Webhook

A webhook is an API call in reverse. Instead of asking every few minutes “are there any new orders?”, the receiving system gives an address, and the sending system calls it as soon as an order appears.

Data is passed along almost instantly and without wasted requests. Payment systems announce a completed payment this way; couriers announce a change in a parcel’s status.

The weak point is reliability. If the receiver is down at that moment the message may never arrive, and sometimes the same message arrives twice. The receiver therefore has to recognise a repeat, and important data is worth reconciling periodically by another route as well.