Kafka plays a very important role in the system. By nature, Kafka is a message broker that stores every received message and replays them reliably. With Kafka as the backbone, requests for action invocation will be reliably delivered to the invokers.

Kafka is formed as a cluster using the ZooKeeper quorum. Kafka is running on port 9092 within the default network. We use the wurstmeister/kafka:0.11.0.1 image for the configuration in this chapter.