Webhook Reliability: Idempotency, Retries, and Dead Letters

Webhooks fail quietly. A receiving endpoint times out, a provider retries, a record is created twice — and nobody notices until the CRM has duplicate deals or a customer is invoiced twice. Because the failure happens between systems, neither side's dashboard shows it. Reliability has to be designed in; it does not come free with the integration.

The first defense is idempotency. Every event needs a stable identifier — the provider's event ID, or a hash of the payload — and the receiver records processed IDs so a retry of the same event becomes a harmless no-op. This one pattern eliminates the most common class of integration bugs: duplicates created by well-meaning retry logic.

The second is honest retry handling. Transient failures deserve retries with exponential backoff; permanent failures — a validation error, a deleted record — should not be retried forever. Events that exhaust their retries need somewhere to land: a dead-letter store with the full payload, the error, and a timestamp, so a human can replay them after the underlying issue is fixed instead of losing them.

The last piece is visibility: a log line per event with its ID, outcome, and duration, and an alert when the dead-letter store grows. None of this is exotic engineering, but it is the difference between an integration that quietly corrupts data and one that can be operated. It is the standard Symbios Lab builds webhook and pipeline work to, and the logs and replay notes are part of the handover.