Webhook
A webhook is an automated message sent from one system to another the moment a specific event happens, letting the receiving system react immediately instead of repeatedly checking for updates.
Instead of one system polling another every few minutes to ask whether anything has changed, a webhook has the source system push a notification the instant something does change — a payment succeeded, a form was submitted, a status flipped. The receiving system exposes a URL; the source system sends an HTTP request to it with the event details whenever the event occurs.
The engineering challenge isn't receiving the webhook — it's handling it correctly under real conditions. Webhooks can arrive out of order, arrive more than once for the same event (most providers explicitly document this and expect the receiver to handle it), or arrive while the receiving system is briefly down. A production-grade webhook handler is idempotent by design, so processing the same event twice doesn't double-charge a customer or double-send a message.
Webhooks are the standard integration point for workflow automation systems reacting to external events — a new lead in a CRM, a payment in Stripe, a message in WhatsApp Business. Reliable webhook handling is one of the specific failure modes we test for before calling a workflow automation build production-ready.