...

Current state: Under Discussion

Discussion threadthreads: https://lists.apache.org/thread/zb5l1fsqw9vj25zkmtnrk6xm7q3dkm1v, https://lists.apache.org/thread/6o3sjvcb8dx1ozqfpltb7p0w76b4nd46

JIRA:

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

KIP-595, KIP-630 and KIP631, introduced the KRaft cluster metadata topic partitions. These are partitions partition. This is a partition with replicas that can achieve consensus on the Kafka log without relying on the Controller or ZK. The KRaft Controllers in KIP-631 use one of these topic partitions (called cluster metadata topic partition) use this partition to order operations on the cluster, commit them to disk and replicate them to other controllers and brokers.

Consensus on the cluster metadata partition was achieved by the voters (Controllerscontrollers). If the operator of a KRaft cluster wanted to make changes to the set of voters, they would have to  to shutdown all of the controllers nodes and manually make changes to the on-disk state of the old controllers and new controllers. If the operator wanted to replace an existing voter because of a disk failure or general hardware failure, they would have to make sure that the new voter node has a superset of the previous voter's on-disk state. Both of these solutions are manual and error prone.

This KIP describes a protocol for extending KIP-595 and KIP-630 so that the operator operators can programmatically update the voter voters set in a way that is safe consistent and is available. There are two important use cases that this KIP supports. One use case is that the operator wants to change the number of controllers by adding or removing a controller.  The other use case is that the operation wants to replace a controller because of a disk or hardware failure.

...

These are the definition of some important terms that are used through the document.

KRaft: The consensus protocol inspired by Raft for Kafka. This protocol is defined by KIP-595: A Raft Protocol for the Metadata Quorum, KIP-630: Kafka Raft Snapshot. The implementation is in the Java raft module.

Controller: The controller or the metadata state machine is the application built using the KRaft consensus layer. The implementation is in the Java metadata module.

Voters: A voter is any replica that can transition to the candidate state and to the leader state. Voters for the KRaft cluster metadata partition are also called controllers. Voters are required to have an ID and UUID.

Voters set: A voters set is the group of voters for a partition. Each replica keeps their own set of voters that Each replica keeps their own set of voters that are part of the topic partition. For a replica, the voters are the replica ID and UUID is in its own voter voters set. A candidate needs to get votes from the majority of its own voter voters set before is it can become the leader of an epoch. When a voter becomes a leader it will use its voter voters set to determine when an offset has been committed.

Observers: An observer is any replica that is not in the voter voters set. This is because they have an ID and UUID which is not in the voter voters set or they don't have an ID or UUID.

...

This KIP is inspired by Chapter 4 of Consensus: Bridging Theory and Practice [2]. The description of this KIP makes the assumption that the reader is familiar with the references enumerated at the bottom of this page. The user -level explanation section explains the feature as the user would interact with it. The reference -level explanation section goes into the implementation details of this feature. 

...

There will be two mechanisms for bootstrapping the KRaft cluster metadata partition. For context, the set of voters in the KRaft cluster metadata partition is currently bootstrapped and configured using the controller.quorum.voters server property. This property is also used to configured the configure the brokers with the set of endpoint endpoints that know the location of the leader if it exists.

This feature will instead persist the set of voters in the cluster metadata partition log, snapshots and quorum state. The new controller.quorum.bootstrap.servers property will be added to configure replicas with a set of endpoints that could be used to discover the set of voters and latest leader.

Bootstrapping with one voter

...

This command will 1) create a meta.properties file in metadata.log.dir with a randomly generated directory.id, 2) create a snapshot at 00000000000000000000-0000000000.checkpoint with the necessary control records (kraft.versionRecord and AddVoterRecordKRaftVersionRecord and VotersRecord) to make this Kafka node the only voter for the quorum.

...

This command is similar to the standalone version but the snapshot at 00000000000000000000-0000000000.checkpoint will instead contain an AddVoterRecord a VotersRecord that includes information for all of the voters specified in --controller-quorum-voters. Just like the controller.quorum.voters properties, it is important that the set of voters is equal in all of the controllers with the same cluster id.

...

