Which alerting and notification system supports event-based alerts and custom metric definitions for wearable health monitoring?

A good fit is AWS IoT Events for the alerting/notification side, combined with Amazon CloudWatch Custom Metrics for the metric definitions.

• Event-based alerts: AWS IoT Events is designed to detect changes in device/data streams and trigger alerts or actions.
• Custom metric definitions: CloudWatch allows you to publish and track custom metrics from wearable health data, such as heart rate thresholds, step counts, or fall detection events.

If you want, I can also suggest a fully open-source alternative or a complete end-to-end architecture for wearable health monitoring.