The service provider can optionally subscribe to callbacks (also known as web hooks) whenever a subscription is created or cancelled, or if a transaction is executed. This can be used for real-time tracking of subscription statuses, without having to query them one by one on a regular basis.
The service provider is expected to implement the 3 callback API, if applicable, and provide the base URL when the service is created (see Getting Started on how to create a service).
The callback endpoint must be HTTPS with TLS 1.3 and require no authentication or BASIC authentication. It is expected to return a HTTP 201 status for POST calls, and a HTTP 204 status for PUT calls. Return payloads are ignored, and any other HTTP status will be interpreted as an error.
If an error occurs while delivering a callback event to the service provider's endpoint, the callback will be requeued and redelivery attempts will occur every 10 minutes for 6 hours until a success HTTP code is received. After that the callback will expire and not be redelivered.