The changes to the protocol and replicated state to implement for this feature are not compatible with the existing KRaft implementation. This feature cannot be enabled unless all of the replicas for the KRaft cluster metadata partition support this feature. The user can upgrade the KRaft version of an existing KRaft cluster with the following command:

...

This command will parse the release-version to the matching MetadataVersion metadata.version and kraft.version features. It will send a UpdateFeatures request to a node with both features set to the matching version. KRaft will write the kraft.versionRecord KRaftVersionRecord control record, if all of the controllers and brokers support the new version. KRaft will use the information in the controller registration, broker registration and add voter records to determine if the new version is compatible.

Add

...

controller

To increase the number of controller the user needs to format a controller node, start the controller node and add that node to the KRaft cluster metadata partition. Formatting a controller node can be done with the following CLI command:

...

kafka-metadata-quorum --bootstrap-server <endpoints> add-controller --config controller.properties

Remove

...

controller

To decrease the number of controller controllers the use user needs to execute the following CLI command:

kafka-metadata-quorum --bootstrap-server <endpoints> remove-controller --replicacontroller-id <replica-id> --replicacontroller-uuid <replica-uuid>

Common

...

scenarios

To better illustrate this feature this section describes two few common scenarios that the Kafka administrator may perform.

Disk Failure Recovery

If one of the replicas encounter a disk failure the operator can replace this disk with a new disk and start the replica.

Let's assume that the cluster has the following voters for the KRaft cluster metadata partition (1, UUID1), (2, UUID2) and (3, UUID3). The first element of the tuples is the replica id. The second element of the tuples is the replica uuid. The replica uuid, or directory id, is generated using the storage tool when the node is formatted.

Automatic joining controllers

Imagine that the user is trying to create KRaft controller quorum with 3 voters (1, 2, 3).

At this point the disk for replica 3 is replaced and formatted. This means that when replica 3 starts it will have new replica uuid (UUID3') and an empty set of voters. Replica 3 will discover the partition leader either using For all of the controller properties files assume that the controller.quorum.auto.join.enable is set to true and the controller.quorum.bootstrap.servers or by the leader sending a BeginQuorumEpoch request to replica 3. Once the new replica, (3, UUID3'), discovers the leader it send the Fetch and FetchSnapshot requests to the leader. At this point the leader's set of voters will be (1, UUID1), (2, UUID2) and (3, UUID3) and the set of observers will include (3, UUID3').

This state can be discovered by the operator by using the DescribeQuorum RPC, the Admin client or the kafka-metadata-quorum CLI. The operator can now decide to add replica (3, UUID3') to the set of voters using the AddVoter RPC, the Admin client or the kafka-metadata-quorum CLI.

When the AddVoter RPC succeeds the voters will be (1, UUID1), (2, UUID2), (3, UUID3) and (3, UUID3'). The operator can remove the failed disk from the voter set by using the RemoveVoter RPC, the Admin client or the kafka-metadata-quorum CLI.

contains the endpoint of controller 1.

On controller 1 the user runs:

kafka-storage format --cluster-id <cluster-id> --release-version 3.8 --standalone --config controller.properties

and starts the controller. When controller 1 starts it sees that is already in the voters set so it eventually becomes leader.

On controller 2 and 3 the user runs:

kafka-storage format --cluster-id <cluster-id> --release-version 3.8 --config controller.properties

and starts the controllers. Notice that neither --standalone or --controller-quorum-voters is used for controller 2 and 3 so the controllers will start as observers. These controllers will discover the leader using controller.quorum.bootstrap.servers and will use the AddVoter RPC to join the voters set.

Disk failure recovery

If one of the replicas encounter a disk failure the operator can replace this disk with a new disk and start the replica.

Let's assume that the cluster has the following voters for the KRaft cluster metadata partition When the RemoveVoter RPC succeeds the voters will be (1, UUID1), (2, UUID2), and (3, UUID3'). At this point the Kafka cluster has successfully recover from a disk failure in a controller node in a consistent way.

Node Failure Recovery

The first element of the tuples is the replica id. The second element of the tuples is the replica uuid. The replica uuid, or directory id, is generated using the storage tool when the node is formatted.

