Master KIP

KIP-500: Replace ZooKeeper with a Self-Managed Metadata Quorum (Accepted)

Status

Current state: in reviewaccepted

Discussion thread: here

JIRA: 

Release: controller-side changes in 2.4, command line changes in 2.6

Motivation

Currently, users initiate replica reassignment by writing directly to a ZooKeeper node named /admin/reassign_partitions.

As explained in KIP-4, ZooKeeper based APIs have many problems.  For example, there is no way to return a helpful error code if an invalid reassignment is partition is proposed.  Adding new features over time is difficult when external tools can access and overwrite Kafka's internal metadata data structures.  Also, ZooKeeper-based APIs inherently lack security and auditability.

...

We would like to provide a well-supported AdminClient API that does not suffer from these problems. Namely, it should support incremental replica reassignments and cancellation (revert) of ongoing reassignments.
This API can be used as a foundation on which to build future improvements.

...

AdminClient APIs

We will add two new admin APIs: alterPartitionAssignments, and listPartitionReassignments.  As the names imply, the alter API modifies partition reassignments, and the list API lists the ones which are ongoing.

...

/**
 * Change the reassignments for one or more partitions.
 *
 * @param reassignments   The reassignments to add, modify, or remove.
 * @param  Providing an empty Optional (e.g via {@link Optional#empty()}) will <bold>cancel</bold> the reassignment for the associated partition.
 *
 * @param reassignments   The reassignments to add, modify, or remove.
 * @param options         The options to use.
 * @return                The result.
 */
public AlterPartitionReassignmentsResult alterPartitionReassignments(
         Map<TopicPartition, Optional<NewPartitionReassignment>> reassignments,
         AlterPartitionReassignmentsOptions options);

/**
 * A new partition reassignment, which can be applied via {@link AdminClient#alterPartitionReassignments(Map)}.
 */
public class NewPartitionReassignment {
    private final List<Integer> targetBrokerstargetReplicas;

    ...
}

class AlterPartitionAssignmentsResult {
  Map<TopicPartition, Future<Void>> futures; // maps partitions to the results for each partition (success / failure)
  Future<Void> all(); /**
     * @throws IllegalArgumentException if no replicas are supplied
	 */
    public NewPartitionReassignment(List<Integer> targetReplicas)

    public List<Integer> targetReplicas()
}

class AlterPartitionAssignmentsResult {
  Map<TopicPartition, KafkaFuture<Void>> values()
  Future<Void> all(); // Throws an exception if any reassignment was rejected
}

class AlterPartitionAssignmentsOptions extends AbstractOptions<> {
  // contains timeoutMs
}

/**
 * List some of the current partition reassignments.
 * 
 * @param optionspartitions     The partitions  The options to useto show reassignments for. Must be non-null.
 * @param partitionsoptions     The partitions to showThe reassignmentsoptions for.  Must be non-nullto use.
 */
public ListPartitionReassignmentsResult listPartitionReassignments(
      ListPartitionReassignmentsOptionsSet<TopicPartition> optionspartitions,
      Set<TopicPartition>ListPartitionReassignmentsOptions partitionsoptions);


/**
 * List all of the current partition reassignments for the given topics.
 .
 * 
 * @param options        The options to use.
 */
public @paramListPartitionReassignmentsResult topicslistPartitionReassignments(
      ListPartitionReassignmentsOptions options);

abstract ListPartitionReassignmentsResult The topics to show reassignments for.  Must be non-null.
 */
ListPartitionReassignmentsResult listPartitionReassignments(
listPartitionReassignments(Optional<Set<TopicPartition>> partitions,
                ListPartitionReassignmentsOptions options,
      Set<String> topics);


/**
 * List all of the current partition reassignments.
 * 
 * @param options        The options to use.
 */
ListPartitionReassignmentsResult listPartitionReassignments(
                     ListPartitionReassignmentsOptions options);

class ListPartitionReassignmentsOptions extends AbstractOptions<> {
  // contains timeoutMs
}

class ListPartitionReassignmentsResult {
  private final KafkaFuture<Map<TopicPartition, PartitionReassignment>> reassignments;
}

/**
 * A partition reassignment, which has been listed via {@link AdminClient#listPartitionReassignments()}.
 */
public class PartitionReassignment {
    /**
     * The brokers which we want this partition tocurrently resideresides on.
     */
    private final List<Integer> replicas;

    /**
     * The brokers that we are adding this partition to as part of a reassignment.
     */
    private final List<Integer> addingReplicas;

    /**
     * The brokers that we are removing this partition from as part of a reassignment.
     */
    private final List<Integer> removingReplicas;
}

...

