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Major changes compared with the v2 proposal.

• leadership Leadership changes are now made by a controller.
• the The controller detects broker failures and elects a new leader for each affected partition.
• each Each leadership change is communicated by the controller to each affected broker.

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1. Leadership change of a partition (each leader can independently update ISR)
2. New topics; deleted topics
3. Replica reassignmentre-assignment

After the controller makes a decision, it publishes the decision permanently in ZK and also informs sends the new decisions through ZKQueue to affected brokers. The published decisions are the source of truth and they are used by clients (for request routing) and for broker startup (to bring all replicas assigned to a broker to the right state). After the broker is started, it picks up new decisions made by the controller from ZKQueue.

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1. Need controller failover.

Paths:

1. Stores the current controller info.

   Code Block
   /controller --> {brokerid, controller epoc} (ephemeral; created by controller)

2. Stores the information of all live brokers.

   Code Block
   /brokers/ids/[broker_id] --> host:port (ephemeral; created by admin)

3. Stores the replication assignment for all partitions in a topic. For each replica, we store the id of the broker to which the replica is assigned. The first replica is the preferred replica. Note that for a given partition, there is at most 1 replica on a broker. Therefore, the broker id can be used as the replica id

   Code Block
   /brokers/topics/[topic] --> {part1: [broker1, broker2], part2: [broker2, broker3] ...} (created by admin)

4. Stores leader and ISR of a partition

   Code Block
   /brokers/topics/[topic]/[partition_id]/leaderAndISR --> {leader: broker_id, ISR: {broker1, broker2}} (updated by controller or current leader; current leader) only updates the ISR part)

5. This path This path is used when we want to reassign some partitions to a different set of brokers. For each partition to be reassigned, it stores a list of new replicas and their corresponding assigned brokers. This path is created by an administrative process and is automatically removed once the partition has been moved successfully

   Code Block
   /brokers/partitions_reassigned/[topic]/[partition_id] --> {broker_id …} (created by admin)

6. ZKQueue: Used to communicate state change information from controller to each broker.

 /brokers/state/[broker_id]/[i] --> { state change requests ... } (created by controller) 

Terminologies:

AR: assigned replicas, ISR: in-sync replicas

A. Failover during broker failure.

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on_broker_change():
The controller keeps in-memory for every partition: leader, AR
1. call change_leaders() on the current list of partitions

change_leaders():
Input: a list of partitions and their leader, AR
1. Read the current live broker list
2. Determine the set of partitions whose leader is not in the live broker list.
3. For each such partition P
3.1 Read the current ISR of P from 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).
    Question A.1, what happens if none of the brokers in AR is alive?
4. For each such partition p, write the new leader and ISR in /brokers/topics/[topic]/[partition_id]/leaderAndISR
5. PublishSend a StateChangeCommand about the new leader/ISR for each affected partition to the ZKQueue of the affected brokers. For efficiency, the controller can write the decision
 for  for all affected partitions in 1 path in ZKQueue.
(Ideally we want to use ZK multi to conditionally write 3 and 4do the writes in step 4 and 5 conditionally in 1 transaction for better latency and correctness.)
 We can also take advantage of ZK multi for reads in step 3.)

Question A.2,. shouldShould the broker send publishthe decisionsStateChangeCommand to the brokers that are currently down? If not, how do we guarantee that no decision changes are lost?
Question A.3, is broker down the only failure scenario that we worry about? Do we worry about 
Technically, we don't need to do that. On startup, the broker can get the latest state of each partition by reading /brokers/topics/[topic]/[partition_id]/leaderAndISR and
receive new StateChange commands from the controller afterwards.

Question A.3, is broker down the only failure scenario that we worry about? Do we worry about leader failure at individual partition level?

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on_leader_assignment_change():
1. Read from this broker's ZKQueue, the list of partitions whose leader/ISR has changed.
2. For each such partition P
2.1 If this broker is the new leader,
2.1.1 stop the fetcher to the current leader
2.1.2 become the leader (This is critical: the leader can only update the ISR in the future/brokers/topics/[topic]/[partition_id]/leaderAndISR in the future if it hasn't been changed by the controller)
2.2 If this broker is following a new leader
2.2.1 stop the fetcher to the current leader
2.2.2 become a follower

C. Creating/deleting topics.

Controller The controller watches child change of /brokers/topics. When the watcher gets triggered, it calls on_topic_change().

on_topic_change():
The controller keeps in memory a list of existing topics.
1. If a new topic is created, read topic's replica assignment.
1.1. call init_leaders() on all newly created partitions.
2. If a topic is deleted, send the stopReplica state change to all affected brokers.

