Definitions
- Service Level Indicator (SLI): A quantified metric of service performance (e.g., % of requests returning success codes within 300ms).
- Service Level Objective (SLO): The target reliability target defined for an SLI (e.g., SLI success rate must stay above 99.9%).
- Service Level Agreement (SLA): The legal contract defining penalties or refunds if the service fails to meet the SLO.
Error Budgets
An error budget represents the allowed rate of failure (100% - SLO). For a 99.9% SLO, a service is allowed 43 minutes of downtime per month. If the error budget is depleted, engineers pause feature releases to focus exclusively on reliability improvements.