check_version sends ApiVersionRequest (Kafka 0.10) first that I think is totally ignored by Kafka 0.9, because max_in_flight_requests_per_connection is set to 1 check_version will not be able to send ListGroupsRequest but it will instead try to receive data from the socket blocking indefinitely (or until the connection is dropped on server side) on the socket recv() (because the server 0.9 won't reply to ApiVersionRequest).