THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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Status
Current state: One of "Under Discussion"Accepted
Discussion Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]
JIRA: here [Change the link from KAFKA-1 to your own ticket]
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
Motivation
The admin client allows users to instruct the controller to attempt to elect the preferred replica on a given set of topic partitions. In addition to preferred replica leader elections the controller also supports three other types of elections. In this proposal we modify the “PreferredLeaderElection” RPC to also support unclean leader election. The new RPC also makes it possible to easily add new type of elections in the future.
Public Interfaces
Network protocol
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PR: https://github.com/apache/kafka/pull/6686
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
Motivation
The admin client allows users to instruct the controller to attempt to elect the preferred replica as leader on a given set of topic partitions. In addition to preferred replica leader elections the controller also supports three other types of elections. One of those other election types is what we call unclean leader election. For the user to enable and trigger unclean leader election they need to either modify the topic configuration or the broker configuration. We believe that this is potentially a dangerous configuration that could lead to data loss. Specially if the user forgets to disable unclean leader election after it completes. Being able to trigger unclean leader election once for a given partition is a much safer mechanism.
In this proposal we modify the “PreferredLeaderElection” RPC to support unclean leader election in addition to preferred leader election. The new RPC definition also makes it possible to easily add new type of elections in the future.
Public Interfaces
Network protocol
Request
Added a top level property called ElectionType with supported values of 0 and 1.
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{
"apiKey": 43,
"type": "request",
"name": "ElectLeadersRequest",
"validVersions": "0-1",
"fields": [
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{ "apiKey": 43, "type": "request", "name": "ElectLeadersRequest", "validVersions": "0-1", "fields": [ { "name": "TopicPartitions", "type": "[]TopicPartitions", "versions": "0+", "nullableVersions": "0+", "about": "The topic partitions to elect leaders.", "fields": [ { "name": "Topic", "type": "string", "versions": "0+", "about": "The name of a topic." }, { "name": "PartitionId", "type": "[]int32", "versions": "0", "about": "The partitions of this topic whose preferred leader should be elected." }, { "name": "PartitionsElectionType", "type": "[]Partitionsint8", "versions": "1+", "about": "The partitionsType of thiselections topic whose leader should be elected.", "fields": [ to conduct for the partition. A value of '0' elects the preferred replica. A value of '1' elects the first live replica if there are no in-sync replica." }, { "name": "PartitionIdTopicPartitions", "type": "int32[]TopicPartitions", "versions": "10+", "nullableVersions": "0+", "about": "The partition id topic partitions to elect leaders." }, "fields": [ { "name": "ElectionTypeTopic", "type": "int8string", "versions": "10+", "about": "TypeThe name of electionsa to conduct for the partition. A value of '0' elects the preferred leader. A value of '1' elects an unclean leader if there are no in-sync leaders." } ] topic." }, { "name": "PartitionId", "type": "[]int32", "versions": "0+", "about": "The partitions of this topic whose leader should be elected." } ] }, { "name": "TimeoutMs", "type": "int32", "versions": "0+", "default": "60000", "about": "The time in ms to wait for the election to complete." } ] } |
AdminClient Abstract Class
Response
Added a top level property called ErrorCode for returning errors that apply to all the topic partitions. The current implementation is using this to return cluster authorization errorsA new method will be added to the AdminClient abstract class to support this new version of the RPC.
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{ "apiKey": 43, "type": "response", "name": "ElectLeadersResponse", "validVersions": "0-1", "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": "1+", "ignorable": false, "about": "The top level response error code." }, { "name": "ReplicaElectionResults", "type": "[]ReplicaElectionResult", "versions": "0+", "about": "The election results, or an empty array if the requester did not have permission and the request asks for all partitions.", "fields": [ { "name": "Topic", "type": "string", "versions": "0+", "entityType": "topicName", "about": "The topic name" }, { "name": "PartitionResult", "type": "[]PartitionResult", "versions": "0+", "about": "The results for each partition", "fields": [package org.apache.kafka.clients.admin; public abstract class AdminClient ... { ... /** * Attempt to elect a new leader for each of the topic partition in {@code partitionElections}. The type of elections supported are * document in the {@link TopicPartitionElection} type. * * If {@code partitionElections} is null, then attempt to elect the preferred replica for all of the partitions. * * @param partitionElections The partitions and the type of elections to conduct. * @param options The options to use when electing the leaders. * @return The ElectLeadersResult. */ public abstract ElectLeadersResult electLeaders( Collection<TopicPartitionElection> partitionElections{ "name": "PartitionId", "type": "int32", "versions": "0+", "about": "The partition ElectLeadersOptions options); } |
TopicPartitionElection Class
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package org.apache.kafka.common; public final class TopicPartitionElection { public static enum ElectionType { id" }, { "name": "ErrorCode", "type": "int16", "versions": "0+", "about": PREFERRED((byte) 0), UNCLEANED((byte) 1); public final byte value; "The result error, or zero if there was no error."}, ElectionType(byte value) { "name": "ErrorMessage", "type": "string", "versions": "0+", "nullableVersions": "0+", this.value = value; "about": "The result message, or null } if there was no error."} public final TopicPartition topicPartition; ]} public]} final ElectionType electionType; ... } |
Admin Command
The command kafka-preferred-replica.election.{sh,bat} will be deprecated and the following command will be added.
]
} |
AdminClient Abstract Class
A new method will be added to the AdminClient abstract class to support this new version of the RPC.
