Fundamentals

Availability

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

Why interviewers ask about it

Dependencies in series multiply. Six services at 99.9% on the critical path give 99.4% - the hidden cost of microservices.

An SLI is the measurement, an SLO is the internal target, an SLA is the contractual promise with penalties.

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

Any component whose failure alone stops the system.

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