Knock, a notification system, employs LaunchDarkly feature flags to control the runtime behavior of its Elixir application, enhancing reliability and flexibility in handling failures and load spikes. By adopting a circuit breaker pattern integrated with feature flags, Knock can start and stop parts of its application, allowing for graceful degradation and recovery from runtime errors. The use of AWS Kinesis as a data stream, consumed by Broadway consumers, presents challenges when shard consumers crash, leading Knock to implement a Circuit Breaker supervisor that monitors child processes and reacts to feature flag changes. This approach prevents cascading failures, maintains uptime for critical infrastructure, and offers a defense-in-depth strategy without relying solely on traditional methods like autoscaling or Kubernetes management. The system also integrates Telemetry for monitoring and logging, providing real-time insights into process states and alerts for necessary interventions. Knock plans to refine this implementation, potentially open-sourcing the tool for broader use in controlling supervision trees at runtime.