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[LI-HOTFIX] Avoid assigning replicas to the preferred controllers or maintenance brokers #111

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[LI-HOTFIX] Avoid assigning replicas to the preferred controllers or maintenance brokers #111

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0536dc7
Fixing bug so that replicas won't be assigned to maintenance brokers …
gitlw Jan 7, 2021
3f16983
Adding test to ensure the bug doesn't happen again
gitlw Jan 7, 2021
caddfe1
Added test and fixes on the broker side to rearrange assignments duri…
gitlw Jan 7, 2021
38e9116
Check supplied topics without considering they are new or not
gitlw Jan 8, 2021
0597342
Reject reassigning partitions to unassignable brokers
gitlw May 27, 2021
821a23e
Adding test to ensure partition reassignment don't like on maintenanc…
gitlw May 28, 2021
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22 changes: 13 additions & 9 deletions core/src/main/scala/kafka/controller/KafkaController.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -880,7 +880,7 @@ class KafkaController(val config: KafkaConfig,
// between the moment this broker started and right now when it becomes controller again.
loadMinIsrForTopics(controllerContext.allTopics)

rearrangePartitionReplicaAssignmentForNewTopics(controllerContext.allTopics.toSet)
rearrangePartitionReplicaAssignmentForNewPartitions(controllerContext.allTopics.toSet)
registerPartitionModificationsHandlers(controllerContext.allTopics.toSeq)
getReplicaAssignmentPolicyCompliant(controllerContext.allTopics.toSet).foreach {
case (topicPartition, replicaAssignment) =>
Expand Down Expand Up @@ -968,25 +968,28 @@ class KafkaController(val config: KafkaConfig,

// Rearrange partition and replica assignment for new topics that get assigned to
// maintenance brokers that do not take new partitions
private def rearrangePartitionReplicaAssignmentForNewTopics(topics: Set[String]) {
private def rearrangePartitionReplicaAssignmentForNewPartitions(topicsToCheck: Set[String]) {
try {
val noNewPartitionBrokers = partitionUnassignableBrokerIds
if (noNewPartitionBrokers.nonEmpty) {
val newTopics = zkClient.getPartitionNodeNonExistsTopics(topics.toSet)
val newTopicsToBeArranged = zkClient.getPartitionAssignmentForTopics(newTopics).filter {
case (_, partitionMap) =>
partitionMap.exists {
val topicsToBeRearranged = zkClient.getPartitionAssignmentForTopics(topicsToCheck.toSet).filter {
case (topic, partitionMap) =>
val existingAssignment = controllerContext.partitionAssignments.getOrElse(topic, mutable.Map.empty)
val newPartitions = partitionMap.filter{case (partitionId, _) => partitionId >= existingAssignment.size}
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could we add some safe check here to ensure the partition state znode doesn't exist for these new partitions. Although I don't see an issue in the current implementation, if this function is used incorrectly, it would be very dangerous. For example, if we use "rearrangePartitionReplicaAssignmentForNewPartitions(topics, false)" when initializing controller context, this may cause all topics to get reassigned since controllerContext.partitionAssignments is an empty set (this will also result in orphan partitions).
Again, I don't see the issue in this implementation, but I think it is worth checking.

In addition, this doesn't address the case when there is controller move right after partition expansion (before the rearrange actually happened). I think it is ok since we are not trying to handle 100% no replicas in the preferred controller. Maybe we can add some comments for this unhandled situations.

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Thanks for the comments. It's true that they are all newPartitions given controllerContext.partitionAssignments is an empty set, but they shouldn't have a replica on the noNewPartitionBrokers, thus they shouldn't be rearranged.

It's a good point that the current implementation cannot handle controller switches. Given it's safe to scan all topics' partitions on controller switch, I think it should be done during a controller switch. Thoughts?

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manual assignment for existing topics can still assign partitions to noNewPartitionBrokers. I think we cannot guarantee that noNewPartitionBrokers won't get replicas. In addition, there are some small-time window that new replicas can still get assigned to preferred controllers due to the fact that preferred controller znode is emphermal znode (see the design doc for more detail).

I think a safer way is to rely on the existence of partition state znode when performing rearrangement.

scan all topics' partitions on controller switch => If we cannot guarantee 100% no replica in the preferred controllers, I think it is ok to not performing special handing during controller switch given the overhead/additional hacky code needed (up to you to make a decision).

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Upon closer look, I find that there is already logic to handle the controller switch over by calling the rearrangePartitionReplicaAssignmentForNewPartitions method inside initializeControllerContext.

Regarding the preferred controller znodes being ephemeral, it's kinda an orthogonal design issue that we could address independently.

newPartitions.exists {
case (_, assignedReplicas) =>
assignedReplicas.replicas.intersect(noNewPartitionBrokers).nonEmpty
}
}
newTopicsToBeArranged.foreach {
topicsToBeRearranged.foreach {
case (topic, partitionMap) =>
val numPartitions = partitionMap.size
val numReplica = partitionMap.head._2.replicas.size
val brokers = controllerContext.liveOrShuttingDownBrokers.map { b => kafka.admin.BrokerMetadata(b.id, b.rack) }.toSeq

