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Fault-tolerance or graceful degradation is the property that enables a system to continue operating properly in the event of the failure of (or one or more faults within) some of its components. If its operating quality decreases at all, the decrease is proportional to the severity of the failure, as compared to a naïvely-designed system in which even a small failure can cause total breakdown. Fault-tolerance is particularly sought-after in high-availability or life-critical systems.
A fault-tolerant design enables a system to continue its intended operation, possibly at a reduced level, rather than failing completely, when some part of the system fails. The term is most commonly used to describe computer systems designed to continue more or less fully operational with, perhaps, a reduction in throughput or an increase in response time in the event of some partial failure. That is, the system as a whole is not stopped due to problems either in the hardware or the software. An example in another field is a motor vehicle designed so it will continue to be drivable if one of the tires is punctured. A structure is able to retain its integrity in the presence of damage due to causes such as fatigue, corrosion, manufacturing flaws, or impact.
Fault-tolerance is not just a property of individual machines; it may also characterise the rules by which they interact. For example, the Transmission Control Protocol (TCP) is designed to allow reliable two-way communication in a packet-switched network, even in the presence of communications links which are imperfect or overloaded. It does this by requiring the endpoints of the communication to expect packet loss, duplication, reordering and corruption, so that these conditions do not damage data integrity, and only reduce throughput by a proportional amount.
Recovery from errors in fault-tolerant systems can be characterised as either roll-forward or roll-back. When the system detects that it has made an error, roll-forward recovery takes the system state at that time and corrects it, to be able to move forward. Roll-back recovery reverts the system state back to some earlier, correct version, for example using checkpointing, and moves forward from there. Roll-back recovery requires that the operations between the checkpoint and the detected erroneous state can be made idempotent. Some systems make use of both roll-forward and roll-back recovery for different errors or different parts of one error.
Within the scope of an individual system, fault-tolerance can be achieved by anticipating exceptional conditions and building the system to cope with them, and, in general, aiming for self-stabilization so that the system converges towards an error-free state. However, if the consequences of a system failure are catastrophic, or the cost of making it sufficiently reliable is very high, a better solution may be to use some form of duplication. In any case, if the consequence of a system failure is so catastrophic, the system must be able to use reversion to fall back to a safe mode. This is similar to roll-back recovery but can be a human action if humans are present in the loop.
If each component, in turn, can continue to function when one of its subcomponents fails, this will allow the total system to continue to operate, as well. Using a passenger vehicle as an example, a car can have "run-flat" tires, which each contain a solid rubber core, allowing them to be used even if a tire is punctured. The punctured "run-flat" tire may be used for a limited time at a reduced speed.
Redundancy is the provision of functional capabilities that would be unnecessary in a fault-free environment. This can be backup components which automatically "kick in" should one component fail. For example, large cargo trucks can lose a tire without any major consequences. They have many tires, and no one tire is critical (with the exception of the front tires, which are used to steer). The idea of incorporating redundancy in order to improve the reliability of a system was pioneered by John von Neumann in the 1950s.
Two kinds of redundancy are possible: space redundancy and time redundancy. Space redundancy provides additional components, functions, or data items that are unnecessary for fault-free operation. Space redundancy is further classified into hardware, software and information redundancy, depending on the type of redundant resources added to the system. In time redundancy the computation or data transmission is repeated and the result is compared to a stored copy of the previous result.
Providing fault-tolerant design for every component is normally not an option. Associated redundancy brings a number of penalties: increase in weight, size, power consumption, cost, as well as time to design, verify, and test. Therefore, a number of choices have to be examined to determine which components should be fault-tolerant:
- How critical is the component? In a car, the radio is not critical, so this component has less need for fault-tolerance.
- How likely is the component to fail? Some components, like the drive shaft in a car, are not likely to fail, so no fault-tolerance is needed.
- How expensive is it to make the component fault-tolerant? Requiring a redundant car engine, for example, would likely be too expensive both economically and in terms of weight and space, to be considered.
