Hello all, I recently had the experience of using the script kafka-consumer-groups.sh from version 2.x of Kafka on a cluster of the same version which was serving consumers using version 0.8 or earlier libraries, hence stored the groups and offsets in Zookeeper.
On --list of the consumer groups, the script returned an empty list and did not immediately realized you cannot list consumer groups which are in Zookeeper with the up-to-date versions of Kafka [1]. What is the reason they are not reported at all, while consumers are still allowed to commit offsets via Zookeeper? Do you think it worth to add a warning in Kafka logs when a user issued a ListGroup command and Kafka detects consumer groups in Zookeeper? [1] https://github.com/apache/kafka/blob/trunk/core/src/main/scala/kafka/server/KafkaApis.scala#L1362-L1387
