MTBF and MTTF (mean time between failures)

MTBF is the average time a system runs between failures; MTTF is the average time until a part that is not repaired fails for good.

Both come from hardware reliability, where a disk or a pump has a rated lifetime. In software they are useful as trends, not as promises: failures depend on releases, traffic and dependencies more than on wear. Teams that ship often usually care more about MTTR, how fast they recover, than about stretching MTBF, because frequent small releases with fast rollback beat rare big ones. Availability = MTBF / (MTBF + MTTR) links the two.

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Related terms

MTTR (mean time to recovery): The average time from a production failure to the service working again for users; one of the DORA metrics of software delivery.
MTTD and MTTA (time to detect and acknowledge): MTTD is the average time from a failure starting to someone or something noticing it; MTTA is the time from the alert to a person taking it on.
SLO and SLI: An SLI is a measured indicator of service quality; an SLO is the internal target for it, stricter than any SLA you sign.
DORA metrics: Research-based measures of software delivery: how fast changes reach production and how often they fail.

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