At this point the disk for replica 3 is replaced and formatted. This means that when replica 3 starts it will have a new replica uuid (UUID3') and an empty set of voters. Replica 3 will discover the partition leader either using To support this scenario the Kafka cluster must be configured to use controller.quorum.bootstrap.servers. Let's assume that the voter set is or by the leader sending a BeginQuorumEpoch request to replica 3. Once the new replica, (3, UUID3'), discovers the leader it send the Fetch and FetchSnapshot requests to the leader. At this point the leader's set of voters will be (1, UUID1), (2, UUID2) and (3, UUID3) .

At this replica 3 has failed and the Kafka operator would like to replace it. The operator would start a new controller with replica id 4. The node 4's metadata log dir would get formatted, generate and persist the directory id UUID4. Replica 4 will discover the leader by sending Fetch and FetchSnapshot request to the servers enumerated in controller.quorum.bootstrap.servers. After a successful Fetch RPC, the leader's set of voters will be (1, UUID1), (2, UUID2), (3, UUID3) and set of observers will include (4, UUID4).

and the set of observers will include (3, UUID3').

This state can be discovered by the operator by using the DescribeQuorum RPC, the Admin client or the kafka-metadata-quorum CLI. The operator can now decide to add replica (43, UUID4UUID3') to the set of voters using the Admin client or the kafka-metadata-quorum CLI.

When the AddVoter RPC . When this operation succeeds the voters set of voters will be   (1, UUID1), (2, UUID2), (3, UUID3) and (43, UUID4UUID3'). The operator can now decided to remove replica (3, UUID3) remove the failed disk from the voters set of voters using the RemoveVoter RPC.

Reference Explanation

The general goal of this KIP is to allow the user to dynamically change the set of voters (also known as controllers) for the KRaft cluster metadata partition. This is achieved by storing the set of voters and their known endpoints in the log instead of the controller.quorum.voters properties. Because the set of voters is stored in the log it allows the leader to replicate this information to all of the fetching replicas (voters and observers). Since old records can be deleted once a snapshot has been replicated, the KRaft snapshots will also contain the set of voters up to the included offset.

Following voters will discover the set of voters and their endpoints by fetching the latest log from the leader but how do new voters discovery the leader's endpoint? The leader will push this information to new voters (or voters that were offline for a long time) using the BeginQuorumEpoch request. The active leader sends BeginQuorumEpoch to all of the voters when it becomes leader for an epoch.

The leader doesn't send BeginQuorumEpoch to observers since this are dynamic and are not included in the KRaft partition log. Observer will instead discover the leader using the controller.quorum.bootstrap.servers. It is important that properties includes at least one of the available voters, else brokers (observers) will not be able to discover the leader of the KRaft cluster metadata partition.

The rest of these section and subsection goes into the detail changes required to implement this new functionality. 

Directory id or replica UUID

In addition to a replica ID each replica assigned to a KRaft topic partition will have a replica UUID. This UUID will be generated once and persisted in the meta.properties file for the metadata.log.dir. Replica UUID and directory UUID was first introduced in KIP-858: Handle JBOD broker disk failure in KRaft.

There are two cases when a directory id will be generated and persisted:

1. kafka-storage format command will generate a directory id for all of the log directories including the metadata log dir.
2. meta.properties exists but it doesn't include a directory.id property. This case will generate and persist a directory id to support upgrade from Kafka versions that don't support this feature to versions that support this feature.

Supported features

This feature can only be enabled if all of the voters (controllers) and observers (brokers) support this feature. This is required mainly because this feature needs two write two new control records (AddVoter and RemoveVoter) to the KRaft cluster metadata partition. All replicas need to be able to read and decode these new records.

There will be a new SupportedFeature added to the ApiVersions response of the Kafka nodes. The name of this new supported feature will be kraft.version. The default value be 0 and represents that only KIP-595: A Raft Protocol for the Metadata Quorum, KIP-630: Kafka Raft Snapshot and KIP-996: Pre-Vote are supported. Version 1 means that this KIP is supported.

When the clients sends a UpdateFeatures RPC to the active controller, if the FeatureUpdates.Feature property is kraft.version, the associated information will be passed to KRaft client. The KRaft client will implement two different algorithm if the upgrade is supported by voters and observers. For voters the KRaft client will comparing the upgraded version against all of the persisted AddVoter and RemoveVoter records for the KRaft cluster metadata partition. The KRaft client cannot do this for observers (brokers) since their supported versions are not persisted in the log. The controller will instead push the broker registration information to the KRaft client.

