Fundamentals

SLI / SLO / SLA

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

Why interviewers ask about it

The SLA must always be looser than the SLO. The error budget - the complement of the SLO, 1 − SLO - turns arguments about risk into arithmetic.

The allowed failure remaining under an SLO. A 99.9% target permits ~43 minutes of failure a month.

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

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