SLO vs SLI vs SLA Differences

An SLI (Service Level Indicator) is a measurement of your service's behavior -- something you can actually observe and put a number on. For example, the percentage of HTTP requests that return in under 300ms, or the ratio of successful responses to total responses. An SLO (Service Level Objective) is a target you set for an SLI. You're saying "we want this SLI to meet this threshold over this time window." For example, "99.9% of requests should succeed over a rolling 30-day window." An SLA (Service Level Agreement) is a contract with your customers that says what happens if you miss your targets. It has business consequences -- refunds, credits, or penalties. Not every service needs an SLA, but every production service should have SLOs. The relationship flows like this: you pick SLIs (what to measure), set SLOs (what target to hit), and if you make a promise to customers, that becomes an SLA. Your SLOs should always be tighter than your SLAs. If your SLA promises 99.9% availability, your internal SLO should be something like 99.95%. That gap gives you a buffer to catch problems before they become contract violations.