Voter Changes

Adding Voters

Voters are added to the cluster metadata partition by sending an AddVoter RPC to the leader. For safety Kafka will only allow one voter change operation at a time. If there are any pending voter change operations the leader will wait for them to finish.

If there are no pending voter change operations the leader send an ApiVersions request to the new voter's endpoint to discover it's kraft.version support features. If the new leader supports the current kraft.version, it will write a AddVoterRecord to the log and immediately update its in-memory quorum state to include this voter as part of the quorum. Any replica that replicates and reads this AddVoterRecord will update their in-memory voter set to include this new voter. Voters will not wait for these records to get committed before updating their voter set.

Once the AddVoterRecord operation has been committed by the majority of the new voter set, the leader can respond to the AddVoter RPC and process new voter change operations.

Removing Voters

Voter are removed from the cluster metadata partition by sending a RemoveVoter RPC to the leader. This works similar to adding a voter. If there are no pending voter change operations the leader will append the RemoveVoterRecord to the log and immediately update its voter set to the new configuration.

Once the RemoveVoterRecord operation has been committed by the majority of the new voter set, the leader can respond to the RPC. If the removed voters is the leader, the leader will resign from the quorum when the RemoveVoterRecord has been committed. To allow this operation to be committed and for the leader to resign the followers will continue to fetch from the leader even if the leader is not part of the new voter set. In KRaft, leader election is triggered when the voter hasn't received a successful response in the fetch timeout.

Bootstrapping

The section describe how the quorum will get bootstrap to support this KIP. There are two configurations that Kafka will support and each will be explain separately. The Kafka cluster can be configured to use the existing controller.quorum.voters and the new property called controller.quorum.bootstrap.servers.

controller.quorum.voters

This is a static quorum configuration where all of the voters' ID, host and port are specified. An example value for this configuration is 1@localhost:9092,2@localhost:9093,3@localhost:9094 .

If the kraft.version is 0, this properties will be used to configure the set of voters for the KRaft cluster metadata partition.

When the kraft.version is upgraded to a version greater 0 and the version is supported by the voters, the leader will write a control record batch that will include the kraft.version record and all of the AddVoter records for all of the voters. This will allow KRaft to ignore the controller.quorum.voters properties and instead rely solely on the log state when it is upgraded to kraft.version 1.

controller.quorum.bootstrap.servers

This configuration describe the set of hosts and ports that can be queried to discover the cluster metadata partition leader. Observers and to-be-added voters will send Fetch requests to this list of servers until the leader is discovered.

When using this configuration for a new cluster, the quorum should be started with only one voter. This voter is bootstrapped by running the storage tool format command. This tool will create the cluster metadata partition and append one AddVoterRecord to it. Additional voters can get added by using the AddVoter RPC as described in this KIP.

Leader Election

by using the Admin client or the kafka-metadata-quorum CLI.

When the RemoveVoter RPC succeeds the voters will be (1, UUID1), (2, UUID2), and (3, UUID3'). At this point the Kafka cluster has successfully recover from a disk failure in a controller node in a consistent way.

Node failure recovery

Let's assume that the voters set is (1, UUID1), (2, UUID2) and (3, UUID3).

At this point replica 3 has failed and the Kafka operator would like to replace it. The operator would start a new controller with replica id 4. The node 4's metadata log dir would get formatted, generate and persist the directory id UUID4. Replica 4 will discover the leader by sending Fetch and FetchSnapshot request to the servers enumerated in controller.quorum.bootstrap.servers. After a successful Fetch RPC, the leader's set of voters will be (1, UUID1), (2, UUID2), (3, UUID3) and set of observers will include (4, UUID4).

The operator can now decide to add replica (4, UUID4) to the set of voters. When this operation succeeds the set of voters will be  (1, UUID1), (2, UUID2), (3, UUID3) and (4, UUID4).

The operator can now decided to remove replica (3, UUID3) from the set of voters.

