SLO

A Service Level Objective (SLO) is an internal target for a measurable property of a service over a window — commonly availability or latency percentiles, such as "99.9% of requests succeed each rolling 30 days." A Service Level Agreement (SLA) is the external, contractual version with penalties.

SLOs earn their keep through error budgets: if the target is 99.9%, the budget is 0.1% of requests or minutes, and the team may spend that budget on launches, maintenance, and experiments. When the budget is exhausted, feature work pauses in favor of reliability work — a negotiated, quantified trade-off.

Good SLOs are few, measured at the user boundary, and slightly looser than the SLA they protect. Setting an availability SLO the same as the contractual SLA leaves no room for controlled maintenance — and guarantees that engineering either burns the SLA or never ships.