² Incomplete
versions of a system can be inspected without additional costs. If a program is
incomplete, then you need to develop specialized test harnesses to test the
parts that are available.
² As
well as searching for program defects, an inspection can also consider broader
quality attributes of a program, such as compliance with standards, portability
and maintainability.
Inspections and testing
² Inspections and testing are complementary and not
opposing verification techniques.
² Both should be used during the V & V process.
² Inspections can check conformance with a
specification but not conformance with the customer’s real requirements.
² Inspections cannot check non-functional
characteristics such as performance, usability, etc.
A
model of the software testing process
Stages
of testing
² Development
testing, where the system is tested during development to discover bugs and defects.
² Release
testing, where a separate testing team test a complete
version of the system before it is released to users.
² User
testing, where users or potential users of a system test the system in their
own environment.
Development
testing
² Development
testing includes all testing activities that are carried out by the team
developing the system.
§
Unit testing, where individual program units or
object classes are tested. Unit testing should focus on testing the
functionality of objects or methods.
§
Component testing, where several individual units
are integrated to create composite components. Component testing should focus
on testing component interfaces.
§
System testing, where some or all of the components
in a system are integrated and the system is tested as a whole. System testing
should focus on testing component interactions.
Unit
testing
² Unit
testing is the process of testing individual components in isolation.
² It
is a defect testing process.
² Units
may be:
§
Individual functions or methods within an object
§
Object classes with several attributes and methods
§
Composite components with defined interfaces used
to access their functionality.
Object class testing
² Complete test coverage of a class involves
§
Testing all operations associated with an object
§
Setting and interrogating all object attributes
§
Exercising the object in all possible states.
² Inheritance makes it more difficult to design
object class tests as the information to be tested is not localised.
The
weather station object interface
Weather
station testing
² Need
to define test cases for reportWeather, calibrate,
test, startup and shutdown.
² Using
a state model, identify sequences of state transitions to be tested and the
event sequences to cause these transitions
² For
example:
§
Shutdown -> Running-> Shutdown
§
Configuring-> Running-> Testing -> Transmitting
-> Running
§
Running-> Collecting-> Running->
Summarizing -> Transmitting -> Running
Automated
testing
² Whenever
possible, unit testing should be automated so that tests are run and checked
without manual intervention.
² In
automated unit testing, you make use of a test automation framework (such as JUnit) to write and run your program tests.
² Unit
testing frameworks provide generic test classes that you extend to create
specific test cases. They can then run all of the tests that you have
implemented and report, often through some GUI, on the success of otherwise of
the tests.
Automated
test components
² A
setup part, where you initialize the system with the test case, namely the
inputs and expected outputs.
² A
call part, where you call the object or method to be tested.
² An
assertion part where you compare the result of the call with the expected
result. If the assertion evaluates to true, the test has been successful if false,
then it has failed.
Unit
test effectiveness
² The
test cases should show that, when used as expected, the component that you are
testing does what it is supposed to do.
² If
there are defects in the component, these should be revealed by test cases.
² This
leads to 2 types of unit test case:
§
The first of these should reflect normal operation
of a program and should show that the component works as expected.
§
The other kind of test case should be based on
testing experience of where common problems arise. It should use abnormal
inputs to check that these are properly processed and do not crash the
component.
Testing
strategies
² Partition
testing, where you identify groups of inputs that have common characteristics
and should be processed in the same way.
§
You should choose tests from within each of these
groups.
² Guideline-based
testing, where you use testing guidelines to choose test cases.
§
These guidelines reflect previous experience of the
kinds of errors that programmers often make when developing components.
Partition testing
² Input data and output results often fall into
different classes where all members of a class are related.
² Each of these classes is an equivalence partition or domain where the program behaves in an
equivalent way for each class member.
² Test cases should be chosen from each partition.
Equivalence
partitioning
Equivalence
partitions
Testing guidelines (sequences)
² Test software with sequences which have only a
single value.
² Use sequences of different sizes in different
tests.
² Derive tests so that the first, middle and last
elements of the sequence are accessed.
² Test with sequences of zero length.
General
testing guidelines
² Choose
inputs that force the system to generate all error messages
² Design
inputs that cause input buffers to overflow
² Repeat
the same input or series of inputs numerous times
² Force
invalid outputs to be generated
² Force
computation results to be too large or too small.
