What is an SLO, SLA, and SLI, and how do they relate to each other?

SLI (Service Level Indicator): An actual measurement of service behavior. Example: the percentage of successful HTTP requests.

SLO (Service Level Objective): The target for your SLI. Example: 99.9% of requests should succeed in the last 30 days.

SLA (Service Level Agreement): A contractual commitment to the SLO with defined consequences for missing it. Example: If availability drops below 99.9%, AWS credits customers.

In practice: define SLIs → set SLO targets → the SLA is what you promise externally. Your internal error budget is 100% - SLO.