Glossary
Failure domain
The set of components that fail together, and therefore must be reasoned about together.
Definition
A failure domain is the set of components that share a failure cause: a region, an availability zone, a connection pool, a worker fleet, a provider account, a DNS resolver. Components in the same domain do not provide redundancy for each other, however many of them there are.
The problem
Redundancy is routinely counted in instances rather than in domains. Two workers on one host, two regions behind one control plane, or two "independent" probes issued by one process are one failure domain wearing several names - and a quorum computed across them confirms a single opinion.
Why it matters
Failure-domain reasoning is what turns "we have redundancy" into a statement that can be checked. It also bounds blast radius: isolating a dependency into its own domain is the difference between a degraded feature and a degraded product.
Practical example
Two scheduling labels emitted by one worker are one observation point. Treating their agreement as a quorum reports one machine’s opinion as independent confirmation, which is why RELIASTRA’s detection policy distinguishes a single observation topology from a multi-point one and applies a different rule to each.
How RELIASTRA approaches it
Every observation carries the origin that produced it, and the incident detection rule is explicit about topology: persistence when there is one observation point, agreement across distinct points when there are genuinely several. Labels are never promoted into independence.
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