SLA / SLO / SLI

The three layers of reliability engineering measurement. SLI (Service Level Indicator) is the measured metric, such as request success rate or latency percentile. SLO (Service Level Objective) is the internal target for that metric (e.g., 99.9% success rate over 30 days). SLA (Service Level Agreement) is the external contractual commitment to customers, with financial penalties for violations. Error budgets are derived from SLOs to guide deployment risk.