Instead of repeatedly asking whether something has changed, a system can register a webhook URL with another service. When an event occurs, the service sends a request to that URL with the details. Payments, e-signatures, form submissions, and repository changes commonly use webhooks.
Webhooks can arrive late, out of order, or more than once, and they can be forged. Receivers should verify signatures, process each event idempotently, respond quickly and do heavy work in the background, and record events so failures can be replayed.