A Service Level Indicator (SLI) is a quantitative measurement of how well a service is behaving from the user's point of view. It is usually expressed as a ratio of good events to total events, for example successful requests divided by all requests.
Why it matters
SLIs are the raw material for reliability targets. An SLO is a goal set on top of an SLI, and an SLA is a contract that usually builds on the SLO. Without a well-chosen SLI, a target such as "99.9%" has no agreed meaning. Good SLIs reflect what users feel: whether pages load, whether checkouts complete, whether answers arrive quickly. CPU load or memory use rarely qualify because users never see them directly.
Example
For an online shop you might define two SLIs over a rolling 30 days:
availability SLI = checks returning HTTP 200 / total checks
latency SLI = requests answered in under 500 ms / total requestsIf 4,320 of 4,330 checks passed, the availability SLI is about 99.77%.
Common mistakes
- Measuring only from inside your own network. A site can look healthy internally while users cannot reach it.
- Using averages for latency. A few very slow requests disappear in the mean, so ratios against a threshold or percentiles are safer.
- Tracking dozens of SLIs. Start with one to three per user-facing journey.
Related
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