Never lose quorum in Apache Kafka® - and what to do when you do

Apache Kafka’s KRaft mode depends on a controller Raft quorum to manage cluster metadata, and losing a majority of controller voters halts control-plane operations because dynamic quorum membership can only be changed through records that require an existing quorum to commit. Aiven describes an emergency recovery process for such rare failures that cannot restore lost data or full consistency, but can restore availability by identifying the longest surviving metadata log across controllers and brokers, selecting it to avoid rolling surviving brokers back to earlier leadership epochs, and offline-editing it to make one controller the sole voter. That controller can then restart, become leader, allow other surviving nodes to replicate the revised membership state, and support replacement voters added through normal operations. The process deliberately discards metadata and potentially partition data that existed only on lost disks, so it is reserved for cases where failed voters are confirmed unrecoverable and is performed manually rather than automatically. The discussion also notes proposed upstream Kafka work on overriding stale voter endpoints, while emphasizing that this does not solve majority-loss recovery because lost voters must be removed from membership records, not merely assigned new addresses.

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