Safety Criteria Flashcards
Define requirements:
Requirements is a description of what a system should do, written from the perspective of a user
Define specification:
Specification is a precise description of what a system or component of a system should do, written from the perspective of a developer of it
How are requirements written?
Requirements are written from the perspective of an outside user.
How are specification written?
Specifications are written in a way that one can verify (using testing or formal verification) whether the system fulfils it or not.
What is requirements specification?
It’s a specification of a system derived from requirements. It makes the requirement precise enough so that one can verify whether the system fulfils it.
What are the requirements of a system?
The descriptions of the services a system should carry out and of the constraints
What are user requirements?
Statements written by the future user of a software product, typically written in natural language or diagrams, of expectations of the system and the constraints it must operate under
What are system requirements?
Precise statements which sets out the system services and constraints in detail.
What is a requirement document?
A document which attempts to describe the requirements of a system in an unambiguous manner.
What typeof requirements might be included in a requirements document?
- Function requirements
- Non-functional requirements
- Domain requirements
- Safety requirements
What do functional requirements describe?
- the functions a system should provide
- how the system should react to particular inputs
- how the system should behave in particular situations
- what the system should not do
What are some function requirements that may be needed for an autopilot system?
- the need to measure acceleration
- the need to compute relative positions
What are non-functional requirements?
Constraints on the services or functions offered by the system e.g.:
- ease of maintenance
- size
- cost
What are domain requirements (Context of Operation)?
These describe the situation in which the system will be used e.g. temperature, education standard of operators
What are safety requirements?
Requirements which ensure adequate safety of a system
What does the safety requirements document set out?
Sets out the safety requirements of a system.
What are the different types of safety requirements?
- functional safety requirements
- non-functional safety requirements
- context of operation
What are functional safety requirements?
Services to be carried out by the system in order to guarantee safety. E.g. turning off a system when temperature exceeds limit
What a non-functional safety requirements?
Other requirements such as reliability and availability
What is context of operation for safety requirements?
Describes the context under which a system is supposed to operate, e.g. the location of a unit, admin who can control the system
What can most accidents in which software is involved be traced to?
Incomplete requirements
What does coding errors effect?
They have more of an effect on reliability and other qualities rather than on safety
Define verification:
The process of verifying that a software product meets its specification
Define validation:
The process of confirming that the specification guarantees the customer requirements AND
The process of confirming that the software product fulfils the customer requirements
What is the validation problem?
The gap between specification and requirements, where we cannot obtain the same level of guarantee as in verification
What is the goal of a specification?
To define in a clear manner, the precise operation of a system
What does a specification include?
- The functionality and performance of a system
- its interaction with other systems
- safety invariants of the system
- constraints of safety invariants on the design
What should an ideal specification be?
- correct
- complete
- consistent
- unambiguous
What are the three problems with natural language specifications?
1) Natural language is often ambiguous
2) Natural language specifications are much longer than mathematical formulations, making it easy to overlook something
3) You cannot apply automatic checks to specifications written in natural language
What is the solution to natural language specifications?
Formal specification languages
What are the two approaches for Formal Specification Languages?
1) Model-based specification languages
2) Algebraic specification languages
What is model-based specification languages?
- Based on a general model for representing programs
- System is constructed in the model using mathematical constructs
- System operations are defined by how they modify the system state
What is algebraic specification languages?
Systems are described in terms of operations and their relationships
What is a fault?
Defect within a system
What is an error?
A deviation from the required operation of the system or subsystem.
What is a system failure?
Occurs when the system fails to perform its required function.
Do faults lead to errors?
A fault may or may not result in an error
What are some examples of faults?
- failure of a hardware component
- coding error
- mistake in the design of a system
Will errors always lead to system failures?
Errors may or may not lead to a system failure depending on whether the system has been built in order to function even in the presence of errors
What is a hazard?
Situation in which there is actual or potential danger to people or to the environment
What is an accident?
Unintended event or sequence of events that cause death, injury, environmental or material damage.
What is an incident or near-miss?
Unintended event or sequence of events that does not result in loss but under different circumstances has the potential to do so.
What is dependability?
Property of a system that justifies placing one’s reliance on it
What is a dependable system?
System that has a high degree of dependability
What are the four main dimensions of dependability?
- Reliability
- Availability
- Safety
- Security
What are the related dimensions to the four main dimensions of dependability?
- Maintainability
- System integrity
- System recovery
- Failsafe operation
- Data integrity
What is reliability (dimension of dependability)?
The probability of a component or system to function correctly continuously over a given period of time under a given set of operation conditions.
What are the three forms of reliability?
- Hardware reliability
- Software reliability
- Operator reliability
What is availability (dimensions of dependability)?
Probability that a system is functioning correctly at any given time. Measured by the proportion of time the device is functioning correctly.
How do we calculate unavailability of a system?
1 - availability
What type of systems are high availability important for?
Systems which are activated in emergency cases only.
What is safety (dimensions of dependability)?
Property of a system that it will not endanger human life or the environment
What is a safety-related system?
System by which the safety of equipment or a plant is guaranteed.
What is security (dimension of dependability)?
Property of a system to protect itself from external attacks that may be intentional or unintentional
What may a lack of security compromise?
Safety, availability and reliability of a system.
Define exposure:
Possible loss or harm in a computing system
Define vulnerability:
Weakness of a computer-based system that may be exploited to cause loss or harm
Define attack:
Exploitation of a system vulnerability
Define threat:
Circumstance that have potential to cause loss or harm
Define control:
Protective measure that reduces system vulnerability
Define survivability:
Ability of a system to continue to deliver service while it is under attack.
What is maintenance (dimensions of dependability)?
Action taken to retain a system in or return it to its designed operating condition
What is maintainability?
The ability of a system to be maintained
What are the metrics relating to maintainability and availability?
- Mean time to repair
- Mean time to failure
- Mean time between failures
What is preventive maintaenance?
Trying to keep a system in good order and remove the effects of wear and ageing before they result in a system failure
What is corrective maintenance?
Aims to restore the system to its designed state following some form of failure
What is integrity of a system?
Ability of a system to detect faults in its own operation and to inform a human operator.
What is system recovery?
The ability of a system to restart itself quick after a failure was detected.
Define recoverability:
The average time to recover from failure
What is failsafe state?
Output state of a critical system, which is absolutely safe and the safety of which is maintained by minimal requirements of the system.
What is data integrity?
Ability of a system to prevent damage to its own database and to detect and possibly correct errors that occur.
What are the classification of Hazards?
- Severity
- Risk
- Nature (can it be controlled)
What are some methods of risk reduction?
- “Design out” the hazard (design systems to avoid hazard)
- Safety devices
- warning signs
- management techniques
What are interlocks?
Mechanisms which ensure that hazardous actions are only performed at when they are safe
What might interlocks control to ensure hazardous actions are only performed at safe time?
- Control equipment directly
- control guards
What are gaurds?
Mechanisms which keep people away from dangerous parts of a system until it is safe
What is a safety case?
A document that sets out the safety justification of a system.
What are safety case also know as?
Safety argument, safety justification or safety assessment report.
Why are standards important?
For case certification is it vital standards have been followed. Also for legal protection in case of an accident
