Architecture
Webhook Delivery Guarantees: Why "At-Least-Once" Changes Everything
Understanding delivery guarantees helps explain why webhook systems behave the way they do.
When developers first work with webhooks, they often assume that every event will arrive exactly once. In reality, most providers use an at-least-once delivery model.
This means an event might be delivered multiple times, especially when the provider is unsure whether your endpoint received the previous attempt.
At-least-once delivery
With this model, the provider guarantees that an event will eventually arrive, but it does not guarantee that it will arrive only once.
If the provider does not receive a clear success response, it may retry the request. Network failures, server restarts, or slow responses can all trigger additional deliveries.
Why exactly-once delivery is difficult
Guaranteeing exactly-once delivery across distributed systems is extremely complex. It would require coordination between both systems and perfect network reliability.
Instead, webhook providers choose reliability: they retry events to make sure nothing is lost. The responsibility for handling duplicates shifts to the receiving application.
Designing systems around retries
Because duplicates are normal, webhook handlers should always assume an event might appear more than once.
- Track event identifiers from providers
- Use database constraints to prevent duplicate processing
- Respond quickly so providers stop retrying
- Monitor delivery failures
Observability completes the picture
Even well-designed webhook handlers benefit from visibility into delivery activity. Monitoring tools help identify patterns like repeated failures, slow responses, or endpoints that suddenly stop acknowledging events.
Reliable systems assume retries will happen — and make them harmless.