This operation requires ALTER on CLUSTER.

Note that even though we have TimeoutMs, this field will be ignored in the implementation for the time being. Its implementation is tracked in 

{
  "apiKey": 45,
  "type": "request",
  "name": "AlterPartitionReassignmentsRequest",
  "validVersions": "0",
  "fields": [
    { "name": "TimeoutMs", "type": "int32", "versions": "0+", "default": "60000",
      "about": "The time in ms to wait for the request to complete." },
    { "name": "Topics", "type": "[]ReassignableTopic", "versions": "0+",
      "about": "The topics to reassign.", "fields": [
      { "name": "Name", "type": "string", "versions": "0+",
        "about": "The topic name." },
      { "name": "Partitions", "type": "[]ReassignablePartition", "versions": "0+",
        "about": "The partitions to reassign.", "fields": [
        { "name": "PartitionIndex", "type": "int32", "versions": "0+",
          "about": "The partition index." },
        { "name": "Replicas", "type": "[]int32", "versions": "0+", "nullableVersions": "0+", "default": "null",
          "about": "The replicas to place the partitions on, or null to cancel a pending reassignment for this partition." }
      ]}
    ]}
  ]
}

...

This is the response from AlterPartitionsReassignmentsRequest.

Possible top-level errors:

• REQUEST_TIMED_OUT: if the request timed out.
• NOT_CONTROLLER: if the node we sent the request to was not the controller.INVALID
• CLUSTER_REPLICAAUTHORIZATION_FAILED: If we didn't have sufficient permission to perform the alteration.

Possible partition-level errors:

• INVALID_REPLICA_ASSIGNMENT - when the replica assignment is not valid (brokers are not part of the cluster, negative replica ids, empty assignment)ASSIGNMENT: if the specified replica assignment was not valid-- for example, if it included negative numbers, repeated numbers, or specified a broker ID that the controller was not aware of.
• NO_REASSIGNMENT_IN_PROGRESS : If - when a cancellation was called on a topic/partition which was not in the middle of a reassignmentCLUSTER
• UNKNOWN_TOPIC_AUTHORIZATION_FAILED: If we didn't have sufficient permission to perform the alteration.OR_PARTITION - when a topic/partition doesn't exist or a topic deletion is in progress

{
  "apiKey": 45,
  "type": "response",
  "name": "AlterPartitionReassignmentsResponse",
  "validVersions": "0",
  "fields": [
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "0+",
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The error code." },
    { "name": "ErrorStringErrorMessage", "type": "string", "versions": "0+", "nullableVersions": "0+",
      "about": "The error stringmessage, or null if there was no error." }
    { "name": "Responses", "type": "[]ReassignableTopicResponse", "versions": "0+",
      "about": "The responses to topics to reassign.", "fields": [
      { "name": "Name", "type": "string", "versions": "0+", "entityType": "topicName",
        "about": "The topic name" },
      { "name": "ErrorCodePartitions", "type": "int16[]ReassignablePartitionResponse", "versions": "0+",
        "about": "The error code." },
 responses to partitions to reassign", "fields": [
          { "name": "ErrorStringPartitionIndex", "type": "stringint32", "versions": "0+", "nullableVersions": "0+",

            "about": "The partition errorindex." string},
 or  null if there was no error." }
      { "name": "PartitionsErrorCode", "type": "[]ReassignablePartitionResponseint16", "versions": "0+",
            "about": "The responseserror to partitions to reassign", "fields": [code." },
          { "name": "PartitionIndexErrorMessage", "type": "int32string", "versions": "0+", "nullableVersions": "0+",
            "about": "The partition index error message, or null if there was no error." },
        ]
  { "name": "ErrorCode", "type": "int16", "versions": "0+",
            "about": "The error code." },
          { "name": "ErrorString", "type": "string", "versions": "0+", "nullableVersions": "0+",
            "about": "The error string, or null if there was no error." }
        ]
      ]
    }
  ]
} ]
    }
  ]
}

ListPartitionReassignmentsRequest

...

It requires DESCRIBE on CLUSTER.

