SLO and SLI

An SLI is a measured indicator of service quality; an SLO is the internal target for it, stricter than any SLA you sign.

A service level indicator is a number you measure, such as the share of requests answered successfully under a set time. A service level objective is the target for it over a window, for example a month. The SLA is the promise in the contract, with consequences. The gap between 100% and the SLO is the error budget: while it lasts you ship, when it runs out you fix reliability first.

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Related terms

SLA (service level agreement): A contract that defines availability, response and fix times per priority, and what happens when they are missed.
Observability (o11y): Being able to tell what a running system is doing, and why, from the data it emits: logs, metrics and traces.
Runbook: A short document that tells someone else how to run a system: where it lives, how to deploy, how to recover.

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