SLOs

SLOs, or service level objectives, are measurable targets that define the expected level of reliability or performance for a service from the user’s perspective. They are typically derived from SLIs, or service level indicators, such as availability, latency, or error rate. An SLO states the target value to be achieved within a specified time window and for a defined scope, and it is used to gauge whether the service meets its reliability goals.

SLOs are distinct from SLAs. SLAs are legal agreements with customers and may include penalties for underperformance, while SLOs are internal goals that guide engineering decisions. SLIs quantify the user experience, and SLOs set the target to be met over a time frame. In practice, organizations use SLOs to balance reliability with development velocity.

Common SLIs include availability (uptime), latency (response time at a given percentile, such as p95 or p99), and error rate. Example SLOs include: API uptime 99.9% over a rolling 30-day window; P95 latency under 200 ms; error rate below 0.1%.

An SLO has three parts: scope (which service and user experience are considered), the metric (the SLI), and the target within a time window (rolling or fixed). The concept of an error budget represents the portion of the SLO not yet consumed by failures, guiding risk-taking and release decisions; burn rate tracks how quickly the budget is used.

Implementation involves selecting appropriate SLIs, setting realistic targets, continuous measurement, alerting, dashboards, and regular reviews. Common pitfalls include misaligned SLIs, overly aggressive or lax targets, and insufficient data quality.