Webhook
A webhook reverses the API model: instead of polling, you receive — the third-party service calls your URL when the event occurs. Payment confirmed, form submitted, build complete: the information arrives on its own.
In practice
Robustness is won at the receiving end: respond fast, process after; verify the sender signature; tolerate duplicates, because retries are part of the contract. A silently broken webhook is an invisible outage — the receipt log belongs in monitoring, not in the nice-to-have column.