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3 minute review of last week -- Critical Systems Specification Reliability metrics are units of measurement of system reliability Availability - relevant for non-stop, continuously running systems like telephone switching systems Probability of failure on demand (POFOD) - relevant for safety-critical systems -- chemical plant Rate of occurrence of failure (ROCOF) - relevant for transaction process systems - credit card processing Mean time to failure - Relevant for systems with long transactions (like CAD systems)

3 minute review of last week -- Hazard and risk analysis stages Hazard identification Risk analysis and hazard classification Hazard decomposition Decompose hazards to discover their potential root causes Risk reduction assessment Define how each hazard must be taken into account when the system is designed

3 minute review of last week -- Critical systems development Fault minimisation Dependable and repeatable development process PL with strict typing and run-time checking Avoid error-prone constructs like goto Fault tolerance Fault recovery Forward recovery -- repair the corrupted system state Backward recovery -- back to the known safe state Fault-tolerant architectures N-version programming Recovery blocks

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 Ch. 19 -- Verification and Validation

Is a whole life-cycle process - V & V must be applied at each stage in the software process. The V & V process

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

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 to provide full V&V coverage Program testing

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 20 Statistical testing tests designed to reflect the frequence of user inputs. Used for reliability estimation. Covered in Chapter 21 Types of testing

V& V goals Verification and validation should establish confidence that the software is fit for purpose Level of 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

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 errors Debugging involves formulating a hypothesis about program behaviour, then testing them to find the errors Testing and debugging

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 19.1 V & V planning

The structure of a software test plan The testing process Requirements traceability To-be- tested items list Testing schedule Test recording procedures Hardware and software requirements Constraints

19.2 Software inspections Very effective technique for discovering errors Involve people examining the source representation to discover 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.) They reuse domain and programming knowledge so reviewers are likely to have seen the types of errors that commonly arise

Inspections and testing Inspections and testing are complementary and not opposing verification techniques Many different defects may be discovered in a single inspection. In testing, one defect may mask another so several executions are required 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.

Inspection pre-conditions A precise specification must be available Syntactically correct code must be available An error checklist should be prepared Team members must be familiar with the organisation standards Management must accept that inspection will increase costs early in the software process Management must not use inspections for staff appraisal

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 noted Modifications are made to repair discovered errors Re-inspection may (or may not) be required

Inspection teams 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

Inspection checklists 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: Initialisation, Constant naming, loop termination, array bounds, etc.

Inspection rate 500 statements/hour during overview 125 source statement/hour during individual preparation 90-125 statements/hour can be inspected Inspecting 500 lines costs about 40 man-hours Inspection is therefore an expensive process but still less than half the testing costs Sessions no longer than 2 hours

19.3 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 Very effective as an aid to inspections. A supplement to but not a replacement for inspections

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

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. Must be used with care.

LINT static analysis 138% more lint_ex.c #include <stdio.h> printarray (Anarray) int Anarray; { printf(“%d”,Anarray); } main () { int Anarray[5]; int i; char c; printarray (Anarray, i, c); printarray (Anarray) ; } 139% cc lint_ex.c 140% lint lint_ex.c lint_ex.c(10): warning: c may be used before set lint_ex.c(10): warning: i may be used before set printarray: variable # of args. lint_ex.c(4) :: lint_ex.c(10) printarray, arg. 1 used inconsistently lint_ex.c(4) :: lint_ex.c(10) printarray, arg. 1 used inconsistently lint_ex.c(4) :: lint_ex.c(11) printf returns value which is always ignored

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

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: Incremental development Formal specification Structured programming Static verification using correctness arguments Statistical testing to determine program reliability. 19.4 Cleanroom software development

Formal specification and inspections The system specification is a state based model and the inspection process checks the program against this model 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

Specification team. Responsible for developing and maintaining the system specification. C-requirements + Formal specifications Development team. Responsible for developing and verifying the software. Software inspection + correctness arguments Certification team. Responsible for developing a set of statistical tests to exercise the software after development. Reliability certification. Cleanroom process teams

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 Cleanroom process evaluation

