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Infrastructure & Operations › Incidents & SRE · also in Observability

SLI

A service level indicator: a measured aspect of service quality.

Backend Engineer · SeniorFrontend Engineer · SeniorData Engineer · Senior

Also known as: SLI, service level indicator, sli definition

An SLI (service level indicator) is a measurement of how well a service is doing from the user’s point of view. It’s a specific number you track: the fraction of requests that succeed, the share served under a latency threshold, the availability of an endpoint. It’s the raw measurement that an SLO sets a target for and an SLA makes a promise about.

SLI: fraction of successful requests (good / total)
SLI: fraction of requests faster than 300ms
SLI: time the service is available / total time

The key word is user’s point of view. The best SLIs measure the experience, not the machine: “99.2% of HTTP requests returned successfully in the last 30 days” is an SLI; “the server’s CPU was 40%” is not. If an SLI looks good while users are unhappy, you’re measuring the wrong thing.

The classic mistakes:

  • Measuring internals instead of outcomes. Healthy servers can serve broken responses. Anchor the SLI to what a user asked for and got — success, latency, correctness.
  • Too many SLIs. A wall of indicators drowns the signal. Pick the few that map to what users actually care about for that service.
  • Averaging over the interesting parts. A single success/failure ratio hides a specific endpoint or region doing badly. Choose the scope deliberately.
  • An indicator with no target. An SLI alone tells you the number; it doesn’t tell you if it’s good enough. That’s the SLO’s job — measure, then set the bar.
  • Only availability. Availability is easy but blunt; latency and correctness often matter more. Use the golden signals as a source of candidate SLIs.

How to use them: define SLIs for each user-facing service, compute them from the same telemetry you already collect (metrics, requests), and let them feed your SLOs and error budgets. A good SLI is one you can calculate continuously and that a user would recognise as “is it working for me?”.