Fault tolerance is the ability of a system to behave in a well-defined manner despite failures. A failure moves the system from a legal state toward an illegal state; intolerable states are beyond the guarantees being considered. section 3.0 section 3.1
A useful mental model treats the system as states connected by normal and failure transitions. Recovery is the sequence of transitions that returns an illegal state to a legal state. section 3.1
Classify tolerance by the guarantees preserved during failure:
- Masking tolerance preserves both safety and liveness. section 3.2
- Nonmasking tolerance preserves liveness while temporarily compromising safety. section 3.2
- Fail-safe tolerance preserves safety while giving up liveness. section 3.2
- Preserving neither property is not fault tolerance. section 3.2.4
A single-variable system
Let process P contain x ∈ {0, 1, 2, 3}. Define safety as x being 1 or 2, and liveness as eventually reaching both values. Normal transitions alternate 1 and 2; repair moves 0 to 1; a failure moves any nonzero value to 0. The system is nonmasking: x = 0 violates safety but can recover and remain live. State 3 is intolerable because no return guarantee exists. section 3.2.4
