Kafka with Spring

Suppose the code that sends confirmation emails runs in its own process, separate from the booking service. Ana books seat A12 while that process is restarting after a deployment. What should happen to her confirmation?

With an application event, the answer is that it disappears, because the event lived only in the memory of one process. The confirmation needs a place to wait that survives restarts on both sides: a server that stores messages until a consumer is ready to read them. That server is a message broker, and Apache Kafka is the broker this course uses.