Enable exactly-once source support on an existing Kafka Connect distributed cluster via the two-phase rollout

Steps

1. On every worker in the distributed Connect cluster, add exactly.once.source.support=preparing to the worker properties file
2. Perform a rolling restart of all workers; in 'preparing' mode workers begin fencing old task generations but do not yet enforce exactly-once writes to Kafka
3. Once all workers report 'preparing' mode (check REST GET /connectors/{name}/status), update every worker config to exactly.once.source.support=enabled
4. Perform a second rolling restart; workers now use transactions to write source records and their offsets atomically, preventing duplicates on producer failure
5. Verify: check the internal Connect offsets topic and confirm records include transactional metadata; monitor for transaction abort metrics on the broker
6. Test failover: kill a worker mid-task and confirm no duplicate records appear in the target topic after the task restarts on another worker