1. CIS 376 Bruce R. Maxim UM-Dearborn
Software Reliability
CIS 376
Bruce R. Maxim
UM-Dearborn

2. Functional and Non-functional Requirements
System functional requirements may specify error checking, recovery features, and system failure protection
System reliability and availability are specified as part of the non-functional requirements for the system.

3. System Reliability Specification
Hardware reliability
probability a hardware component fails
Software reliability
probability a software component will produce an incorrect output
software does not wear out
software can continue to operate after a bad result
Operator reliability
probability system user makes an error

4. 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

5. Functional Reliability Requirements
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.

6. Non-functional Reliability Specification
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.

7. Hardware Reliability Metrics
Hardware metrics are not suitable for software since its metrics are based on notion of component failure
Software failures are often design failures
Often the system is available after the failure has occurred
Hardware components can wear out

8. 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

9. Reliability Metrics - part 1
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

10. Reliability Metrics - part 2
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)

11. Time Units Raw Execution Time Calendar Time Number of Transactions
non-stop system
Calendar Time
If the system has regular usage patterns
Number of Transactions
demand type transaction systems

12. 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)

13. 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

14. 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

15. 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)

16. Failure Consequences - part 1
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

17. Failure Consequences - part 2
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

18. 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

19. 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

20. Examples

21. Specification Validation
It is impossible to empirically validate high reliability specifications
No database corruption really means POFOD class < 1 in 200 million
If each transaction takes 1 second to verify, simulation of one day’s transactions takes 3.5 days

22. 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)

23. 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

24. 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

25. Safety Life Cycle - part 1
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

26. Safety Life Cycle - part 2
Deployment
safety validation
installation and commissioning
Operation and maintenance
System decommissioning

27. 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

28. 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

29. Fault-tree Analysis
Hazard analysis method that starts with an identified fault and works backwards to the cause of the fault
Can be used at all stages of hazard analysis
It is a top-down technique, that may be combined with a bottom-up hazard analysis techniques that start with system failures that lead to hazards

30. Fault-tree Analysis Steps
Identify hazard
Identify potential causes of hazards
Link combinations of alternative causes using “or” or “and” symbols as appropriate
Continue process until “root” causes are identified (result will be an and/or tree or a logic circuit) the causes are the “leaves”

31. How does it work?
What would a fault tree look like for a fault tree describing the causes for a hazard like “data deleted”?

32. 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)

33. 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)

34. 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

35. 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

36. Security Specification Stages - part 1
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

37. Security Specification Stages - part 2
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