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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 if P doesn't exist locally, call startReplica()
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 call 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 call becomeFollower()
3. If the command has a flag INIT, delete all local partitions not in set_p.

becomeLeader(r: Replica, command)
{
  stop the ReplicaFetcherThread to the old leader   //after this, no more messages from the old leader can be appended to r  r.leaderAndISRZKVersion = command.leaderAndISRZKVersion
  r.ISR = command.ISR   stop the HW checkpoint thread for r
  r.isLeader = true                                 //enables reads/writes to this partition on this broker
  start a commit thread on r
}
Note that the new leader's HW could be behind the HW of the previous leader. Therefore, immediately after the leadership transition,
it is possible for a consumer (client) to ask for an offset larger than the new leader's HW. To handle this case, if the consumer is
asking for an offset between the leader's HW and LEO, the broker could just return an empty set. If the requested offset is larger
than LEO, the broker would still return OffsetOutofRangeException.


becomeFollower(r: Replica)
{

  stop the ReplicaFetcherThread to the old leader  r.isLeader = false                                //disables reads/writes to this partition on this broker
  stop the commit thread, if any
  truncate the log to r.hw
  start a new ReplicaFetcherThread to the current leader of r, from offset r.leo
}


startReplica(r: Replica)
{
  create the partition directory locally, if not present
  start the 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 call stopReplica() on p



stopReplica(r: Replica){
  stop the ReplicaFetcherThread associated with r, if any.
  stop the HW checkpoint thread for r
  delete the partition directory locally, if present
}