Fundamentals

MTBF / MTTR

Mean time between failures and mean time to recovery. Availability = MTBF / (MTBF + MTTR).

Why interviewers ask about it

MTTR is usually the cheaper lever. A system failing weekly but recovering in 20 seconds beats one failing yearly and taking six hours.

The fraction of time a system serves requests successfully, expressed in nines. 99.9% ≈ 43 minutes downtime per month.

Recovery Time Objective is how long until service returns; Recovery Point Objective is how much data may be lost.

This term is part of the free System Design interview preparation module - browse the full glossary for every definition.