Service Level Indicator (SLI)
A service level indicator is a precise measurement of a service's behavior, such as availability or latency, that forms the basis for reliability objectives and error budgets.
A service level indicator (SLI) is a carefully defined quantitative measure of some aspect of the service a system provides. It is the metric against which reliability targets are set and judged.
How It Works
An SLI is usually expressed as a ratio of good events to total events, which yields a value between 0 and 100 percent that is easy to reason about. Common SLIs include availability, the fraction of requests that succeed; latency, the fraction of requests served faster than a threshold; and freshness or correctness for data systems. Choosing a good SLI means measuring what users actually experience, often at the point where requests enter the system, rather than internal proxies that may not reflect user impact. The SLI is then measured continuously from telemetry.
Why It Matters
SLIs ground reliability discussions in objective data. Without a precise indicator, terms like "available" or "fast" are ambiguous. A well-chosen SLI captures the user's actual experience and becomes the foundation for a service level objective and its associated error budget. Poorly chosen SLIs, by contrast, can show green dashboards while users suffer, so the choice of indicator is one of the most important decisions in reliability engineering.
Related Terms
An SLI is the measurement underlying a service level objective and an error budget. It is a specialized application of metrics within observability.