What is a Kafka broker?

A broker is a single Kafka server — a process that stores partition data on disk, serves produce and fetch requests from clients, and (in KRaft mode) can also serve as one of the cluster's controller nodes if configured with that role. A Kafka cluster is simply a group of these brokers working together, typically anywhere from a handful to hundreds of nodes depending on scale.

kafka-broker-api-versions.sh --bootstrap-server localhost:9092

Each broker hosts some subset of the cluster's partitions — for a given partition, one broker acts as the leader (handling all reads and writes for it) while other brokers hosting replicas of that same partition act as followers, continuously fetching from the leader to stay in sync. This leader/follower split, repeated across every partition in the cluster and spread across all the brokers, is what balances both storage and request load across the whole cluster rather than concentrating it on one machine.

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