Azure Event Grid follows Durable Event delivery mechanisms. This means that Azure Event Grid delivers each event published through Event Grid at least once to the event listener registered through the validation subscription.
If a registered subscription listener does not acknowledge the event emitted through Event Grid, Event Grid will try to deliver the event again. This might be the case when a registered event subscribing endpoint is unavailable and throwing up a 404 (resource not found) exception. Azure Event Grid's event delivery exponential retry intervals are described here:
- 10 seconds
- 30 seconds
- 1 minute
- 5 minutes
- 10 minutes
- 30 minutes
- 1 hour
Events that are not delivered within 24 hours of event publication expire and become dead lettered. To make event retries more flexible, we can also customize event retry policies when creating the subscription; some of those customizations can also be done through the Azure portal:
The maximum number of delivery attempts that we can set is 30, and the event time-to-live is 1,440 minutes. If the message is not delivered with these retries or within the time left before expiry, we can also configure the dead-lettering feature, which we will discuss in the next section.