SLO (service level objective)

A service level objective (SLO) is a target for how well a service should perform, measured over a time window. It is set on a service level indicator (SLI), which is a measured ratio of good events to valid events. A service level agreement (SLA) is a contract with customers that carries a penalty when it is missed.

How to calculate it

An SLI is good events divided by valid events. Suppose a service handles 1,000,000 requests in 30 days and 999,500 succeed. The availability SLI is 99.95%. If the SLO is 99.9%, the service met its target with room to spare. Latency SLOs work the same way, for example 99% of requests complete in under 400 ms. SLAs are usually set looser than the internal SLO, so the team gets warned before a contract is at risk.

Why it matters in production

An SLO turns "is the service reliable enough?" into a number everyone can check. It sets how much failure is acceptable, which is the basis for error budgets and for alerts that page only when users are affected.