What is an SLO, SLI, and error budget?

Answer

An SLI (Service Level Indicator) is a measured metric of service health, such as request success rate or latency. An SLO (Service Level Objective) is the target you set for that indicator, for example 99.9% of requests succeed over 30 days. The error budget is the allowed amount of failure—the gap between 100% and your SLO—that the service may consume before changes are halted to focus on reliability. Together they give teams a data-driven way to balance shipping features against maintaining reliability.