Question C1. How to deal with repeated topic deletion/creation? A broker can be down for a long time during which a topic can be deleted and recreated. When the broker comes up, the topic it has locally may not match the content of the newly created topic. There are a couple of ways of dealing with this. (1) Simply let the broker with the outdated topic become a follower and figure out the right offset from which it can sync up with the leader. (2) Keep a version ID for each topic/partition. Delete a partition on broker startup if the partition version is outdated.

init_leaders():
Input: a list of partitions and their AR
0. Read the current live broker list
1. For each partition P
1.1 Select one of the live brokers in AR as the new leader and set all live brokers in AR as the new ISR.
2. For each such partition p, write the new leader and ISR in /brokers/topics/[topic]/[partition_id|partition_id]/leaderAndISR
3. Publish the new leader/ISR for each affected partition to the ZKQueue of the affected brokers. Again, for efficiency, the controller can write the decision for all affected partitions in 1 path in ZKQueue.

init_leaders():
Input: a list of partitions and their AR
0. Read the current live broker list
1. For each partition P
1.1 Select one of the live brokers in AR as the new leader and set all live brokers in AR as the new ISR.
2. For each such partition p, write the new leader and ISR in /brokers/topics/[topic]/[partition_id|partition_id]/leaderAndISR
3. Publish the new leader/ISR for each affected partition to the ZKQueue of the affected brokers. Again, for efficiency, the controller can write the decision
   for all affected partitions in 1 path in ZKQueue.
(Ideally we want to use ZK multi to do the writes in step 2 and 3 conditionally in 1 transaction for better latency and correctness.)

Question C1. How to deal with repeated topic deletion/creation? A broker can be down for a long time during which a topic can be deleted and recreated. When the broker comes up, the topic it has locally may not match the content of the newly created topic. There are a couple of ways of dealing with this. (1) Simply let the broker with the outdated topic become a follower and figure out the right offset from which it can sync up with the leader. (2) Keep a version ID for each topic/partition. Delete a partition on broker startup if the partition version is outdated.(Ideally we want to use ZK multi to conditionally write 2 and 3 in 1 transaction for better latency and correctness.)

D. Handling controller failure.

Each broker sets an exists watch on /controller (ephemeral). When the watcher gets triggered, it calls on_controller_failover(). Basically, the controller needs to inform each of the brokers all decisions that it has made in the history (since it's not sure if there is any decision lost during the controller failover). A broker can ignore decisions that it has followed already.

on_controller_failover():
1. create /controller \-> {this broker id; new broker epoc)
2. if successful
2.1 write all published decisions (leader/ISR for each partition) to ZKQueue to all brokers.
2.2 change_leaders()
2.3 for the list of partitions without a leader, call init_leaders().

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on_broker_startup():
1. read all /brokers/topics/[topic]
2. read /broker/leader_assignment/brokers/topics/[topic]/[partition_id|partition_id]/leaderAndISR
3. for each replica assigned to this broker
3.1 start replica
3.2 if this broker is a leader of this partition, become leader. (shouldn't happen in general)
3.3 if this broker is a follower of this partition, become follower.
4. subscribes to changes in ZKQueue for this broker.

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on_partitions_reassigned():
1. read /brokers/partitions_reassigned/[topic]
2. issue StartReplica command to the right brokers.
3. periodically check ISR of affected partitions
3.1 if ISR == AR+RAR, update ISR (??? need to do that in individual ISR and leader path), and send , and send StartReplica (to inform the leader of the new ISR) and StopReplica command to the right brokers.
3.2 update /brokers/topics/[topic] to change AR to the new replica set
3.3 delete /brokers/partitions_reassigned/[topic]
(An alternative approach to 3 is to set watches on ISR and do the check only when ISR is changed.)
4. inform the current leader of the ISR change by write ISRState change in ZKQueue

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In the critical path, the most time consuming operation is step 3 where we need to write 1 ZK path per partition. Assuming that during a broker failover we need to change leader for 10K partitions and each ZK write takes 2ms4ms, this could take 20 40 secs. One possibility is to use the multi() support in ZK 3.4 to batch those writes in 1 ZK operation.

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Occasionally, it's possible for multiple brokers to simultaneous assume that they are the leader of a partition. For example, broker A is the initial leader of a partition and the ISR of that partition is {A,B,C}.. Then, broker A goes into GC and losses its ZK registration. The controller assumes that broker A is dead, assigns the leader of the partition to broker B and sets the new ISR in ZK to {B,C}. Broker B becomes the leader and at the same time, Broker A wakes up from GC but hasn't acted on the leadership change decision made command sent by the controller. Now, both broker A and B think they are the leadersleader. It would be bad if we allow both broker A and B to commit new messages since the data among replicas will be out of sync. In fact, our Our current design actually will prevent broker A from committing any new message in the this from happening in this situation. Here is why. The claim is that after broker B becomes the new leader, broker A can no longer commit new messages any more. For broker A to commit a message m, it needs every replica in ISR to receive m. At the moment, broker A still thinks the ISR is {A,B,C} (its local copy; although the ISR in ZK has changed). Broker B will never receive message m. This is because by becoming the new leader, it must have first stopped fetching data from the previous leader. Therefore broker A can't commit message m without shrinking the ISR first. In order to shrink ISR, broker A has to write the new ISR in ZK. However, it can't do that because it will realize that the leaderAndISR node in ZK is not on a version that it assumes to be (since it has already been changed by the controller). At this moment, broker A will realize that it's no longer the leader any more.