Reference explanation

The general goal of this KIP is to allow the user to dynamically change the set of voters (also known as controllers) for the KRaft cluster metadata partition. This is achieved by storing the set of voters and their known endpoints in the log instead of the controller.quorum.voters properties. Because the set of voters is stored in the log it allows the leader to replicate this information to all of the fetching replicas (voters and observers). Since old log segments (records) can be deleted once a snapshot has been created, the KRaft snapshots will also contain the set of voters up to the included offset.

The fetching (following) voters will discover the set of voters and their endpoints by fetching the latest log from the leader. How do new voters discovery the leader's endpoint? The leader will push this information to new voters (or voters that were offline for a long time) using the BeginQuorumEpoch request. The active leader sends BeginQuorumEpoch to all of the voters in the voters set when it becomes leader for an epoch.

The leader doesn't send BeginQuorumEpoch to observers since this are dynamic and are not included in the KRaft partition log. Observer will instead discover the leader using the controller.quorum.bootstrap.servers. It is important that this property includes at least one of the available voters, else brokers (observers) will not be able to discover the leader of the KRaft cluster metadata partition.

To make changes to the voters set consistent it is required that the majority of the competing voters sets commit the voter changes. In this design the competing voters sets are the current voters set and new voters set. Since this design only allows one voter change at a time the majority of the new configuration always overlaps (intercepts) the majority of the old configuration. This is done by the leader committing the current epoch when it becomes leader and committing single voter changes with the new voters set before accepting another voter change.

The rest of these section and subsection goes into the detail changes required to implement this new functionality. 

Directory id or replica UUID

In addition to a replica ID each replica assigned to a KRaft topic partition will have a replica UUID. This UUID will be generated once and persisted in the meta.properties file for the metadata.log.dir. Replica UUID and directory id was first introduced in KIP-858: Handle JBOD broker disk failure in KRaft.

There are two cases when a directory id will be generated and persisted:

1. kafka-storage format command will generate a directory id for all of the log directories including the metadata log dir.
2. meta.properties exists but it doesn't include a directory.id property. This case will generate and persist a directory id to support upgrade from Kafka versions that don't support this feature to versions that support this feature.

Supported features

This feature can only be enabled if all of the voters (controllers) and observers (brokers) support this feature. This is required mainly because this feature needs to write a new control record (VotersRecord) to the KRaft cluster metadata partition. All replicas need to be able to read and decode these new control records.

There will be a new SupportedFeature added to the ApiVersions response of the Kafka nodes. The name of this new supported feature will be kraft.version. The default value be 0 and represents that only KIP-595: A Raft Protocol for the Metadata Quorum, KIP-630: Kafka Raft Snapshot and KIP-996: Pre-Vote are supported. Version 1 means that this KIP is supported.

When the clients sends a UpdateFeatures RPC to the active controller, if the FeatureUpdates.Feature property is kraft.version, the associated information will be passed to KRaft client. The KRaft client will implement two different algorithms to check if the upgrade is supported by voters and observers. For voters, the KRaft leader will compare the upgraded version against all of the voters' supported kraft.version. The KRaft leader will learn about the supported versions through the UpdateVoter RPC.

The KRaft leader doesn't know the kraft.version for the observers (brokers) because observers don't send the UpdateVoter request to the leader. The controller state machine will instead push the brokers' kraft.version information to the KRaft client (RaftClient in the implementation). The KRaft will use the controller provided kraft.version to check if an upgrade is supported.

Any UpdateFeature RPC that attempts to downgrade the kraft.version from 1 to 0 will be rejected.

Voter changes

Adding voters

Voters are added to the cluster metadata partition by sending an AddVoter RPC to the leader. For safety Kafka will only allow one voter change operation at a time. If there are any pending voter change operations the leader will wait for them to finish.

If there are no pending voter change operations the leader send an ApiVersions request to the new voter's endpoint to discover it's kraft.version support features. If the new leader supports the current kraft.version, it will write a VotersRecord to the log, with the current voters set plus the voter getting added, and immediately update its in-memory quorum state to include this voter as part of the quorum. Any replica that replicates and reads this VotersRecord will update their in-memory voters set to include this new voter. Voters will not wait for these records to get committed before updating their voters set.

Once the VotersRecord operation has been committed by the majority of the new voters set, the leader can respond to the AddVoter RPC and process new voter change operations.

