Federico Valeri

Cross-cluster replication is essential for disaster recovery, geographic distribution, and data migration. Today, MirrorMaker 2 is the standard tool for this job, but it comes with well-known trade-offs: a separate Connect cluster to deploy and monitor, lossy offset translation that causes duplicate processing during failover, etc. KIP-1279 takes a different approach. Cluster Mirroring embeds cross-cluster replication directly into the Kafka broker runtime, eliminating external infrastructure entirely. But getting this right is harder than it looks. The design had to solve several subtle problems: - Unclean leader elections on either cluster can silently diverge replica logs, yet failover and failback must remain consistent. - Transactional records carry producer state that becomes stale the moment a mirror topic turns writable, yet idempotent producers must be able to resume cleanly. - Leader epochs on source and destination can drift apart, yet consumers must be able to fetch from their last committed offset without falling into infinite metadata refresh loops. - Source clusters can run versions as old as Kafka 2.1, long before leader epochs or modern fetch features existed, yet…