Webhook
A webhook is an automated HTTP callback: when an event happens in one system, it sends a POST request with a payload to a URL you control. Unlike polling, webhooks push data the moment something occurs, making them the standard way services notify your app of events in real time.
Polling asks 'is it done yet?' on a loop, wasting requests and adding lag. A webhook flips that: you register a URL once, and the source system calls it the instant something happens — a payment succeeds, an upload finishes, a file is processed. Your endpoint receives a POST with a JSON body describing the event.
Reliable webhook handling has a few non-negotiables. Verify authenticity with a signature header so attackers can't forge events. Respond fast with a 2xx and do heavy work asynchronously, because senders often time out and retry. And make handlers idempotent — retries mean you will sometimes receive the same event twice.
Webhooks pair naturally with direct uploads and async pipelines: instead of holding a request open while a file processes, you accept it, return immediately, and fire a webhook when the work is done.