Removing voters

Voter are removed from the cluster metadata partition by sending a RemoveVoter RPC to the leader. This works similar to adding a voter. If there are no pending voter change operations the leader will append the VotersRecord to the log, the the current voters set minus the voter getting removed, and immediately update its voters set to the new configuration.

Once the VotersRecord operation has been committed by the majority of the new voters set, the leader can respond to the RPC. If the removed voters is the leader, the leader will resign from the quorum when the VotersRecord has been committed. To allow this operation to be committed and for the leader to resign the followers will continue to fetch from the leader even if the leader is not part of the new voters set. In KRaft, leader election is triggered when the voter hasn't received a successful response in the fetch timeout.

Endpoints information

Replicas need to be able to connect and send RPCs to each others to implement the KRaft protocol. There are two important sets of endpoints that are needed to allow the replicas to connect to each other.

Voters set

The voters set is the collection of replicas and their endpoints that are part that are voters and can become leaders. The voters set is stored in the snapshot and log using the VotersRecord control record. If the partition doesn't contain a VotersRecord control record then the value stored in the controller.quorum.voters property will be used.

This set of endpoints and replicas will be use by the Vote, BeginQuorumEpoch and EndQuorumEpoch RPCs. In other words, replicas will use the voters set to establish leadership and to propagate leadership information to all of the voters.

Bootstrap servers

When the leader is known Fetch requests will be sent to the leader's endpoint. The leader's endpoint can be discovered from many of the KRaft RPCs but typically it will be discover from the BeginQuorumEpoch request for voters (controllers) or the Fetch response for observers (brokers).

In the case that the leader is not known the replica (observer) will send the Fetch RPC to the endpoints in the bootstrap servers configuration. The set of endpoints in the bootstrap servers configuration will come from the controller.quorum.bootstrap.servers configuration property. If that property is not specified the endpoints in controller.quorum.voters will be used.

Leader election

It is possible for the leader to add a new voter to the voters set, write the VotersRecord to the log and only replicate It is possible for the leader to write an AddVoterRecord to the log and replica it to some of the voters in the new configuration. If the leader fails before this record has been replicated to the new voter it is possible that a new leader cannot be elected. This is because voters reject vote request from replicas that are not in the voter voters set. This check will be removed and replicas will reply to votes Vote request when the candidate is not in the voter voters set or the voting replica is not in the voter voters set. The candidate must still have a longer log (offset and epoch) before the voter will grant a vote to itthe candidate.e

Once a leader is elected, leader will propagate this information to all of the voters using the BeginQuorumEpoch RPC. The leader will continue to send the BeginQuorumEpoch requests to a voter if the voter doesn't send a Fetch or FetchSnapshot request within the "check quorum" timeout.

First leader

When a KRaft voter becomes leader it will write a KRaftVersionRecord and VotersRecord to the log if the log or the latest snapshot doesn't contain any VotersRecord. This is done to make sure that the voters set in the bootstrap snapshot gets replicated to all of the voters and to not rely on all of the voters being configured with the same bootstrapped voters set.

Automatic endpoint and directory Automatic endpoint and directory id discovery

To improve the usability of this feature it would beneficial for the leader of the KRaft cluster metadata leader partition to automatically rediscover the voters' endpoints. This makes it possible for the operator to update the endpoint of a voter without having to use the kafka-metadata-quorum tool. When a voter becomes a follower and discovers a new leader, it will always send an UpdateVoter RPC to the leader. This request instructs the leader to update the endpoints of the matching replica id and replica uuid. When at a voter becomes a leader it will also write an AddVoter record for itself if the endpoints have changeda VotersRecord control record with the updated endpoints and kraft.version feature.

The directory id, or replica uuid, will behave differently. The quorum shouldn't automatically update the directory id, since different values means that the disk was replaced. For directory id, the leader will only override it if it was not previously set. This behavior is useful for when a cluster gets upgraded to a kraft.version greater than 10.

High

...

watermark

As describe described in KIP-595, the high-watermark will be calculated using the fetch offset of the majority of the voters. When a replica is removed or added it is possible for the high-watermark to decrease. The leader will not allow the high-watermark to decrease and will guarantee that is is monotonically increasing for both the state machines and the remote replicas.

