Fault tolerance
Introduction
Fault tolerance is a property of a system that enables it to continue functioning in the event of the failure of 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 naively designed system, in which even a small failure can cause total system breakdown. Fault tolerance is particularly sought after in high-availability or life-critical systems.
Overview
The ability of maintaining functionality when portions of a system break down is referred to as fault tolerance. It is a crucial aspect of computer science, electrical engineering, and many other fields where high availability is a necessity, such as in life-critical systems. The quality of a system's operation may decrease in the event of a failure, but the decrease is often proportional to the severity of the failure. This is in contrast to a system that is not fault-tolerant, where even a minor failure could lead to a complete system breakdown.
Principles of Fault Tolerance
Fault tolerance revolves around two key concepts: redundancy and diversity. Redundancy involves the provision of multiple interchangeable components so that in the event of a component failure, another can take its place. This can be achieved through various means, including hardware redundancy, software redundancy, and information redundancy.
Diversity, on the other hand, involves using different approaches to achieve the same result. This way, if one approach fails, another can be used. This can be achieved through various means, such as design diversity and environmental diversity.
Redundancy
Redundancy is a common method of achieving fault tolerance. It involves having multiple interchangeable components in a system so that if one component fails, another can take its place. There are several types of redundancy, including hardware redundancy, software redundancy, and information redundancy.
Hardware Redundancy
Hardware redundancy involves having duplicate pieces of hardware that perform the same functions. If one piece of hardware fails, the system can switch to the redundant hardware. This can be achieved through various means, such as standby redundancy, where the redundant hardware is kept idle until needed, and parallel redundancy, where all hardware is active and the output from the failed component is simply ignored.
Software Redundancy
Software redundancy involves having duplicate pieces of software that perform the same functions. If one piece of software fails, the system can switch to the redundant software. This can be achieved through various means, such as version redundancy, where different versions of the software are used, and module redundancy, where different software modules are used.
Information Redundancy
Information redundancy involves having duplicate pieces of information in a system. If one piece of information is lost or corrupted, the system can use the redundant information. This can be achieved through various means, such as error detection and correction codes, and data mirroring.
Diversity
Diversity is another method of achieving fault tolerance. It involves using different approaches to achieve the same result, so that if one approach fails, another can be used. There are several types of diversity, including design diversity and environmental diversity.
Design Diversity
Design diversity involves using different designs to achieve the same result. This can involve using different algorithms, different hardware designs, or different software designs. If one design fails, the system can switch to a different design.
Environmental Diversity
Environmental diversity involves using different environments to achieve the same result. This can involve using different operating systems, different hardware platforms, or different physical locations. If one environment fails, the system can switch to a different environment.
Fault Tolerance Techniques
There are several techniques that can be used to achieve fault tolerance. These include error detection, error recovery, and fault masking.
Error Detection
Error detection involves identifying errors that occur in a system. This can be achieved through various means, such as parity checks, checksums, and cyclic redundancy checks.
Error Recovery
Error recovery involves correcting errors that have been detected. This can be achieved through various means, such as retransmission of data, use of error correction codes, and rollback of transactions.
Fault Masking
Fault masking involves preventing errors from affecting the system's output. This can be achieved through various means, such as voting systems, where the output of a system is determined by the majority of its components, and error correcting codes, where errors can be corrected without the need for retransmission of data.
Fault Tolerance in Practice
Fault tolerance is a critical aspect of many systems, particularly those that are life-critical or high-availability. Examples of such systems include air traffic control systems, nuclear power plants, and data centers.
Air Traffic Control Systems
Air traffic control systems are a prime example of a life-critical system that requires fault tolerance. These systems must be able to continue operating even in the event of a component failure, as any downtime could have catastrophic consequences.
Nuclear Power Plants
Nuclear power plants are another example of a life-critical system that requires fault tolerance. These systems must be able to continue operating even in the event of a component failure, as any downtime could lead to a nuclear meltdown.
Data Centers
Data centers are an example of a high-availability system that requires fault tolerance. These systems must be able to continue operating even in the event of a component failure, as any downtime could lead to significant financial loss.