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1. If the co-ordinator fails at step #1 after receiving a notification but not getting a chance to act on it, the new co-ordinator has to be able to detect the need for a rebalance operation on completing the failover. During failover, the co-ordinator reads a group's metadata from zookeeper, including the list of topics the group has subscribed to and the previous partition ownership decision. If the # of topics or # of partitions for the subscribed topics are different from the ones in the previous partition ownership decision, the new co-ordinator detects the need for a rebalance and initiates one for the group. Similarly if the consumers that connect to the new co-ordinator are different from the ones in the group's generation in zookeeper, it initiates a rebalance for the group. For example, if a consumer that is not in the current generation sends a HeartbeatRequest or
2. If the co-ordinator fails at step #2
   

Consumer id assignment