When publishing events to Event Hubs, timeouts or other transient failures may introduce ambiguity into the understanding of whether a batch of events was received by the service. Because Event Hubs has an at-least-once guarantee for delivery and consumers are strongly encouraged to be idempotent in their processing, the common approach is to resend any batch where the status of receipt was unknown.
In some specialized scenarios, producers have a need to make efforts to avoid publishing duplicate events. To support these scenarios, the Event Hubs service is adding support for annotating messages with metadata to indicate the sequence in which events were intended to be published and to use that as an indicator of which events were already received by a the target partition. This functionality comes at a performance cost, however, and requires producers to follow a strict set of semantics to allow the service to perform server-side deduplication based on the intent