This will come up in SRE interviews.
- SLI (Service Level Indicator): A measurement. Example: "99.5% of requests in the last 30 days completed in under 200ms."
- SLO (Service Level Objective): Your target. Example: "99% of requests must complete in under 200ms over a 30-day window."
- SLA (Service Level Agreement): Contractual obligation to customers, usually with financial penalties. Example: "If availability drops below 99.5% in a calendar month, customers receive a 10% credit."
SLI is what you measure. SLO is what you aim for. SLA is what you promise externally. SLO is always stricter than SLA — you give yourself headroom.
Error budget: If your SLO is 99.9%, your error budget is 0.1%. You can "spend" that budget on releases, maintenance, and inevitable failures. When the budget is exhausted, you stop shipping features and focus on reliability.