Apache Pulsar depends upon message acknowledgments to provide at-least-once or exactly-once processing guarantees. With these guarantees, any transmission between the broker, producers, and consumers requires acknowledgment.

But what happens if an acknowledgment is not received? Resending the message introduces the potential of duplicate processing and increases the likelihood of out or order processing. Therefore, it is critical to understand the Pulsar message redelivery semantics in order to prevent either of these conditions.

In this talk, we will walk you through the redelivery semantics of Apache Pulsar and highlight some of the control mechanisms available to application developers to control this behavior. Finally, we will present best practices for configuring message redelivery to suit various use cases.