Note that even though we have TimeoutMs, this field will be ignored in the implementation for the time being. Its implementation is tracked in 

{
 

{
  "apiKey": 46,
  "type": "request",
  "name": "ListPartitionReassignmentsRequest",
  "validVersions": "0",
  "fields": [
    { "name": "TimeoutMs", "type": "int32", "versions": "0+", "default": "60000",
      "about": "The time in ms to wait for the request to complete." }
    { "name": "Topics", "type": "[]ListPartitionReassignmentsTopics", "versions": "0+", "nullableVersions": "0+",
      "about": "The topics to list partition reassignments for, or null to list everything.", "fields": [
      { "name": "PartitionIndexesName", "type": "[]int32string", "versions": "0+", "entityType": "topicName",
        "about": "The partitions to list partition reassignments fortopic name" },
    ]}
   ]
}{ "name": "PartitionIndexes", "type": "[]int32", "versions": "0+",
        "about": "The partitions to list partition reassignments for" }
    ]}
  ]
}

ListPartitionReassignmentsResponse

Possible errors:

• REQUEST_TIMED_OUT: if the request timed out.
• NOT_CONTROLLER: if the node we sent the request to is not the controller.
• CLUSTER_AUTHORIZATION_FAILED: if we didn't have sufficient permissions.

If the top-level error code is set, no responses will be provided.

If a partition is not going through a reassignment, its AddingReplicas and RemovingReplicas fields will simply be empty.

If a partition doesn't exist, no response will be returned for it.

{

{
  "apiKey": 46,
  "type": "response",
  "name": "ListPartitionReassignmentsResponse",
  "validVersions": "0",
  "fields": [
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "0+",
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The top-level error code, or 0 on success." },
    { "name": "ErrorMessage", "type": "string", "versions": "0+", "nullableVersions": "0+",
      "about": "The top-level error message, or null on success." },
    { "name": "Topics", "type": "[]OngoingTopicReassignment", "versions": "0+",
      "about": "The ongoing reassignments for each topic.", "fields": [
        { "name": "Name", "type": "string", "versions": "0+",
          "about": "The topic name." },
        { "name": "Partitions", "type": "[]OngoingPartitionReassignment", "versions": "0+",
          "about": "The ongoing reassignments for each partition.", "fields": [
          { "name": "PartitionIndex", "type": "int32", "versions": "0+",
            "about": "The index of the partition." },
          { "name": "Replicas", "type": "[]int32", "versions": "0+",
            "about": "The current replica set." },
          { "name": "AddingReplicas", "type": "[]int32", "versions": "0+",
            "about": "The set of replicas we are currently adding." },
          { "name": "RemovingReplicas", "type": "[]int32", "versions": "0+",
            "about": "The set of replicas we are currently removing." }
      ]}
    ]}
  ]
}

...

LeaderAndIsrRequest

We will add a two new field fields to LeaderAndIsrRequest named TargetReplicas.  This field will AddingReplicas and RemovingReplicas. These fields will, respectively, contain the replicas which will exist once the partition reassignment is complete.are being added and removed from a partition's assignment.

These fields allow the controller to propagate reassignment information to the leader/follower brokers, paving the way for some of the improvements outlined in the Future Work section of this KIP. We will not make use of these fields as of this KIP.

diff --

diff --git a/clients/src/main/resources/common/message/LeaderAndIsrRequest.json b/clients/src/main/resources/common/message/LeaderAndIsrRequest.json
index b8988351c..d45727078 100644
--- a/clients/src/main/resources/common/message/LeaderAndIsrRequest.json
+++ b/clients/src/main/resources/common/message/LeaderAndIsrRequest.json
@@ -20,6 +20,8 @@
   // Version 1 adds IsNew.
   //
   // Version 2 adds broker epoch and reorganizes the partitions by topic.
+  //
+  // Version 3 adds AddingReplicas and RemovingReplicas.
   "validVersions": "0-3",
   "fields": [
     { "name": "ControllerId", "type": "int32", "versions": "0+",
@@ -48,6 +50,8 @@
           "about": "The ZooKeeper version." },
         { "name": "Replicas", "type": "[]int32", "versions": "0+",
           "about": "The replica IDs." },
+        { "name": "AddingReplicas", "type": "[]int32", "versions": "3+",
+          "about": "The replica IDs that we are adding this partition to." },
+        { "name": "RemovingReplicas", "type": "[]int32", "versions": "3+",
+          "about": "The replica IDs that we are removing this partition from." },
         { "name": "IsNew", "type": "bool", "versions": "1+", "default": "false", "ignorable": true, 
           "about": "Whether the replica should have existed on the broker or not." }
       ]}

...

We propose supporting the "cancellation" of an on-going partition rebalance.   Cancellation is not the same as a revert. It is essentially a rollback of a reassignment.
To concretize the notion - a cancellation of a rebalance reassignment means stopping stopping any on-going partition movements . A partition reassignment which is half done will not have the new replicas that have entered the ISR be reverted, it will only stop the in-flight movements for replicas that are not in ISR.  Note that it is possible to have a partition's replication factor become higher than configured if a reassignment is stopped half-way. We do not propose a clear way to guard against this nor revert it – users are expected to run another reassignment.and restoring the replica set to the one it was prior to the reassignment being initiated.
There is no guarantee that the reverted replica set will be in the same order as it was previously.

kafka-reassign-partitions.sh

kafka-reassign-partitions.sh is a script to create, inspect, or modify partition reassignments.