Component
testing
² Software
components are often composite components that are made up of several
interacting objects.
§
For example, in the weather station system, the
reconfiguration component includes objects that deal with each aspect of the
reconfiguration.
² You
access the functionality of these objects through the defined component
interface.
² Testing
composite components should therefore focus on showing that the component
interface behaves according to its specification.
§
You can assume that unit tests on the individual
objects within the component have been completed.
Interface
testing
² Objectives are to detect faults due to interface
errors or invalid assumptions about interfaces.
² Interface types
§
Parameter interfaces Data passed from one method or procedure to another.
§
Shared memory interfaces Block of memory is shared between procedures or functions.
§
Procedural interfaces Sub-system encapsulates a set of procedures to be called by other
sub-systems.
§
Message passing interfaces Sub-systems request services from other sub-systems
Interface errors
² Interface misuse
§
A calling component calls another component and makes an error in its
use of its interface e.g. parameters in the wrong order.
² Interface misunderstanding
§
A calling component embeds assumptions about the behaviour of the called
component which are incorrect.
² Timing errors
§
The called and the calling component operate at different speeds and
out-of-date information is accessed.
Interface testing guidelines
² Design tests so that parameters to a called
procedure are at the extreme ends of their ranges.
² Always test pointer parameters with null pointers.
² Design tests which cause the component to fail.
² Use stress testing in message passing systems.
² In shared memory systems, vary the order in which
components are activated.
System
testing
² System
testing during development involves integrating components to create a version
of the system and then testing the integrated system.
² The
focus in system testing is testing the interactions between components.
² System
testing checks that components are compatible,
interact correctly and transfer the right data at the right time across their
interfaces.
² System
testing tests the emergent behaviour of a system.
System
and component testing
² During
system testing, reusable components that have been separately developed and
off-the-shelf systems may be integrated with newly developed components. The
complete system is then tested.
² Components
developed by different team members or sub-teams may be integrated at this
stage. System testing is a collective rather than an individual process.
§
In some companies, system testing may involve a
separate testing team with no involvement from designers and programmers.
Use-case
testing
² The
use-cases developed to identify system interactions can be used as a basis for
system testing.
² Each
use case usually involves several system components so testing the use case
forces these interactions to occur.
² The
sequence diagrams associated with the use case documents the components and
interactions that are being tested.
Collect
weather data sequence chart
Testing
policies
² Exhaustive
system testing is impossible so testing policies which define the required
system test coverage may be developed.
² Examples
of testing policies:
§
All system functions that are accessed through
menus should be tested.
§
Combinations of functions (e.g. text formatting)
that are accessed through the same menu must be tested.
§
Where user input is provided, all functions must be
tested with both correct and incorrect input.
Test-driven
development
² Test-driven
development (TDD) is an approach to program development in which you
inter-leave testing and code development.
² Tests
are written before code and ‘passing’ the tests is the critical driver of
development.
² You
develop code incrementally, along with a test for that increment. You don’t
move on to the next increment until the code that you have developed passes its
test.
² TDD
was introduced as part of agile methods such as Extreme Programming. However,
it can also be used in plan-driven development processes.
TDD
process activities
² Start
by identifying the increment of functionality that is required. This should
normally be small and implementable in a few lines of code.
² Write
a test for this functionality and implement this as an automated test.
² Run
the test, along with all other tests that have been implemented. Initially, you
have not implemented the functionality so the new test will fail.
² Implement
the functionality and re-run the test.
² Once
all tests run successfully, you move on to implementing the next chunk of
functionality.
Benefits
of test-driven development
² Code
coverage
§
Every code segment that you write has at least one
associated test so all code written has at least one test.
² Regression
testing
§
A regression test suite is developed incrementally
as a program is developed.
² Simplified
debugging
§
When a test fails, it should be obvious where the
problem lies. The newly written code needs to be checked and modified.
² System
documentation
§
The tests themselves are a form of documentation
that describe what the code should be doing.
Regression
testing
² Regression
testing is testing the system to check that changes have not ‘broken’
previously working code.
² In
a manual testing process, regression testing is expensive but, with automated
testing, it is simple and straightforward. All tests are rerun every time a
change is made to the program.
² Tests
must run ‘successfully’ before the change is committed.
