# What is Fault Tolerance?

> Fault tolerance is a system property that allows a required function to continue correctly when one or more anticipated hardware or software faults occur.

- Canonical URL: https://yellowcube.eu/glossary/fault-tolerance/
- Publisher: Yellow Cube
- Language: en
- Contact: hello@yellowcube.eu

## Content

A fault-tolerant design detects, contains, masks, or reconfigures around faults so operation remains within stated correctness, capacity, timing, and safety limits, sometimes at a deliberately degraded service level.

The requirement must define which faults, combinations, and durations the system should tolerate. Designs may use replication, voting, error correction, independent power or communication paths, diverse implementations, state reconciliation, and graceful degradation, supported by monitoring and repair.

### Key points

- **Define correct service:** Specify essential functions, permitted degradation, fault assumptions, detection time, recovery behavior, and the point at which safe shutdown is preferable to continued operation.
- **Create independence:** Separate replicas and supporting power, networks, identities, administration, and locations where the threat model requires it; consider diversity against common defects.
- **Protect state:** Control replication, ordering, quorum, consistency, corruption detection, and reintegration so redundant components do not spread bad data or create conflicting actions.
- **Important limitation:** Redundancy alone is not fault tolerance. Common-mode failures, shared dependencies, design defects, correlated attacks, exhausted capacity, and incorrect failure detection can defeat every replica, while added complexity can introduce new faults.

### Related terms

[Failover](<https://yellowcube.eu/glossary/failover/>) · [Cyber resilience](<https://yellowcube.eu/glossary/cyber-resilience/>) · [Business continuity](<https://yellowcube.eu/glossary/business-continuity/>) · [Disaster recovery](<https://yellowcube.eu/glossary/disaster-recovery/>) · [Recovery time objective (RTO)](<https://yellowcube.eu/glossary/recovery-time-objective/>)

### Sources

[NIST, Fault Tolerant](https://csrc.nist.gov/glossary/term/fault_tolerant) · [NIST SP 800-160 Vol. 2 Rev. 1, Developing Cyber-Resilient Systems](https://csrc.nist.gov/pubs/sp/800/160/v2/r1/final) · [NIST SP 800-82 Rev. 3, Guide to Operational Technology Security](https://csrc.nist.gov/pubs/sp/800/82/r3/final)

## Attribution and scope

This Markdown representation is generated from the same approved content records as the canonical HTML page. Cite the canonical URL above when referencing this material. Product and service descriptions are informational; confirm project-specific requirements with Yellow Cube.