What is a callback?

Push instead of poll

Without callbacks, a system that wants to know about events has to ask repeatedly — poll the API every minute in case something happened. That is wasteful when nothing has, and slow when something has.

A callback inverts it. You register a URL; the platform calls you when the event occurs. No polling, and the notification arrives when it happens rather than at the next poll.

The terms callback, webhook and postback describe the same mechanism in different contexts. Postback is the term of art in mobile attribution specifically.

What a callback endpoint must handle

• Duplicates. Delivery is usually at-least-once, so the same event will arrive twice. De-duplicate on an event identifier.
• Out-of-order arrival. Network timing means events can arrive in a different order from the one they occurred in.
• Verification. An unauthenticated endpoint accepting conversion notifications is an obvious target; signature verification is not optional.
• Fast response. Most senders time out quickly — acknowledge immediately and process asynchronously.
• Failure visibility. Silent callback failure loses data permanently, so a dead-letter queue and alerting matter more than they seem to.

Where they are used

Throughout the measurement chain: an MMP notifying a network that its click produced an install, a payment provider confirming settlement, a platform reporting a conversion for optimisation.

The reliability of a marketing stack is frequently determined by how well its callback endpoints are built, and it is consistently the least-reviewed part of an integration.