Systems
Ian Critical Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 1
Objectives
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To explain what is meant by a critical system where system failure can have severe human or economic consequence. To explain four dimensions of dependability availability, reliability, safety and security. To explain that, to achieve dependability, you need to avoid mistakes, detect and remove errors and limit damage caused by failure.
Software Engineering, 7th edition. Chapter 3 Slide 2
Ian Sommerville 2004
Topics covered
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A simple safety-critical system System dependability Availability and reliability Safety Security
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 3
Critical Systems
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Safety-critical systems
Failure results in loss of life, injury or damage to the environment; Chemical plant protection system; Failure results in failure of some goal-directed activity; Spacecraft navigation system; Failure results in high economic losses; Customer accounting system in a bank;
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Mission-critical systems
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Business-critical systems
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 4
System dependability
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For critical systems, it is usually the case that the most important system property is the dependability of the system. The dependability of a system reflects the user's degree of trust in that system. It reflects the extent of the user's confidence that it will operate as users expect and that it will not `fail' in normal use. Usefulness and trustworthiness are not the same thing. A system does not have to be trusted to be useful.
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 5
Importance of dependability
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Systems that are not dependable and are unreliable, unsafe or insecure may be rejected by their users. The costs of system failure may be very high. Undependable systems may cause information loss with a high consequent recovery cost.
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 6
Development methods for critical systems
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The costs of critical system failure are so high that development methods may be used that are not cost-effective for other types of system. Examples of development methods
Formal methods of software development Static analysis; examine program without execution; Effigy External quality assurance
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 7
Socio-technical critical systems
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Hardware failure
Hardware fails because of design and manufacturing errors or because components have reached the end of their natural life. Software fails due to errors in its specification, design or implementation. Contingency. Human operators make mistakes. Now perhaps the largest single cause of system failures.
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Software failure
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Operational failure
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 8
A software-controlled insulin pump
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Used by diabetics to simulate the function of the pancreas which manufactures insulin, an essential hormone that metabolises blood glucose. Measures blood glucose (sugar) using a micro-sensor and computes the insulin dose required to metabolise the glucose.
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 9
Insulin pump organisation
Insulin reservoir Needle assembly
Pump
Clock
Sensor
Controller
Alarm
Display1
Display2
Power supply
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 10
Insulin pump data-flow
Blood Blood parameters Blood sugar sensor Blood sugar analysis Blood sugar level Insulin requirement computation Insulin Insulin pump Pump control commands Insulin delivery controller Insulin requirement
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 11
Dependability requirements
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The system shall be available to deliver insulin when required to do so. The system shall perform reliability and deliver the correct amount of insulin to counteract the current level of blood sugar; biological variability. The essential safety requirement is that excessive doses of insulin should never be delivered as this is potentially life threatening.
Software Engineering, 7th edition. Chapter 3 Slide 12
Ian Sommerville 2004
Dependability
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The dependability of a system equates to its trustworthiness. A dependable system is a system that is trusted by its users. Principal dimensions of dependability are:
Availability; Reliability; Safety; Security
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 13
Dimensions of dependability
Dependability
Availability
Reliability
Safety
Security
The ability of the system to deliver services when requested
The ability of the system to deliver services as specified
The ability of the system to operate without catastrophic failure
The ability of the system to protect itelf against accidental or deliberate intrusion
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 14
Real World Example
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Infusion pump Potential problems Biological variability
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 15
Other dependability properties
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Repairability
Reflects the extent to which the system can be repaired in the event of a failure Reflects the extent to which the system can be adapted to new requirements; Reflects the extent to which the system can deliver services whilst under hostile attack; Reflects the extent to which user input errors can be avoided and tolerated.
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Maintainability
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Survivability
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Error tolerance
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 16
Maintainability
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A system attribute that is concerned with the ease of repairing the system after a failure has been discovered or changing the system to include new features Very important for critical systems as faults are often introduced into a system because of maintenance problems (regression analysis, human error) Maintainability is distinct from other dimensions of dependability because it is a static and not a dynamic system attribute. I do not cover it in this course.
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 17
Survivability
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The ability of a system to continue to deliver its services to users in the face of deliberate or accidental attack This is an increasingly important attribute for distributed systems whose security can be compromised Survivability subsumes the notion of resilience - the ability of a system to continue in operation in spite of component failures
Software Engineering, 7th edition. Chapter 3 Slide 18
Ian Sommerville 2004
Dependability vs performance
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Untrustworthy systems may be rejected by their users System failure costs may be very high It is very difficult to tune systems to make them more dependable It may be possible to compensate for poor performance Untrustworthy systems may cause loss of valuable information
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 19
Dependability costs
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Dependability costs tend to increase exponentially as increasing levels of dependability are required There are two reasons for this
The use of more expensive development techniques and hardware that are required to achieve the higher levels of dependability The increased testing and system validation that is required to convince the system client that the required levels of dependability have been achieved
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 20
Costs of increasing dependability
Low
Medium
High
V ery high
Ultra-high
Dependability
Ian Sommerville 2004 Software Engineering, 7th edition. Chapter 3 Slide 21
Dependability economics
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Because of very high costs of dependability achievement, it may be more cost effective to accept untrustworthy systems and pay for failure costs However, this depends on social and political factors. A reputation for products that can't be trusted may lose future business Depends on system type - for business systems in particular, modest levels of dependability may be adequate
Software Engineering, 7th edition. Chapter 3 Slide 22
Ian Sommerville 2004
Availability and reliability
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Reliability
The probability of failure-free system operation over a specified time in a given environment for a given purpose The probability that a system, at a point in time, will be operational and able to deliver the requested services
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Availability
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Both of these attributes can be expressed quantitatively
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 23
Availability and reliability
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It is sometimes possible to subsume system availability under system reliability
Obviously if a system is unavailable it is not delivering the specified system services
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However, it is possible to have systems with low reliability that must be available. So long as system failures can be repaired quickly and do not damage data, low reliability may not be a problem, telephone system Availability takes repair time into account
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 24
Reliability terminology
Term System failure Description An event that occurs at some point in time when the system does not deliver a service as expected by its users An erroneous system state that can lead to system behaviour that is unexpected by system users. A characteristic of a software system that can lead to a system error. For example, failure to initialise a variable could lead to that variable having the wrong value when it is used. Human behaviour that results in the introduction of faults into a system.
