Service-Level Indicator (SLI)
An SLI is the measured quantity used to describe a service’s reliability or performance.
What is Service-Level Indicator (SLI)?
A service-level indicator is an observed measurement of service behavior. Examples include the percentage of valid requests that succeed, the proportion completed under a latency threshold, or the freshness of data. An SLO sets the target for that indicator; the SLI supplies the evidence about actual performance.
Example in SaaS
A reporting service measures the proportion of scheduled reports delivered within five minutes of their promised time. If 9,950 of 10,000 reports qualify, the observed SLI is 99.5%. This better describes the customer’s experience than merely checking whether the web server responds to a health probe.
What to watch for
The indicator needs a clear numerator and denominator, consistent instrumentation, and a rule for missing data. Aggregation can hide a badly affected customer or region even when the overall figure looks healthy. Distinguish request-based measures from time-based uptime. Choose a small set of indicators that reflect critical journeys, and verify monitoring captures the failures customers actually experience.
Source and further reading
Related terms: Service-level objective, Uptime.