An example of a component that passes all the tests is a car's occupant restraint system. While we do not normally think of the primary occupant restraint system, it is gravity. If the vehicle rolls over or undergoes severe g-forces, then this primary method of occupant restraint may fail. Restraining the occupants during such an accident is absolutely critical to safety, so we pass the first test. Accidents causing occupant ejection were quite common before seat belts, so we pass the second test. The cost of a redundant restraint method like seat belts is quite low, both economically and in terms or weight and space, so we pass the third test. Therefore, adding seat belts to all vehicles is an excellent idea. Other "supplemental restraint systems", such as airbags, are more expensive and so pass that test by a smaller margin.
The basic characteristics of fault tolerance require:
- No single point of failure - If a system experiences a failure, it must continue to operate without interruption during the repair process
- Fault isolation to the failing component - When a failure occurs, the system must be able to isolate the failure to the offending component. This requires the addition of dedicated failure detection mechanisms that exist only for the purpose of fault isolation. Recovery from a fault condition requires classifying the fault or failing component. The National Institute of Standards and Technology (NIST) categorizes faults based on Locality, Cause, Duration and Effect.
- Fault containment to prevent propagation of the failure - Some failure mechanisms can cause a system to fail by propagating the failure to the rest of the system. An example of this kind of failure is the "Rogue transmitter" which can swamp legitimate communication in a system and cause overall system failure. Mechanisms that isolate a rogue transmitter or failing component to protect the system are required.
- Availability of reversion modes
In addition, fault tolerant systems are characterized in terms of both planned service outages and unplanned service outages. These are usually measured at the application level and not just at a hardware level. The figure of merit is called availability and is expressed as a percentage. For example, a five nines system would statistically provide 99.999% availability.
Fault-tolerant systems are typically based on the concept of redundancy.
Spare components addresses the first fundamental characteristic of fault-tolerance in three ways:
- Replication: Providing multiple identical instances of the same system or subsystem, directing tasks or requests to all of them in parallel, and choosing the correct result on the basis of a quorum;
- Redundancy: Providing multiple identical instances of the same system and switching to one of the remaining instances in case of a failure (failover);
- Diversity: Providing multiple different implementations of the same specification, and using them like replicated systems to cope with errors in a specific implementation.
A lockstep fault-tolerant machine uses replicated elements operating in parallel. At any time, all the replications of each element should be in the same state. The same inputs are provided to each replication, and the same outputs are expected. The outputs of the replications are compared using a voting circuit. A machine with two replications of each element is termed Dual Modular Redundant (DMR). The voting circuit can then only detect a mismatch and recovery relies on other methods. A machine with three replications of each element is termed Triple Modular Redundancy (TMR). The voting circuit can determine which replication is in error when a two-to-one vote is observed. In this case, the voting circuit can output the correct result, and discard the erroneous version. After this, the internal state of the erroneous replication is assumed to be different from that of the other two, and the voting circuit can switch to a DMR mode. This model can be applied to any larger number of replications.
Lockstep fault tolerant machines are most easily made fully synchronous, with each gate of each replication making the same state transition on the same edge of the clock, and the clocks to the replications being exactly in phase. However, it is possible to build lockstep systems without this requirement.
Bringing the replications into synchrony requires making their internal stored states the same. They can be started from a fixed initial state, such as the reset state. Alternatively, the internal state of one replica can be copied to another replica.
One variant of DMR is pair-and-spare. Two replicated elements operate in lockstep as a pair, with a voting circuit that detects any mismatch between their operations and outputs a signal indicating that there is an error. Another pair operates exactly the same way. A final circuit selects the output of the pair that does not proclaim that it is in error. Pair-and-spare requires four replicas rather than the three of TMR, but has been used commercially.
