Webhook

A webhook inverts the direction of an integration. Instead of your system repeatedly asking another whether anything has happened, the other system calls a URL of yours the moment it does. Payment providers, source control and messaging platforms all deliver events this way.

It removes both the delay and the wasted traffic of polling, at the price of an endpoint that is publicly reachable and receives whatever arrives. Three defences are not optional: verify the signature so you know the sender is genuine, respond quickly and do the real work afterwards, and treat every delivery as possibly duplicated.

That last point is where webhook integrations most often fail in production. Providers retry when a call times out, so the same event legitimately arrives twice, and an endpoint that is not idempotent will cheerfully charge a customer or send an email a second time.