Software Reliability

Software Reliability
Leralyn S. Quitaneg
Carl Angelo S. Pamplona

Software Reliability
 Software Reliability Testing is a field of software that relates to testing a software's ability to
function, given environmental conditions, for a particular amount of time. Software reliability
testing helps discover many problems in the software design and functionality.
 Software Reliability is an important to attribute of software quality, together with functionality,
usability, performance, serviceability, capability, installability, maintainability, and
documentation.
 Software reliability is the probability that software will work properly in a specified environment
and for a given amount of time. Using the following formula, the probability of failure is
calculated by testing a sample of all available input states.
 Probability = No. of failing cases / Total no. of cases
 The set of all possible input states is called the input space. To find reliability of software, we
need to find output space from given input space and software.
Functional and Non-functional Requirements
 Functional Requirements maybe calculations, technical details, data manipulation and
processing and other specific functionality that define what a system is supposed to accomplish.
 The system will check the all operator inputs to see that they fall within their
required ranges.
 The system will check all disks for bad blocks each time it is booted.
 The system must be implemented in using a standard implementation of Ada.
 Non-Functional Requirements specifies criteria that can be used to judge the operation of a
system rather than specific behaviors reliability and availability are specified as part of the non-
functional requirements for the system.
 The required level of reliability must be expressed quantitatively.
 Reliability is a dynamic system attribute.
 Source code reliability specifications are meaningless (e.g. N faults/1000 LOC)
 An appropriate metric should be chosen to specify the overall system reliability.
Failure Probabilities
 If there are two independent components in a system and the operation of the system depends
on them both then
P(S) = P(A) + P(B)
 If the components are replicated then the probability of failure is
P(S) = P(A)
n

meaning that all components fail at once
Software Reliability Metrics
 Reliability metrics are units of measure for system reliability
 System reliability is measured by counting the number of operational failures and relating these
to demands made on the system at the time of failure
 A long-term measurement program is required to assess the reliability of critical systems
Reliability Metrics
 Probability of Failure on Demand (POFOD)
 POFOD = 0.001
 For one in every 1000 requests the service fails per time unit
 Rate of Fault Occurrence (ROCOF)
 ROCOF = 0.02
 Two failures for each 100 operational time units of operation
 Mean Time to Failure (MTTF)
 average time between observed failures (aka MTBF)
 Availability = MTBF / (MTBF+MTTR)
 MTBF = Mean Time Between Failure
 MTTR = Mean Time to Repair
 Reliability = MTBF / (1+MTBF)
Time Units
 Raw Execution Time
 non-stop system
 Calendar Time
 If the system has regular usage patterns
 Number of Transactions
 demand type transaction systems
Availability
 Measures the fraction of time system is really available for use
 Takes repair and restart times into account
 Relevant for non-stop continuously running systems (e.g. traffic signal)
Probability of Failure on Demand
 Probability system will fail when a service request is made
 Useful when requests are made on an intermittent or infrequent basis
 Appropriate for protection systems service requests may be rare and consequences can be
serious if service is not delivered
 Relevant for many safety-critical systems with exception handlers
Rate of Fault Occurrence
 Reflects rate of failure in the system
 Useful when system has to process a large number of similar requests that are relatively
frequent
 Relevant for operating systems and transaction processing systems
Mean Time to Failure
 Measures time between observable system failures
 For stable systems MTTF = 1/ROCOF
 Relevant for systems when individual transactions take lots of processing time (e.g. CAD or WP
systems)
Failure Consequences
 Reliability does not take consequences into account
 Transient faults have no real consequences but other faults might cause data loss or corruption
 May be worthwhile to identify different classes of failure, and use different metrics for each
 When specifying reliability both the number of failures and the consequences of each matter
 Failures with serious consequences are more damaging than those where repair and recovery is
straightforward
 In some cases, different reliability specifications may be defined for different failure types
Failure Classification
 Transient - only occurs with certain inputs
 Permanent - occurs on all inputs
 Recoverable - system can recover without operator help
 Unrecoverable - operator has to help
 Non-corrupting - failure does not corrupt system state or data
 Corrupting - system state or data are altered
Building Reliability Specification
 For each sub-system analyze consequences of possible system failures
 From system failure analysis partition failure into appropriate classes
 For each class send out the appropriate reliability metric
Statistical Reliability Testing
 Test data used, needs to follow typical software usage patterns
 Measuring numbers of errors needs to be based on errors of omission (failing to do the right
thing) and errors of commission (doing the wrong thing)
Difficulties with Statistical Reliability Testing
 Uncertainty when creating the operational profile
 High cost of generating the operational profile
 Statistical uncertainty problems when high reliabilities are specified
Safety Specification
 Each safety specification should be specified separately
 These requirements should be based on hazard and risk analysis
 Safety requirements usually apply to the system as a whole rather than individual components
 System safety is an an emergent system property
Safety Life Cycle
 Concept and scope definition
 Hazard and risk analysis
 Safety requirements specification
 safety requirements derivation
 safety requirements allocation
 Planning and development
 safety related systems development
 external risk reduction facilities
 Deployment
 safety validation
 installation and commissioning
 Operation and maintenance
 System decommissioning
Safety Processes
 Hazard and risk analysis
 assess the hazards and risks associated with the system
 Safety requirements specification
 specify system safety requirements
 Designation of safety-critical systems
 identify sub-systems whose incorrect operation can compromise entire system safety
 Safety validation
 check overall system safety
Hazard Analysis Stages
 Hazard identification
 identify potential hazards that may arise
 Risk analysis and hazard classification
 assess risk associated with each hazard
 Hazard decomposition
 seek to discover potential root causes for each hazard
 Risk reduction assessment
 describe how each hazard is to be taken into account when system is designed
Risk Assessment
 Assess the hazard severity, hazard probability, and accident probability
 Outcome of risk assessment is a statement of acceptability
 Intolerable (can never occur)
 ALARP (as low as possible given cost and schedule constraints)
 Acceptable (consequences are acceptable and no extra cost should be incurred to
reduce it further)

Risk Acceptability
 Determined by human, social, and political considerations
 In most societies, the boundaries between regions are pushed upwards with time (meaning risk
becomes less acceptable)
 Risk assessment is always subjective (what is acceptable to one person is ALARP to another)

Risk Reduction
 System should be specified so that hazards do not arise or result in an accident
 Hazard avoidance
 system designed so hazard can never arise during normal operation
 Hazard detection and removal
 system designed so that hazards are detected and neutralized before an accident can
occur
 Damage limitation
 system designed to minimized accident consequences
Security Specification
 Similar to safety specification
 not possible to specify quantitatively
 usually stated in “system shall not” terms rather than “system shall” terms
 Differences
 no well-defined security life cycle yet
 security deals with generic threats rather than system specific hazards
Security Specification Stages
 Asset identification and evaluation
 data and programs identified with their level of protection
 degree of protection depends on asset value
 Threat analysis and risk assessment
 security threats identified and risks associated with each is estimated
 Threat assignment
 identified threats are related to assets so that asset has a list of associated threats
 Technology analysis
 available security technologies and their applicability against the threats
 Security requirements specification
 where appropriate these will identify the security technologies that may be used to
protect against different threats to the system