System error System fault
Human error or mistake
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 25
Faults and failures
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Failures are a usually a result of system errors that are derived from faults in the system However, faults do not necessarily result in system errors
The faulty system state may be transient and `corrected' before an error arises The error can be corrected by built-in error detection and recovery The failure can be protected against by built-in protection facilities. These may, for example, protect system resources from system errors
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Errors do not necessarily lead to system failures
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 26
Perceptions of reliability
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The formal definition of reliability does not always reflect the user's perception of a system's reliability
The assumptions that are made about the environment where a system will be used may be incorrect
Usage of a system in an office environment is likely to be quite different from usage of the same system in a university environment
The consequences of system affects failures the perception of reliability
Unreliable windscreen wipers in a car may be irrelevant in a dry climate Failures that have serious consequences (such as an engine breakdown in a car) are given greater weight by users than failures that are inconvenient rural areas, etc
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 27
Reliability achievement
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Fault avoidance
Development technique are used that either minimise the possibility of mistakes or trap mistakes before they result in the introduction of system faults Verification and validation techniques that increase the probability of detecting and correcting errors before the system goes into service are used Run-time techniques are used to ensure that system faults do not result in system errors and/or that system errors do not lead to system failures
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Fault detection and removal
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Fault tolerance
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 28
Reliability modelling
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You can model a system as an input-output mapping where some inputs will result in erroneous outputs The reliability of the system is the probability that a particular input will lie in the set of inputs that cause erroneous outputs Different people will use the system in different ways so this probability is not a static system attribute but depends on the system's environment
Software Engineering, 7th edition. Chapter 3 Slide 29
Ian Sommerville 2004
Input/output mapping
Inputs causing erroneous outputs Input set Ie
Program
Erroneous outputs Output set Oe
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 30
Reliability perception
Possible inputs User 1 Erroneous inputs
User 3
User 2
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 31
Reliability improvement
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Removing X% of the faults in a system will not necessarily improve the reliability by X%. A study at IBM showed that removing 60% of product defects resulted in a 3% improvement in reliability Program defects may be in rarely executed sections of the code so may never be encountered by users. Removing these does not affect the perceived reliability A program with known faults may therefore still be seen as reliable by its users
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 32
Safety
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Safety is a property of a system that reflects the system's ability to operate, normally or abnormally, without danger of causing human injury or death and without damage to the system's environment It is increasingly important to consider software safety as more and more devices incorporate software-based control systems Safety requirements are exclusive requirements i.e. they exclude undesirable situations rather than specify required system services
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 33
Safety criticality
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Primary safety-critical systems
Embedded software systems whose failure can cause the associated hardware to fail and directly threaten people. Systems whose failure results in faults in other systems which can threaten people
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Secondary safety-critical systems
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Discussion here focuses on primary safety-critical systems
Secondary safety-critical systems can only be considered on a one-off basis
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 34
Safety and reliability
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Safety and reliability are related but distinct
In general, reliability and availability are necessary but not sufficient conditions for system safety - what is sufficient?
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Reliability is concerned with conformance to a given specification and delivery of service Safety is concerned with ensuring system cannot cause damage irrespective of whether or not it conforms to its specification
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 35
Unsafe reliable systems
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Specification errors
If the system specification is incorrect then the system can behave as specified but still cause an accident Hard to anticipate in the specification
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Hardware failures generating spurious inputs
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Context-sensitive commands i.e. issuing the right command at the wrong time
Often the result of operator error
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 36
Safety terminology
Term Accident (or mishap) Hazard Damage Hazard severity Hazard probability Risk Definition An unplanned event or sequence of events which results in human death or injury, damage to property or to the environment. A computer-controlled machine injuring its operator is an example of an accident. A condition with the potential for causing or contributing to an accident. A failure of the sensor that detects an obstacle in front of a machine is an example of a hazard. A measure of the loss resulting from a mishap. Damage can range from many people killed as a result of an accident to minor injury or property damage. An assessment of the worst possible damage that could result from a particular hazard. Hazard severity can range from catastrophic where many people are killed to minor where only minor damage results. The probability of the events occurring which create a hazard. Probability values tend to be arbitrary but range from probable (say 1/100 chance of a hazard occurring) to implausible (no conceivable situations are likely where the hazard could occur). This is a measure of the probability that the system will cause an accident. The risk is assessed by considering the hazard probability, the hazard severity and the probability that a hazard will result in an accident.
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 37
Safety achievement
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Hazard avoidance
The system is designed so that some classes of hazard simply cannot arise. The system is designed so that hazards are detected and removed before they result in an accident The system includes protection features that minimise the damage that may result from an accident
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Hazard detection and removal
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Damage limitation
Ian Sommerville 2004
Software Engineering, 7th edition. Chapter 3
Slide 38
Normal accidents
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Accidents in complex systems rarely have a single cause as these systems are designed to be resilient to a single point of failure
Designing systems so that a single point of failure does not cause an accident is a fundamental principle of safe systems design
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Almost all accidents are a result of combinations of malfunctions It is probably the case that anti...