Explain the Kafka controller's responsibilities in KRaft mode.

In KRaft mode a quorum of controller nodes manages the cluster metadata. One controller is active and the others are standbys that can take over if the active one fails. The controller is responsible for managing broker registration and, when a broker fails, electing new partition leaders (from the in-sync replicas; with Eligible Leader Replicas, the default on new 4.1+ clusters, leaders are not chosen only from the ISR). Metadata changes such as topic creation and partition assignment are recorded in the cluster metadata log (the __cluster_metadata topic), which the controllers keep consistent among themselves using the Raft protocol.

Expected key concepts: quorum, standby, broker registration, metadata log, __cluster_metadata, Raft, ISR, Eligible Leader Replicas