Webhook

An HTTP request one system sends to another when something happens, instead of the other system asking repeatedly.

A payment provider calls your URL when a payment succeeds; a form tool calls it when someone submits. Webhooks arrive late, twice or out of order, and anyone who knows the URL can send one. A solid receiver verifies the signature, stores the event, answers quickly and does the real work in the background.

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Related terms

Idempotency (idempotency key): An operation is idempotent if doing it twice has the same effect as doing it once. Retries become safe.
Runtime validation (Zod): Checking at run time that data from outside (requests, APIs, files, models) matches the type your code assumes.
KSeF (Polish National e-Invoice System): Poland’s central system for structured e-invoices: invoices are sent as FA(3) XML and receive a KSeF number.

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