Fault-tolerant design's advantages are obvious, while many of its disadvantages are not:
- Interference with fault detection in the same component. To continue the above passenger vehicle example, it may not be obvious to the driver when a tire has been punctured, with either of the fault-tolerant systems. This is usually handled with a separate "automated fault detection system". In the case of the tire, an air pressure monitor detects the loss of pressure and notifies the driver. The alternative is a "manual fault detection system", such as manually inspecting all tires at each stop.
- Interference with fault detection in another component. Another variation of this problem is when fault-tolerance in one component prevents fault detection in a different component. For example, if component B performs some operation based on the output from component A, then fault-tolerance in B can hide a problem with A. If component B is later changed (to a less fault-tolerant design) the system may fail suddenly, making it appear that the new component B is the problem. Only after the system has been carefully scrutinized will it become clear that the root problem is actually with component A.
- Reduction of priority of fault correction. Even if the operator is aware of the fault, having a fault-tolerant system is likely to reduce the importance of repairing the fault. If the faults are not corrected, this will eventually lead to system failure, when the fault-tolerant component fails completely or when all redundant components have also failed.
- Test difficulty. For certain critical fault-tolerant systems, such as a nuclear reactor, there is no easy way to verify that the backup components are functional. The most infamous example of this is Chernobyl, where operators tested the emergency backup cooling by disabling primary and secondary cooling. The backup failed, resulting in a core meltdown and massive release of radiation.
- Cost. Both fault-tolerant components and redundant components tend to increase cost. This can be a purely economic cost or can include other measures, such as weight. Manned spaceships, for example, have so many redundant and fault-tolerant components that their weight is increased dramatically over unmanned systems, which don't require the same level of safety.
- Inferior components. A fault-tolerant design may allow for the use of inferior components, which would have otherwise made the system inoperable. While this practice has the potential to mitigate the cost increase, use of multiple inferior components may lower the reliability of the system to a level equal to, or even worse than, a comparable non-fault-tolerant system.
Hardware fault-tolerance sometimes requires that broken parts can be taken out and replaced with new parts while the system is still operational (in computing known as hot swapping). Such a system implemented with a single backup is known as single point tolerant, and represents the vast majority of fault-tolerant systems. In such systems the mean time between failures should be long enough for the operators to have time to fix the broken devices (mean time to repair) before the backup also fails. It helps if the time between failures is as long as possible, but this is not specifically required in a fault-tolerant system.
Fault-tolerance is notably successful in computer applications. Tandem Computers built their entire business on such machines, which used single point tolerance to create their NonStop systems with uptimes measured in years.
Fail-safe architectures may encompass also the computer software, for example by process replication (computer science).
Data formats may also be designed to degrade gracefully. HTML for example, is designed to be forward compatible, allowing new HTML entities to be ignored by Web browsers which do not understand them without causing the document to be unusable.
There is a difference between fault tolerance and systems that rarely have problems. For instance, the Western Electric crossbar systems had failure rates of two hours per forty years, and therefore were highly fault resistant. But when a fault did occur they still stopped operating completely, and therefore were not fault-tolerant.
- Byzantine fault tolerance
- Capillary routing
- Cluster (computing)
- Control reconfiguration
- Data redundancy
- Defence in depth
- Elegant degradation
- Error detection and correction
- Error-tolerant design
- Fail soft
- Failure transparency[clarification needed]
- fall back and forward
- Fault-tolerant computer systems
- Graceful degradation
- Intrusion tolerance
- List of system quality attributes
- Progressive Enhancement
- Resilience (ecology)
- Resilience (network)
- Rhizome (Philosophy)
- Safe-life design
- Separation of protection and security
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- Dubrova, E. (2013). "Fault-Tolerant Design", Springer, 2013, ISBN 978-1-4614-2112-2
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- P. J. Denning (December 1976). "Fault tolerant operating systems". ACM Computing Surveys (CSUR) 8 (4): 359–389. doi:10.1145/356678.356680. ISSN 0360-0300.
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