How do you choose good SLIs for a service?

Summary

Start from what users care about for each critical user journey: for request-driven services, availability and latency; for pipelines, freshness, coverage and correctness; for storage, durability and throughput. Express each as a ratio of good events to valid events, measured where it reflects user experience.

Longer Answer

Good SLIs are few (two to four per journey), meaningful to users, and directly actionable. Pitfalls: averaging latency (hides the tail; use thresholds such as "under 400 ms" or percentiles), counting health-check traffic, measuring only at the server (misses DNS, load balancer and network failures), and SLIs on internal components that users never notice.

Example for a checkout API: proportion of POST /checkout requests that return non-5xx in under 1 second, measured at the load balancer, excluding requests rejected for invalid input (4xx).

Key Points

• Based on user journeys.
• Good events / valid events.
• Latency via thresholds, not averages.
• Measure near the user.