An alert should prompt action; an SLI is a measurement, an SLO a target, and an SLA a contractual commitment.

How to apply it

Define eligible traffic and measurement windows. Use error budgets to inform reliability trade-offs. Runbooks include symptoms, dashboards, safe commands, rollback, escalation, and recovery verification.

Worked example

A 99.9% time-based availability target allows roughly 43 minutes of unavailability in a 30-day month; request-based SLOs require request-based calculations.

Common mistakes to avoid

Alerts with no owner or never testing recovery procedures.

Practice task

Implement this concept in the capstone. Add one positive test, one negative or failure-path test, and a short note explaining the trade-off you chose.