A Service Level Indicator (SLI) is the actual measurement of one aspect of service — usually availability, latency, error rate, or freshness. SLIs are the numbers you compare against SLOs to decide whether the service is meeting its reliability target.
Good SLIs are ratio-based: good events divided by total events. 'Percentage of homepage requests returning HTTP 2xx within 400ms' is a strong SLI. 'Average response time' is a weak one — averages hide the tail latency users actually feel.
Every SLO needs a matching SLI wired into monitoring. Lemwatch surfaces the raw SLI numbers per check (uptime %, P50/P95 latency, error rate) so teams can define SLOs against them directly.
Without a measured SLI, an SLO is aspirational. Wire the SLI into the alerting stack before publishing the SLO — otherwise the team can neither prove compliance nor detect breach.