Fouttolerant

Fouttolerant refers to the ability of a system to continue operating correctly in the presence of faults. A fault is an abnormal condition that can cause components to fail or function incorrectly. A fouttolerant design aims for continuous availability and correctness, even when some parts malfunction, and is often contrasted with systems that fail completely or degrade rapidly under stress.

Key design principles include redundancy so spare components can take over, error detection and masking to prevent faults from spreading, isolation to prevent cascading failures, and mechanisms for failover or graceful degradation to maintain essential functions. Robustness is frequently achieved through systematic checks, fault containment, and recovery processes.

Common techniques span hardware and software. Hardware approaches include redundancy such as RAID, duplicate power supplies, and ECC memory. Software methods include checkpointing and restart, write-ahead logs or transaction logs, and watchdog timers. In networks and storage, replication, erasure coding, and consensus mechanisms are used to tolerate node or link failures. In distributed systems, fault tolerance often relies on replication and majority voting to maintain service despite multiple faulty components.

Assessing fouttolerance involves metrics such as availability, reliability, and recovery characteristics like MTBF (mean time between failures) and MTTR (mean time to repair), as well as data-loss and recovery objectives such as RPO (recovery point objective) and RTO (recovery time objective). Trade-offs include cost, complexity, latency, and the risk of correlated failures. Fouttolerant designs are common in critical infrastructure, data centers, aerospace, finance, and safety‑critical software.