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

Key points Program inspections are very effective in discovering errors Program code in inspections is 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

10 minute break - a test devised by a 13 years old Can you find out the incorrect multiplications without using a calculator ? a) 67896 x 321 ------------ 27094616 b) 34675 x 603 ------------ 20909025 c) 47183 x 369 ------------ 17401527

10 minute break - a test devised by a 13 years old (X +|-|* Y) mod 9 = X mod 9 +|-|* Y mod 9 a) 67896 x 321 ------------ 27094616 b) 34675 x 603 ------------ 20909025 c) 47183 x 369 ------------ 17401527 0 6 8 7 0 0 6 0 0 It can prove the presence not the absence of errors.

Ch 20 -- Software testing Component testing (by developer) Testing of individual program components Tests are derived from the developer’s experience Integration testing (by testing team) Testing of groups of components integrated to create a system or sub-system Tests are based on a system specification

20.1 Defect testing The goal of defect testing is to discover defects in programs A successful defect test is a test which causes a program to behave in an anomalous way Tests show the presence not the absence of defects Only exhaustive testing can show a program is free from defects. However, exhaustive testing is impossible Tests should exercise a system's capabilities rather than its components

The defect testing process Test data: Inputs which have been devised to test the system Test cases: Inputs to test the system and the expected outputs for these inputs if the system operates according to its specification

Black-box testing An approach to testing where the program is considered as a ‘black-box’ The program test cases are based on the system specification Experience of Test engineers helps here.

Equivalence partitioning Input data (and output results) often fall into different classes -- all members of a class are related Each of these classes is an equivalence partition where the program behaves in an equivalent way for each class member Test cases should be chosen from each partition

Program accepts 4 to 10 five-digit integer inputs Partition system inputs and outputs into ‘equivalence sets’ The equivalence partitions are < 10,000, 10,000-99,999 and > 100,000 Choose test cases at the boundary of these sets 00000, 09999, 10000, 99999, 100001 Equivalence partitioning

Search routine specification procedure Search (Key : ELEM ; T: ELEM_ARRAY; Found : in out BOOLEAN; L: in out ELEM_INDEX) ; Pre-condition -- the array has at least one element T’FIRST <= T’LAST Post-condition -- the element is found and is referenced by L ( Found and T (L) = Key) or -- the element is not in the array ( not Found and not ( exists i, T’FIRST >= i <= T’LAST, T (i) = Key ))

Inputs which conform to the pre-conditions Inputs where a pre-condition does not hold The key element is a member of the array The key element is not a member of the array Use sequences of different sizes in different tests Derive tests so that the first, middle and last elements of the sequence are accessed Test with sequences of zero length Test with sequences which have only a single value Search routine - input partitions

Search routine - input partitions

Sometime called white-box testing Derivation of test cases according to program structure. Knowledge of the program is used to identify additional test cases Objective is to exercise all program statements (not all path combinations) Structural testing

Pre-conditions satisfied, key element in array Pre-conditions satisfied, key element not in array Pre-conditions unsatisfied, key element in array Pre-conditions unsatisfied, key element not in array Input array has a single value Input array has an even number of values Input array has an odd number of values Binary search - equiv. partitions

Binary search equiv. partitions

Path testing The objective of path testing is to ensure that the set of test cases is such that each path through the program is executed at least once The starting point for path testing is a program flow graph that shows nodes representing program decisions and arcs representing the flow of control Statements with conditions are therefore nodes in the flow graph

Describe the program control flow. Program flow graphs

1, 2, 3, 8, 9 1, 2, 3, 4, 6, 7, 2 1, 2, 3, 4, 5, 7, 2 1, 2, 3, 4, 6, 7, 2, 8, 9 Test cases should be derived so that all of these paths are executed A dynamic program analyser may be used to check that paths have been executed Independent paths

20.2 Integration testing Tests complete systems or subsystems composed of integrated components Integration testing should be black-box testing with tests derived from the specification Main difficulty is localising errors Incremental integration testing reduces this problem

