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on_broker_change():
1. Get the current live brokers from BrokerPath in ZK
2. Determine set_p, a set of partitions whowhose leader is no longer live or whose ISR will change because a broker is down.
3. For each partition P in set_p
3.1 Read the current ISR of P from LeaderAndISR path in ZK
3.2 Determine the new leader and the new ISR of P:
If ISR has at least 1 broker in the live broker list, select one of those brokers as the new leader. The new ISR includes all brokers in the current ISR that are alive.
Otherwise, select one of the live brokers in AR as the new leader and set that broker as the new ISR (potential data loss in this case).
Finally, if none of the brokers in AR is alive, set the new leader to \-1.
3.3 Write the new leader, ISR and a new epoc (increase current epoc by 1) in /brokers/topics/[topic]/[partition_id|partition_id]/leaderAndISR.
This write has to be done conditionally. If the version of the LeaderAndISR path has changed btw 1.1 and 1.3, go back to 1.1.
4. Send a LeaderAndISRCommand (containers the new leader/ISR and the ZK version of the LeaderAndISR path) for each partition in set_p to the affected brokers.
For efficiency, we can put multiple commands in one RPC request.
(Ideally we want to use ZK multi to do the reads and writes in step 3.1 and 3.3.)