When the partition parameter is null the controller will attempt the election type on all of partitions that are legible for election. This means that the RPC will only return success or failure on partition that eligible for election as oppose to all of the partitions in the cluster as the previous implemented behaved.
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package org.apache.kafka.clients.admin;
public abstract class AdminClient ... {
...
/**
* ...
* @deprecated Since 2.4.0. Use {@link #electLeaders}.
*/
@Deprecated
public abstract ElectPreferredLeadersResult electPreferredLeaders(Collection<TopicPartition> partitions,
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$ bin/kafka-leader-election.sh --help This tool attempts to elect a new leader for a set of topic partitions. The type of elections supported are preferred replicas and unclean replica. Option Description ------ ----------- --admin.config <String: config file> Admin client config properties file to ElectPreferredLeadersOptions options); /** * ... * * @param electionType pass to the admin client when -- The type of election. * @param partitions The partitions for which to conduct elections. * @param options bootstrap-server is given. --bootstrap-server <String: host:port> A host name and port for theThe brokeroptions to use when electing the leaders. * @return The ElectLeadersResult. */ connect to, inpublic theabstract form host:port.ElectLeadersResult electLeaders( ElectionType electionType, Set<TopicPartition> partitions, ElectLeadersOptions options); } |
ElectLeaderResult Class
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final public ElectLeadersResult Multiple{ comma-separated URLs can be... public KafkaFuture<Map<TopicPartition, Optional<Throwable>>> partitions() { ... } public KafkFuture<Void> all() { ... given. REQUIRED unless --zookeeper} } |
ElectionType Enumeration
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package org.apache.kafka.common; public enum ElectionType { PREFERRED((byte) 0), UNCLEAN((byte) 1); public final byte value; ElectionType(byte value) { this.value = value; } } |
Admin Command
The command kafka-preferred-replica-election.{sh,bat} will be deprecated and the following command will be added.
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$ bin/kafka-leader-election.sh --help This tool attempts to elect a new leader for a set of topic partitions. The type of elections supported are preferred replicas and unclean replica. Option is given. --help Print usage information. --path-to-json-file <String: list of The JSON file with the list of partitions for which replica Description ------ partitions for which leader election leader election needs to be should be done. Supported elections triggered> ----------- --admin.config <String: config file> Admin client config properties file to pass to the admin are 0 for preferred and 1 for uncleaned. client when --bootstrap-server is given. --all-topic-partitions If anPerform election ison notall specified, of the topic partitions. Not allowed if preferred is the default. This is an --topic, --partition or --path-to-json-file is specified. --bootstrap-server <String: host:port> A host name and port for the broker to example format. connect to, in the form host:port. {"partitions": Multiple comma-separated URLs can be [{"topic": "foo", "partition": 1}, given. REQUIRED unless --zookeeper {"topic": "foobar", "partition": 2, "election": 1}] is given. --election-type <String: election> Type of election to attempt. Possible values are "preferred" for preferred election or "unclean" for unclean }election. Defaults to all existing partitions REQUIRED. --topic <String: topic> Name of topic for which to perform an election. REQUIRED if --partition is specified. Not allowed if --path-to-json-file or --all-topic-partitions is specified. --partition <Integer: partition id> Partition id for which to perform an election. REQUIRED if --topic is specified. Not allowed if --path-to-json-file or --all-topic-partitions is specified. --help Print usage information. --path-to-json-file <String: list of The JSON file with the list of partitions for which replica partitions for which leader election leader election needs to be should be done. Supported elections triggered> are 0 for preferred and 1 for unclean. If an election is not specified, preferred is the default. This is an example format. {"partitions": [{"topic": "foo", "partition": 1}, {"topic": "foobar", "partition": 2}] } Not allowed if --all-topic-partitions, --topic or --partition flags are specified. |
Metrics
The following metrics were added:
- kafka.server:type=DelayedOperationPurgatory,name=NumDelayedOperations,delayedOperation=ElectLeader
- kafka.server:type=DelayedOperationPurgatory,name=PurgatorySize,delayedOperation=ElectLeader
While the following metrics were removed:
- kafka.server:type=DelayedOperationPurgatory,name=NumDelayedOperations,delayedOperation=ElectPreferredLeader
- kafka.server:type=DelayedOperationPurgatory,name=PurgatorySize,delayedOperation=ElectPreferredLeader
Proposed Changes
In addition to the protocol and client changes enumerated above, the Controller will be extended to allow unclean leader election requests to come from the admin client. Currently the controller only support preferred leader elections from the admin client. Unclean leader election can only be enabled through either a topic configuration change or a broker configuration change.supports preferred leader election from the admin client. Unclean leader election can only be enabled through either a topic configuration change or a broker configuration change.
When performing unclean leader election on a topic partition through the admin client the "unclean leader election" topic configuration and broker configuration will be ignored. In the code this will result is code similar to the one below:
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uncleanElectionFromAdminClient || logConfigs(partition.topic).uncleanLeaderElectionEnable.booleanValue() |
Compatibility, Deprecation, and Migration Plan
- What impact (if any) will there be on existing users?
- If we are changing behavior how will we phase out the older behavior?
- If we need special migration tools, describe them here.
- When will we remove the existing behavior?
Rejected Alternatives
- The kafka-preferred-replica-elections.{sh,bat} scripts will be deprecated.
- The electPreferredLeaders method in AdminClient will be deprecated.
- The DelayedOperationPurgatory metrics ElectPreferredLeader were replaced with ElectLeader
Rejected Alternatives
Not applicableIf there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.