Incremental integration testing

Approaches to integration testing Top-down testing Start with high-level system and integrate from the top-down replacing individual components by stubs where appropriate Bottom-up testing Integrate individual components in levels until the complete system is created In practice, most integration involves a combination of these strategies

Testing approaches Architectural validation Top-down integration testing is better at discovering errors in the system architecture System demonstration Top-down integration testing allows a limited demonstration at an early stage in the development Test implementation Often easier with bottom-up integration testing Test observation Problems with both approaches. Extra code may be required to observe tests

Takes place when modules or sub-systems are integrated to create larger systems Objectives are to detect faults due to interface errors or invalid assumptions about interfaces Particularly important for object-oriented development as objects are defined by their interfaces Interface testing

Interface types Parameter interfaces Data passed from one component to another Shared memory interfaces Block of memory is shared between sub-systems Procedural interfaces Sub-system encapsulates a set of procedures to be called by other sub-systems. E.g., ADTs, classes Message passing interfaces Sub-systems request services from other sub-systems. E.g., client-server systems.

Interface errors 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, wrong type.. Interface misunderstanding A calling component embeds assumptions about the behaviour of the called component which are incorrect. E.g., binary search fails on unordered array. Timing errors The called and the calling component operate at different speeds and out-of-date information is accessed

Interface testing guidelines Design tests so that parameters to a called procedure are at the extreme ends of their ranges Always test pointer parameters with null pointers Design tests which cause the component to fail Use stress testing in message passing systems In shared memory systems, vary the order in which components are activated

Stress testing Exercises the system beyond its maximum design load. Stressing the system often causes defects to come to light Stressing the system test failure behaviour .. Systems should not fail catastrophically. Stress testing checks for unacceptable loss of service or data Particularly relevant to distributed systems which can exhibit severe degradation as a network becomes overloaded

The components to be tested are object classes that are instantiated as objects Larger grain than individual functions so approaches to white-box testing have to be extended No obvious ‘top’ to the system for top-down integration and testing 20.3 Object-oriented testing

Testing levels Testing operations associated with objects Testing object classes Testing clusters of cooperating objects Testing the complete OO system

Object class testing 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 Inheritance makes it more difficult to design object class tests as the information to be tested is not localised

Weather station object interface Test cases are needed for all operations Use a state model to identify state transitions for testing Examples of testing sequences Shutdown  Waiting  Shutdown Waiting  Calibrating  Testing  Transmitting  Waiting Waiting  Collecting  Waiting  Summarising  Transmitting  Waiting

Object integration Levels of integration are less distinct in object-oriented systems Cluster testing is concerned with integrating and testing clusters of cooperating objects Identify clusters using knowledge of the operation of objects and the system features that are implemented by these clusters

Approaches to cluster testing Use-case or scenario testing Testing is based on a user interactions with the system Has the advantage that it tests system features as experienced by users Thread testing Tests the systems response to events as processing threads through the system

Scenario-based testing -- Collect weather data

Weather station testing Thread of methods executed CommsController:request  WeatherStation:report  WeatherData:summarise Inputs and outputs Input of report request, the associated acknowledgement and a final output of a report Can be tested by creating raw data and ensuring that it is summarised properly Use the same raw data to test the WeatherData object

20.4 Testing workbenches Testing is an expensive process phase. Testing workbenches provide a range of tools to reduce the time required and total testing costs Most testing workbenches are open systems because testing needs are organisation-specific Difficult to integrate with closed design and analysis workbenches

Tetsing workbench adaptation Scripts may be developed for user interface simulators and patterns for test data generators Test outputs may have to be prepared manually for comparison Special-purpose file comparators may be developed

Key points Test parts of a system which are commonly used rather than those which are rarely executed Equivalence partitions are sets of test cases where the program should behave in an equivalent way Black-box testing is based on the system specification Structural testing identifies test cases which cause all paths through the program to be executed

Key points Test coverage measures ensure that all statements have been executed at least once. Interface defects arise because of specification misreading, misunderstanding, errors or invalid timing assumptions To test object classes, test all operations, attributes and states Integrate object-oriented systems around clusters of objects

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