DevOps
What is SLA, SLO, and SLI?
A hierarchy of reliability targets: SLIs measure behavior, SLOs set internal goals, and SLAs are contractual promises with penalties.
Definition
These three acronyms form a reliability hierarchy. An SLI (service level indicator) is a measurement — say, the fraction of requests served successfully in under 300 ms. An SLO (objective) is the internal target for that indicator, such as 99.9% over 30 days. An SLA (agreement) is the external contract promising customers a level of service, usually looser than the SLO and backed by refunds or credits. The gap between SLO and 100% is the error budget: while budget remains, teams ship features; when it's spent, they prioritize reliability. Google's SRE book codified this practice.