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The source is opt in and would requirement require users to implement code changes. It is possible for the MultiClusterKafkaSource builder to make it easier to convert, code wise.

In the same vein as the migration from FlinkKafkaConsumer and KafkaSource, the source state is incompatible between KafkaSource and MultiClusterKafkaSource so we would recommend it is recommended to reset all state or reset partial state by setting a different uid and starting the application from nonrestore state.

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Test Plan

This will be tested by unit and integration tests. The work will extend existing KafkaSource test utilities in Flink to exercise multiple clusters.

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