SLIs, SLOs and SLAs: Error Budgets and Measuring What Users Experience
Three terms that get used interchangeably and mean quite different things. An SLI is a measurement, an SLO is an internal target for that measurement, and an SLA is a contractual promise with consequences attached. Getting them apart is most of the topic. The three, precisely A Service Level Indicator is a metric that reflects service quality: the proportion of successful requests, the latency at a given percentile, the freshness of data, the fraction of time a service is reachable. It is a number you measure. A Service Level Objective is a target for an SLI over a window — 99.9 percent of requests succeed over 30 days, or the 95th percentile latency stays under 300 milliseconds. Internal, and set by you. A Service Level Agreement is a contract with a customer stating a level and the remedy if you miss it, usually service credits. External, and legally meaningful. The practical rule: set SLOs stricter than SLAs. The gap is your margin — you want to be alerting a...