Deprecation of --zookeeper

We will deprecate We will remove the --zookeeper option from in this script.  New operations introduced by this KIP (cancellation, list) will not be supported by the ZK flag.

All old operations and new will be supported Instead, all operations will be performed through the admin client.  This will be initialized by the --bootstrap-server flag, which already exists.

The --list flag

There will be a new flag, --list, which will list all of the current replica reassignments that are going on in the cluster.  They will be output in the same JSON format that is accepted by --reassignment-json-fileFor each reassignment, we will list the replica set, the adding replicas, and the removing replicas.  If there are no ongoing replica reassignments, an empty JSON object "No partition reassignments found." will be printed.

Cancellation with the tool

The --additional flag

Like the underlying KIP-455 API, the The reassign-partitions tool will now work in an incremental fashion – any .  Any new reassignment will be added on top of the current ones. It will continue to output a rollback JSON as part of any reassignment in order to let the user revert the changes.

Additionally, we will support a --create-cancellation-plan flag.  This flag will create a JSON file that, if applied, will cancel all on-going partition reassignments.

Not that there is a slight race condition here when using the tool.  Someone may create a new reassignment in between creating the cancellation plan and applying it.  This new reassignment would not be cancelled by the original cancellation plan.  In general, we expect system administrators to stop any process or person that is creating new reassignments before cancelling all pending reassignments, so we don't expect this to be a problem in practice.  If such processes are not stopped, the effect of cancelling all pending reassignments will be negated anyway, by the creation of new reassignments.

kafka-topics.sh

When describing partitions, kafka-topics.sh will show two additional fields, "addingReplicas," and "removingReplicas."  These will have information about the adding and removing replicas for the given partition. kafka-topics.sh will get this information from listReassigningPartitions.

Implementation

ZooKeeper Changes

/brokers/topics/[topic]

We will store the reassignment data for each partition in this znode, as that is where we already store the current replicaSet for every partition.
This node will store two new maps, named addingReplicas and removingReplicas. Each key is the partition and the value is an array of integers denoting the replicas we should add or remove for that partition.

...

However, to avoid confusing users, the tool will decline to create a new reassignment if there is already one in progress.  Users can choose to create additional reassignments even if there is one in progress by specifying the new --additional flag.  This flag is only supported when --bootstrap-server is provided.

There is also one slight behavior change to --execute that is worth noting here.  Previously, when --execute declined to create a new reassignment because one was already in progress, it would still apply the provided throttle, if any.  In a world of concurrent reassignments, this behavior no longer makes sense, and has been removed.  (However, users can still modify the throttle by simply supplying the --additional flag)

Cancellation with the tool

Additionally, we will support a --cancel flag.  This flag will create a JSON file that, if applied, will cancel all on-going partition reassignments.

Not that there is a slight race condition here when using the tool.  Someone may create a new reassignment in between creating the cancellation plan and applying it.  This new reassignment would not be cancelled by the original cancellation plan.  In general, we expect system administrators to stop any process or person that is creating new reassignments before cancelling all pending reassignments, so we don't expect this to be a problem in practice.  If such processes are not stopped, the effect of cancelling all pending reassignments will be negated anyway, by the creation of new reassignments.

kafka-topics.sh

When describing partitions, kafka-topics.sh will show two additional fields, "addingReplicas," and "removingReplicas."  These will have information about the adding and removing replicas for the given partition. kafka-topics.sh will get this information from listReassigningPartitions.

Implementation

ZooKeeper Changes

/brokers/topics/[topic]

We will store the reassignment data for each partition in this znode, as that is where we already store the current replicaSet for every partition.
This node will store two new maps, named addingReplicas and removingReplicas. Each key is the partition and the value is an array of integers denoting the replicas we should add or remove for that partition.

If both maps are empty, there is no on-going reassignment for that topic.

{
  "version": 2,
  

{
  "version": 1,
  "partitions": {"0": [1, 4, 2, 3] },
  "addingReplicas": {"0": [4] } # <----- NEW
  "removingReplicas": {"0": [1] }, # <---- NEW
}

Because we are not making incompatible changes to the znode, we will not be bumping up the inter-broker protocol versionThe znode will now be at version 2 rather than 1.  This change is backwards-compatible-- older brokers can read the new ZNode.