val replicaAssignment = adminZkClient.assignReplicasToAvailableBrokers(brokers, noNewPartitionBrokers.toSet, numPartitions, numReplica)
val existingAssignment = controllerContext.partitionAssignments.getOrElse(topic, mutable.Map.empty)
val partitionsToAdd = numPartitions - existingAssignment.size
val replicaAssignment = adminZkClient.assignReplicasToAvailableBrokers(brokers, noNewPartitionBrokers.toSet, partitionsToAdd, numReplica, -1, existingAssignment.size)
adminZkClient.writeTopicPartitionAssignment(topic, replicaAssignment.mapValues(ReplicaAssignment(_)).toMap, true)
info(s"Rearrange partition and replica assignment for topic [$topic]")
}
Expand Down Expand Up @@ -1697,7 +1700,7 @@ class KafkaController(val config: KafkaConfig,
val newTopics = topics -- controllerContext.allTopics
val deletedTopics = controllerContext.allTopics -- topics
controllerContext.allTopics = topics
rearrangePartitionReplicaAssignmentForNewTopics(newTopics)
rearrangePartitionReplicaAssignmentForNewPartitions(newTopics)

registerPartitionModificationsHandlers(newTopics.toSeq)
val addedPartitionReplicaAssignment = getReplicaAssignmentPolicyCompliant(newTopics)
Expand Down Expand Up @@ -1755,6 +1758,7 @@ class KafkaController(val config: KafkaConfig,
}

if (!isActive) return
rearrangePartitionReplicaAssignmentForNewPartitions(immutable.Set(topic))
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does it conflict with partition reassignment?
say if a partition gets reassigned to replica ( 1, 2, 5, 6 ) ==> if one of this replica is maintenance brokers, would this reassignment complete if we automatically changing zk node to disallow placing replicas on maintenance brokers.

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That's a good point. I've updated the PR to avoid assignment replicas to the undesirable hosts during partition reassignment. Please take another look. Thanks!

val partitionReplicaAssignment = zkClient.getReplicaAssignmentForTopics(immutable.Set(topic))
val partitionsToBeAdded = partitionReplicaAssignment.filter { case (topicPartition, _) =>
controllerContext.partitionReplicaAssignment(topicPartition).isEmpty
Expand Down
4 changes: 2 additions & 2 deletions core/src/main/scala/kafka/zk/AdminZkClient.scala
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Expand Up @@ -53,7 +53,7 @@ class AdminZkClient(zkClient: KafkaZkClient) extends Logging {
topicConfig: Properties = new Properties,
rackAwareMode: RackAwareMode = RackAwareMode.Enforced): Unit = {
val brokerMetadatas = getBrokerMetadatas(rackAwareMode)
val noNewPartitionBrokerIds = getMaintenanceBrokerList()
val noNewPartitionBrokerIds = getMaintenanceBrokerList() ++ zkClient.getPreferredControllerList
val replicaAssignment = assignReplicasToAvailableBrokers(brokerMetadatas, noNewPartitionBrokerIds.toSet, partitions, replicationFactor)
createTopicWithAssignment(topic, topicConfig, replicaAssignment)
}
Expand Down Expand Up @@ -235,7 +235,7 @@ class AdminZkClient(zkClient: KafkaZkClient) extends Logging {
numPartitions: Int = 1,
replicaAssignment: Option[Map[Int, Seq[Int]]] = None,
validateOnly: Boolean = false): Map[Int, Seq[Int]] = {
val noNewPartitionBrokerIds = getMaintenanceBrokerList()
val noNewPartitionBrokerIds = getMaintenanceBrokerList() ++ zkClient.getPreferredControllerList
addPartitions(topic, existingAssignment, allBrokers, numPartitions, replicaAssignment, validateOnly, noNewPartitionBrokerIds.toSet)
}

Expand Down
28 changes: 28 additions & 0 deletions core/src/test/scala/unit/kafka/server/PreferredControllerTest.scala
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Expand Up @@ -33,6 +33,7 @@ import org.scalatest.Assertions.fail

import scala.collection.JavaConverters._
import scala.collection.Map
import scala.collection.immutable

class PreferredControllerTest extends ZooKeeperTestHarness {

Expand Down Expand Up @@ -71,6 +72,33 @@ class PreferredControllerTest extends ZooKeeperTestHarness {
client.close()
}

@Test
def testPartitionCreatedByAdminZkClientShouldNotBeAssignedToPreferredControllers(): Unit = {
val brokerConfigs = Seq((0, false), (1, true), (2, false))
createBrokersWithPreferredControllers(brokerConfigs, true)

TestUtils.waitUntilControllerElected(zkClient)
// create topic using admin client
val topic = "topic1"
TestUtils.createTopic(zkClient, topic, 3, 2, brokers)

assertTrue("topic1 should not be in broker 1", ensureTopicNotInBrokers("topic1", Set(1)))

val existingAssignment = zkClient.getFullReplicaAssignmentForTopics(immutable.Set(topic)).map {
case (topicPartition, assignment) => topicPartition.partition -> assignment
}
val allBrokers = adminZkClient.getBrokerMetadatas()
val newPartitionsCount = 5
adminZkClient.addPartitions(topic, existingAssignment, allBrokers, 5)
(0 until newPartitionsCount).map { i =>
TestUtils.waitUntilMetadataIsPropagated(brokers, topic, i)
i -> TestUtils.waitUntilLeaderIsElectedOrChanged(zkClient, topic, i)
}

assertTrue("topic1 should not be in broker 1 after increasing partition count",
ensureTopicNotInBrokers("topic1", Set(1)))
}

@Test
def testElectionWithoutPreferredControllersAndNoFallback(): Unit = {
val brokerConfigs = Seq((0, false), (1, false), (2, false))
Expand Down