Apache ZooKeeper is a naming registry for distributed applications as well as a distributed, open-source configuration and synchronization service. It keeps track of the Kafka cluster nodes' status, as well as Kafka topics, partitions, and so on.
ZooKeeper is used by Kafka brokers to maintain and coordinate the Kafka cluster. When the topology of the Kafka cluster changes, such as when brokers and topics are added or removed, ZooKeeper notifies all nodes. When a new broker enters the cluster, for example, ZooKeeper notifies the cluster, as well as when a broker fails.
ZooKeeper also allows brokers and topic partition pairs to elect leaders, allowing them to select which broker will be the leader for a given partition (and server read and write operations from producers and consumers), as well as which brokers contain clones of the same data. When the cluster of brokers receives a notification from ZooKeeper, they immediately begin to coordinate with one another and elect any new partition leaders that are required. This safeguards against the unexpected absence of a broker.