Release
testing
² Release
testing is the process of testing a particular release of a system that is
intended for use outside of the development team.
² The
primary goal of the release testing process is to convince the supplier of the
system that it is good enough for use.
§
Release testing,
therefore, has to show that the system delivers its specified functionality,
performance and dependability, and that it does not fail during normal use.
² Release
testing is usually a black-box testing process where tests are only derived
from the system specification.
Release
testing and system testing
² Release
testing is a form of system testing.
² Important
differences:
§
A separate team that has not been involved in the
system development, should be responsible for release
testing.
§
System testing by the development team should focus
on discovering bugs in the system (defect testing). The objective of release
testing is to check that the system meets its requirements and is good enough
for external use (validation testing).
Requirements
based testing
² Requirements-based
testing involves examining each requirement and developing a test or tests for
it.
² MHC-PMS
requirements:
§
If a patient is known to be allergic to any
particular medication, then prescription of that medication shall result in a
warning message being issued to the system user.
§
If a prescriber chooses to ignore an allergy
warning, they shall provide a reason why this has been ignored.
Requirements
tests
² Set
up a patient record with no known allergies. Prescribe medication for allergies
that are known to exist. Check that a warning message is not issued by the
system.
² Set
up a patient record with a known allergy. Prescribe the medication to that the
patient is allergic to, and check that the warning is issued by the system.
² Set
up a patient record in which allergies to two or more drugs are recorded.
Prescribe both of these drugs separately and check that the correct warning for
each drug is issued.
² Prescribe
two drugs that the patient is allergic to. Check that two warnings are
correctly issued.
² Prescribe
a drug that issues a warning and overrule that warning. Check that the system
requires the user to provide information explaining why the warning was
overruled.
Features
tested by scenario
² Authentication
by logging on to the system.
² Downloading
and uploading of specified patient records to a laptop.
² Home
visit scheduling.
² Encryption
and decryption of patient records on a mobile device.
² Record
retrieval and modification.
² Links
with the drugs database that maintains side-effect information.
² The
system for call prompting.
A
usage scenario for the MHC-PMS
Performance
testing
² Part
of release testing may involve testing the emergent properties of a system,
such as performance and reliability.
² Tests
should reflect the profile of use of the system.
² Performance
tests usually involve planning a series of tests where the load is steadily
increased until the system performance becomes unacceptable.
² Stress
testing is a form of performance testing where the system is deliberately
overloaded to test its failure behavior.
User
testing
² User
or customer testing is a stage in the testing process in which users or
customers provide input and advice on system testing.
² User
testing is essential, even when comprehensive system and release testing have
been carried out.
§
The reason for this is that influences from the
user’s working environment have a major effect on the reliability, performance,
usability and robustness of a system. These cannot be replicated in a testing
environment.
Types
of user testing
² Alpha
testing
§
Users of the software work with the development
team to test the software at the developer’s site.
² Beta
testing
§
A release of the software is made available to
users to allow them to experiment and to raise problems that they discover with
the system developers.
² Acceptance
testing
§
Customers test a system to decide whether or not it
is ready to be accepted from the system developers and deployed in the customer
environment. Primarily for custom systems.
The
acceptance testing process
Stages
in the acceptance testing process
² Define
acceptance criteria
² Plan
acceptance testing
² Derive
acceptance tests
² Run
acceptance tests
² Negotiate
test results
² Reject/accept
system
Agile
methods and acceptance testing
² In
agile methods, the user/customer is part of the development team and is
responsible for making decisions on the acceptability of the system.
² Tests
are defined by the user/customer and are integrated with other tests in that
they are run automatically when changes are made.
² There
is no separate acceptance testing process.
² Main
problem here is whether or not the embedded user is ‘typical’ and can represent
the interests of all system stakeholders.
Key
points
² When testing software, you should try to ‘break’
the software by using experience and guidelines to choose types of test case
that have been effective in discovering defects in other systems.
² Wherever possible, you should write automated
tests. The tests are embedded in a program that can be run every time a change
is made to a system.
² Test-first development is an approach to
development where tests are written before the code to be tested.
² Scenario testing involves inventing a typical usage
scenario and using this to derive test cases.
² Acceptance testing is a user testing process where
the aim is to decide if the software is good enough to be deployed and used in
its operational environment.