With this KIP, it is possible for the leader to not be part of the voter voters set when the replica removed is the leader. In this case the leader will continue to handle Fetch and FetchSnapshot request as normal but it will not count itself when computing the high watermark.

...

The snapshot generation code needs to be extended to include these new KRaft specific control record for AddVoterVotersRecord. Before this KIP the snapshot didn't include any KRaft generated control records.

Internal Listener

When the state machine (controller or broker) calls RaftClient.createSnapshot, the KRaft client will write the SnapshotHeaderRecord, KRaftVersionRecord and VotersRecord controller records at the beginning of the snapshot.

Note that the current KRaft implementation already writes the SnapshotHeaderRecord. The new KRaft implementation will write KRraftVersionRecord and VotersRecord, if the kraft.version is greater than 0.

To efficiently implement this feature the KRaft implementation will keep track of all voters sets between the latest snapshot and the LEO. The replicas will append the offset and voters set as it reads VotersRecord control records from the snapshot or log. Whenever the replica generates a snapshot for a given offset it can remove any additional entry that has a smaller offset.

Internal listener

This KIP requires that The KRaft implementation and protocol describe in KIP-595 and KIP-630 never read from the log or snapshot. This KIP requires the KRaft implementation now read and decode uncommitted data from log and snapshot to discover the voter voters set. This also means that the KRaft implementation needs to handle this uncommitted state voter sets getting truncated and reverted.

Public Interfaces

Configuration

There only two configurations for this feature.

controller.quorum.voters

This is an existing configuration. This configuration describes the state of the quorum and will only be used if the kraft.version feature is 0.

controller.quorum.bootstrap.servers

This is a list of nodes that brokers and new controllers can use to discover the quorum leader. Brokers and new controllers (observers) will send Fetch requests to all of the nodes in this configuration until they discover the quorum leader and the Fetch request succeeds. The quorum voters and their configuration will be learned by fetching and reading the records from the log and snapshot. This includes committed and uncommitted records.

If this configuration is specified, observers will not use the controller.quorum.voters endpoints to discover the leader. 

Log and Snapshot Control Records

Two new control records will be added to the log and snapshot of a KRaft partition.

LeaderChangeMessage

Add an optional VoterUuid to Voter. This change is not needed for correctness but it is nice to have for tracing and debugging. The leader will write version 0 if the kraft.version is 0. The leader will write version 1 if the kraft.version is 1.

removed from the log. As the internal listener reads new voters sets and endpoints it needs to update the network client with this new information.

Controller auto joinning

To make it easier for users to operate a KRaft controller cluster, they can have KRaft controllers automatically join the cluster's voters set by setting the controller.quorum.auto.join.enable to true. In this case, the controller will remove any voter in the voters set that matches its replica id but doesn't match its directory id (replica uuid) by sending the RemoveVoter RPC. Once the controller has remove all duplicate replica uuid it will add itself to the voters set by sending a AddVoter RPC to the leader, if its replica id and replica uuid is not in the voters set. The TimoutMs for the AddVoter RPC will be set to 30 seconds.

For an example, imagine that the user is trying to create KRaft controller quorum with 3 voters (1, 2, 3).

On controller 1 the user runs:

kafka-storage format --cluster-id ABC --release-version "3.8" --standalone --config ...

and starts the controller. When controller 1 starts it sees that is already in the voters set so it doesn't perform any AddVoter and RemoveVoter RPC. Controller 1 eventually becomes leader.

On controller 2 and 3 the user runs:

kafka-storage format --cluster-id ABC --release-version "3.8" --config ...

and starts the controllers. Notice that neither --standalone or --controller-quorum-voters is used for controller 2 and 3 so the controllers start as observers. These controllers will discover the leader using controller.quorum.bootstrap.servers and will use the RemoveVoter and AddVoter RPC as described in the beginning of this section.

Public Interfaces

Configuration

There only two configurations for this feature.

controller.quorum.voters

This is an existing configuration. This configuration describes the state of the quorum and will only be used if the kraft.version feature is 0.

controller.quorum.bootstrap.servers

This is a list of nodes that brokers and new controllers can use to discover the quorum leader. Brokers and new controllers (observers) will send Fetch requests to all of the nodes in this configuration until they discover the quorum leader and the Fetch request succeeds. The quorum voters and their configuration will be learned by fetching and reading the records from the log and snapshot. This includes committed and uncommitted records.

