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For LeaderAndISRCommand, it calls on_LeaderAndISRCommand().
on_LeaderAndISRCommand(command):
1. Read the set of partitions set_P from command.
2. For each partition P in set_p
2.0 create the partition locally if not present
2.1 If the command asks this broker to be the new leader for P and this broker is not already the leader for P,
2.1.1 Stop the fetcher to the current leader
2.1.2 Become the leader and remembers the ZK version of the LeaderAndISR pathcall becomeLeader()
2.2 If the command asks this broker to following a leader L and the broker is not already following L
2.2.1 stop the fetcher to the current leader
2.2.2 become a follower to Lcall becomeFollower()
3. If the command has a flag INIT, delete all local partitions not in set_p.
has a flag INIT, delete all local partitions not in set_p.


becomeLeader(r: Replica, command)
{
   r.partition.leaderAndISRZKVersion = command.leaderAndISRZKVersion   r.partition.ISR = command.ISR   stop the HW checkpoint thread for r
   wait until every replica in ISR catches up to r.leo   r.hw = r.leo
   r.partition.leader = r                  // this enables reads/writes to this partition on this broker
   start a commit thread on r.partition
   start HW checkpoint thread for r
}

becomeFollower(r: Replica)
{
  // this is required if this replica was the last leader
  stop the commit thread, if any
  stop the current ReplicaFetcherThread, if any
  truncate the log to r.hw
  start a new ReplicaFetcherThread to the current leader of r, from offset r.leo
  start HW checkpoint thread for r
}
For StopReplicaCommand, it calls on_StopReplicaCommand().
on_StopReplicaCommand(command):
1. Read the list of partitions from command.
2. For each such partition P
2.1 delete p from local storage, if present.