At the end of an Airnode's run (every minute), Airnode can make an HTTP POST request to a specified URL. This is both to signal that the Airnode is alive and working (useful especially right after the deployment) and also as a notification every time it runs (self-operates).
Airnode gathers on-chain requests targeting the API operations it supports.
The required API operation for each request is called.
A response is sent to each request.
Finally Airnode makes a request to the heartbeat URL (HTTP POST). This could be to an operation within the API Airnode supports or to any cloud REST endpoint such as a monitoring service.
Turn on the optional heartbeat functionality by setting all fields in the config.json section for nodeSettings.heartbeat.