Service level objectives and error budgets

What it is

The SRE book distinguishes indicators (SLIs: measured quantities such as the fraction of successful requests or the 99th-percentile latency), objectives (SLOs: target values for indicators over a window) and agreements (SLAs: contracts with consequences). The error budget is 1 - SLO: with a 99.9% monthly availability objective, about 43 minutes of downtime per month are acceptable.

Why it matters

Objectives turn "reliable" into a number that engineering and product can negotiate. The error budget converts that number into a policy: while budget remains, releases and experiments may proceed; once it is spent, reliability work takes priority over features.

How to apply

• Choose few indicators that reflect user experience (success ratio, latency at a percentile, freshness).
• Measure them at the edge the user sees, not inside the service.
• Set objectives slightly below what the system currently achieves, then tighten deliberately.
• Alert on budget burn rate rather than on every blip.

Pitfalls

100% is not an objective; it forbids all change. Averages hide tail latency; use percentiles. Objectives without an agreed policy for exhausted budgets are decoration.