Define SLI, SLO and SLA, and explain how they relate.

Summary

An SLI (service level indicator) is a measurement of service behaviour, such as the proportion of successful requests. An SLO (objective) is a target for an SLI over a window, such as 99.9% of requests successful over 28 days. An SLA (agreement) is a contract with customers that specifies consequences, usually credits, if a level is missed.

Longer Answer

SLIs are usually ratios of good events to valid events: availability (non-5xx responses / all responses), latency (requests faster than 300 ms / all requests), freshness, correctness, throughput.

SLOs are internal targets that drive engineering decisions; SLAs are business commitments and should be looser than SLOs so you get warning before breaching a contract. Many internal services have SLOs but no SLA. Measure SLIs as close to the user as practical (load balancer or client), because server-side metrics miss failures that never reach the server.

Key Points

• SLI = measurement, SLO = target, SLA = contract.
• SLIs as good/valid ratios.
• SLA looser than SLO.
• Measure close to the user.