Webhook

Quick definition: "When this happens, you'll know about it." A mechanism that automatically sends an HTTP request when an event occurs.

Polling vs. webhook

Previously, to get data from a system, you used polling: "Any new data?" every 5 minutes. Inefficient.

Webhooks flip this: "When there's new data, call me." You register a URL; the system sends a POST request to that URL when an event occurs.

Polling = phone calls; webhook = text messages. The second is vastly more efficient.

E-commerce: Order received, payment completed, shipment dispatched.

Onremo: system.purchased, user.invited, payment.completed.

Slack: Message received, channel created.

GitHub: Push completed, pull request opened.

1. Be idempotent. Receiving the same webhook twice should cause no harm (network errors happen).

2. Respond fast. Return 200 OK within 5 seconds. Do the real work asynchronously.

3. Think about retry logic. Systems like Onremo retry 3 times (after 1, 5, and 60 minutes).

4. Verify signatures. Use a signature to verify the webhook actually came from that system.