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What is Fault Tolerance?

Fault tolerance is a process that enables an operating system to respond to a failure in hardware or software. This fault-tolerance definition refers to the system’s ability to continue operating despite failures or malfunctions.

An operating system that offers a solid definition for faults cannot be disrupted by a single point of failure. It ensures business continuity and the high availability of crucial applications and systems regardless of any failures. 

How Does Fault Tolerance Work?

Fault tolerance can be built into a system to remove the risk of it having a single point of failure. To do so, the system must have no single component that, if it were to stop working effectively, would result in the entire system failing.

Fault tolerance is reliant on aspects like load balancing and failover, which remove the risk of a single point of failure. It will typically be part of the operating system’s interface, which enables programmers to check the performance of data throughout a transaction. 

A fault-tolerance process follows two core models:

Normal Functioning

This describes a situation when a fault-tolerant system encounters a fault but continues to function as usual. This means the system sees no change in performance metrics like throughput or response time.  

Graceful Degradation

Other types of fault-tolerant systems will go through graceful degradation of performance when certain faults occur. That means the impact the fault has on the system’s performance is proportionate to the fault severity. In other words, a small fault will only have a small impact on the system’s performance rather than causing the entire system to fail or have major performance issues.

Components of a Fault-tolerance System

The key benefit of fault tolerance is to minimize or avoid the risk of systems becoming unavailable due to a component error. This is particularly important in critical systems that are relied on to ensure people’s safety, such as air traffic control, and systems that protect and secure critical data and high-value transactions.

The core components to improving fault tolerance include:


If a system’s main electricity supply fails, potentially due to a storm that causes a power outage or affects a power station, it will not be possible to access alternative electricity sources. In this event, fault tolerance can be sourced through diversity, which provides electricity from sources like backup generators that take over when a main power failure occurs.

Some diverse fault-tolerance options result in the backup not having the same level of capacity as the primary source. This may, in some cases, require the system to ensure graceful degradation until the primary power source is restored.


Fault-tolerant systems use redundancy to remove the single point of failure. The system is equipped with one or more power supply units (PSUs), which do not need to power the system when the primary PSU functions as normal. In the event the primary PSU fails or suffers a fault, it can be removed from service and replaced by a redundant PSU, which takes over system function and performance. 

Alternatively, redundancy can be imposed at a system level, which means an entire alternate computer system is in place in case a failure occurs.


Replication is a more complex approach to achieving fault tolerance. It involves using multiple identical versions of systems and subsystems and ensuring their functions always provide identical results. If the results are not identical, then a democratic procedure is used to identify the faulty system. Alternatively, a procedure can be used to check for a system that shows a different result, which indicates it is faulty. 

Replication can either take place at the component level, which involves multiple processors running simultaneously, or at the system level, which involves identical computer systems running simultaneously.

Elements of Fault-tolerant Systems

Fault-tolerant systems also use backup components, which automatically replace failed components to prevent a loss of service. These backup components include:

Hardware Systems

Hardware systems can be backed up by systems that are identical or equivalent to them. A typical example is a server made fault-tolerant by deploying an identical server that runs in parallel to it and mirrors all its operations, such as the redundant array of inexpensive disks (RAID), which combines physical disk components to achieve redundancy and improved performance.

Software Systems

Software systems can be made fault-tolerant by backing them up with other software. A common example is backing up a database that contains customer data to ensure it can continuously replicate onto another machine. As a result, in the event that a primary database fails, normal operations will continue because they are automatically replicated and redirected onto the backup database.

Power Sources

Power sources can also be made fault-tolerant by using alternative sources to support them. One approach is to run devices on an uninterruptible power supply (UPS). Another is to use backup power generators that ensure storage and hardware, heating, ventilation, and air conditioning (HVAC) continue to operate as normal if the primary power source fails.

Factors To Consider in Fault Tolerance

There are several factors that affect organizations’ decision to implement a fault-tolerant system, including:


The biggest disadvantage of adopting a fault-tolerant approach is the cost of doing so. Organizations must think carefully about the cost elements of a fault-tolerant or highly available system.

Fault-tolerant systems require organizations to have multiple versions of system components to ensure redundancy, extra equipment like backup generators, and additional hardware. These components need regular maintenance and testing. They also take up valuable space in data centers. 

Quality Degradation

One way around the cost of fault tolerance is to opt for more cost-effective but lower-quality redundant components. This approach can inadvertently increase maintenance and support costs and make the system less reliable. To avoid such a situation, organizations must monitor the performance of individual components and keep an eye on their lifespan in relation to their cost.

Testing and Fault-detection Difficulties

Fault tolerance inevitably makes it more difficult to know if components are performing to the expected level because failures do not automatically result in the system going down. As a result, organizations will require additional resources and expenditure to continuously test and monitor their system health for faults. 

Additionally, they may need to acquire or develop custom software and procedures to carry out these detection and testing tasks. 

How Fortinet Can Help?

Fortinet helps organizations achieve fault tolerance through its FortiGate next-generation firewalls (NGFWs). As an example, Fortinet NGFWs have been core to delivering fault-tolerant RingCentral access, which allowed unified communications provider RingCentral to achieve fault tolerance across its global data centers.

This was made possible through the implementation of a highly fault-tolerant network of active and backup virtual private networks (VPNs). The solutions deployed included classic FortiGate network security features, failover from a primary to backup wide-area network (WAN), failover between data centers, and many more.