Concept / Term · Established
Service Level Indicator / Objective
An SLI is a measured aspect of service quality, such as latency or error rate; an SLO is the reliability target set for that measure.
Field note
An SLI is a measured indicator of service quality, such as successful requests or response time. An SLO is the target the team promises itself for that indicator over a stated period.
The language was popularised by Site Reliability Engineering to make reliability an explicit, measurable product decision rather than a vague ambition to achieve “100% uptime”.
A team might measure the proportion of requests completed in under 300 milliseconds and set an objective that 99.9% meet that threshold each month.
Clear objectives align engineering effort with the level of reliability users actually need and the business is prepared to fund.
In a sentence“Our latency SLO is 99.9% of checkout requests under 300 ms over 30 days.”