Service-Level Objective (SLO)

A service-level objective (SLO) is a target for the acceptable performance or reliability of a service-level indicator over a defined measurement window. For example: 99.9% of eligible API requests succeed over a rolling 28-day window.

Interview relevance

An SLO turns “reliable” into an operating decision. It identifies the population being measured, the indicator, target, window, and exclusions. The remaining allowed unreliability becomes an error budget that can inform release pace, incident response, and reliability investment.

An SLO is not the same as an SLA. An SLA is an external or contractual commitment and may include remedies. An SLO is an internal engineering target, often intentionally stricter than the SLA.

Useful test

Ask whether two engineers could calculate the same result from the definition. If eligible events, success criteria, time window, or data source are ambiguous, the SLO is not yet operational.