Controller Changes

When the controller starts up, it will load all of the currently active reassignments from the `topics/[topic]' znodes and the `admin/reassign_partitions` znode, with the latter taking precedence. This will not impose any additional startup overhead, because the controller already reads these znodes during start up.
The reason for having the `reassign_partitions` take precedence is that we cannot know for sure whether the Controller has taken action on that znode, so we allow it to overwrite the API-assigned reassignments in the event of a failover.

...

The controller will handle the AlterPartitionAssignmentsResponse RPC by modifying the /brokers/topics/[topic] znode.  Once the node is modified, the controller will send out LeaderAndIsrRequest to start all the movements.Once the controller is notified that a new replica has entered the ISR for a particular partition, it will remove it from the "addingReplicas" field in `by modifying the /brokers/topics/[topic] `. If "addingReplicas" becomes emptyznode.  Once the node is modified, the controller will send a new wave of LeaderAndIsr requests to retire all the old replicas and, if applicable, elect new leadersout LeaderAndIsrRequest to start all the movements.

Algorithm

We will always empty out the "addingReplicas" field before starting to act on the "removingReplicas" field.always add all the replicas in the "addingReplicas" field before starting to act on the "removingReplicas" field.

# Illustrating the rebalance of a single partition.
# R is the current replicas, I is the ISR list, AR is addingReplicas and RR is removingReplicas

# Illustrating the rebalance of a single partition.
# R is the current replicas, I is the ISR list, AR is addingReplicas and RR is removingReplicas
R: [1, 2, 3], I: [1, 2, 3], AR: [], RR: []
# Reassignment called via API with targetReplicas=[4,3,2]

R: [1, 4, 3, 2], I: [1, 2, 3], AR: [4], RR: [1] # Controller sends LeaderAndIsr requests 
# (We ignore RR until AR is empty)
R: [1, 42, 3, 2], I: [1, 2, 3, 4], AR: [4], RR: [1] # Leader 1 adds 4 to ISR
# ControllerReassignment pickscalled upvia onAPI the change and removes 4 from AR.
# (in-memory) - with targetReplicas=[4,3,2]

R: [1, 4, 3, 2], I: [1, 2, 3, 4], AR: [4], RR: [1] 
# Controller realizes AR is empty and starts work on RR. Removes all of RR from R and sends LeaderAndIsr requestssends LeaderAndIsr requests 
# (We ignore RR until AR is empty)
R: [1, 4, 3, 2], I: [1, 2, 3, 4], AR: [4], RR: [1] # Leader 1 adds 4 to ISR
# The controller atrealizes thisthat pointall the reassignment is considered complete
# Leaders change, 1 retires and 4 starts leading. 4 shrinks the ISR replicas in AR have been added and starts work on RR. Removes all of RR from R and from the ISR, and sends StopReplica/LeaderAndIsr requests
R: [4, 3, 2], I: [2, 3, 4], AR: [], RR: [] # at this point the reassignment is considered complete

When all of the replicas in AR have been added, we remove RR from R and empty out AR/We iteratively shrink the AR collection. When AR is empty, we empty out RR in one step. 

If a new reassignment is issued during an on-going one, we cancel the current one by emptying out both AR and RR, constructing them one, we cancel the current one by emptying out both AR and RR, send StopReplica requests to the replicas that were part of the old reassignment but not part of the new one, construct AR/RR from (the updated from the last-reassignment) R and TR, and starting start anew.

In general this algorithm is consistent with the current Kafka behavior - other brokers still get the full replica set consisting of both the original replicas and the new ones.

Note that because of iteratively shrinking the AR collection, the server will not know how to create a "revert" of a reassignment, as it is always seeing it in motion. To keep the server logic simpler, we defer to clients for potential reverts. Here is an example of a cancellation:

# Illustrating the rebalance of a single partition.
# R is the current replicas, I is the ISR list, RR is removingReplicas and AR is addingReplicas
R: [1, 2, 3], I: [1, 2, 3], AR: [], RR: []
# Reassignment called via API with targetReplicas=[3, 4, 5]
R: [1, 2, 3, 4, 5], I: [1, 2, 3], AR: [4, 5], RR: [1, 2]
R: [1, 2, 3, 4, 5], I: [1, 2, 3, 4], AR: [4, 5], RR: [1, 2]
# Cancellation called
R: [1, 2, 3, 4], I: [1, 2, 3, 4], AR: [], RR: []

Essentially, once a cancellation is called we subtract AR from R, empty out both AR and RR, and send StopReplica/LeaderAndIsr requests to cancel the replica movements that have not yet completed.

...