Combine unit tests, API and contract tests, integration tests against real dependencies where practical, load and stress tests, soak tests, and failure injection. Validate overload, dependency timeouts, retries, broker outages, replica lag, and recovery after process or zone failure. Use production-like data shape and traffic patterns, define pass/fail thresholds, and verify dashboards and runbooks during exercises. A successful happy-path test is not evidence of resilience.