An SLI (service level indicator) is the actual measurement behind a reliability target. If an SLO says "the API should be available 99.95% of the time," the SLI is the number that tells you whether you're , and by how much. Without an SLI, an SLO is just a wish written on a whiteboard.
Most SLIs boil down to a ratio: good events divided by total events over a time window. For availability, that's successful requests over total requests. For latency, it might be requests completed under 200ms over total requests. The ratio produces a percentage, and that percentage is what you compare against the SLO threshold.
Common SLI types and what they measure
The four SLIs you'll run into most are availability (did the request succeed?), latency (how fast?), throughput (how many requests per second?), and error rate (what fraction failed?). A single service usually tracks , not all four. An internal batch processor cares about availability and error rate. A user-facing API cares more about latency and availability. Picking the right SLI matters more than tracking every possible metric.
SLIs and monitoring
Uptime monitoring generates one of the simplest SLIs: the percentage of checks that returned a healthy response over a given period. Uptime reports turn that raw data into the actual number you compare against your SLO. When the SLI dips below the target, the error budget , and that's when alerting should fire, not when a single check fails.
Related terms: SLO, SLA, uptime, observability
WatchCron monitors availability around the clock and reports the actual uptime percentage — the SLI you need to measure against your SLO. Free plan available.