If this configuration is specified, observers will not use the controller.quorum.voters endpoints to discover the leader. 

controller.quorum.auto.join.enable

Controls whether a KRaft controller should automatically join the cluster metadata partition for its cluster id. If the configuration is set to true the controller must be stopped before removing the controller with kafka-metadata-quorum remove-controller. The default value is false.

Log and snapshot control records

A few new control records will be added to the log and snapshot of a KRaft partition.

KRaftVersionRecord

Control record for recording the latest KRaft protocol version. The KRaft protocol version is used to determine which version of LeaderChangeMessage to write. It is also used to determine if the VotersRecord control record can be written to the topic partition.

The ControlRecordType is TBD and will be updated when the code is committed to Kafka.

{
  "type": "data",
  "name": "KRaftVersionRecord",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "Version

git diff upstream/trunk clients/src/main/resources/common/message/LeaderChangeMessage.json
diff --git a/clients/src/main/resources/common/message/LeaderChangeMessage.json b/clients/src/main/resources/common/message/LeaderChangeMessage.json
index fdd7733388..2b019a2a80 100644
--- a/clients/src/main/resources/common/message/LeaderChangeMessage.json
+++ b/clients/src/main/resources/common/message/LeaderChangeMessage.json
@@ -16,7 +16,7 @@
 {
   "type": "data",
   "name": "LeaderChangeMessage",
-  "validVersions": "0",
+  "validVersions": "0-1",
   "flexibleVersions": "0+",
   "fields": [
     {"name": "Version", "type": "int16", "versions": "0+",
@@ -30,7 +30,8 @@
   ],
   "commonStructs": [
     { "name": "Voter", "versions": "0+", "fields": [
-      {"name": "VoterId", "type": "int32", "versions": "0+"}
+      { "name": "VoterId", "type": "int32", "versions": "0+" },
+      { "name": "VoterUuid", "type": "int32int16", "versions": "10+" }
     ]}
 ,
  ]
 } 

AddVoterRecord

A control record for instructing the voters to add a new voter to the topic partition. This record can exist in both the log and the snapshot of a topic partition.

The ControlRecordType is TBD and will be updated when the code is committed to Kafka.

{
  "typeabout": "data",
The version "name": "AddVoterRecord",
  "validVersions": "0",
  "flexibleVersions": "0+"of the kraft version record" },
  "fields": [
    { "name": "VersionKRaftVersion", "type": "int16", "versions": "0+",
      "about": "The versionkraft of the add voter recordprotocol version" },
    { "name": "VoterId", "type": "int32", "versions": "0+", "entityType": "brokerId",
      "about": "The replica id of the voter getting added to the topic partition" },
    { "name": "VoterUuid", "type": "uuid", "versions": "0+",
      "about": "The directory id of the voter getting added to the topic partition" },
    { "name": "EndPoints
  ]
}

Handling

When KRaft replicas read this record from the log they will update their finalized kraft.version once the HWM shows this record as committed.

LeaderChangeMessage

Add an optional VoterUuid to Voter. This change is not needed for correctness but it is nice to have for tracing and debugging. The leader will write version 0 if the kraft.version is 0. The leader will write version 1 if the kraft.version is 1.

git diff upstream/trunk clients/src/main/resources/common/message/LeaderChangeMessage.json
diff --git a/clients/src/main/resources/common/message/LeaderChangeMessage.json b/clients/src/main/resources/common/message/LeaderChangeMessage.json
index fdd7733388..2b019a2a80 100644
--- a/clients/src/main/resources/common/message/LeaderChangeMessage.json
+++ b/clients/src/main/resources/common/message/LeaderChangeMessage.json
@@ -16,7 +16,7 @@
 {
   "type": "data",
   "name": "LeaderChangeMessage",
-  "validVersions": "0",
+  "validVersions": "0-1",
   "flexibleVersions": "0+",
   "fields": [
     {"name": "Version", "type": "[]Endpointint16", "versions": "0+",
@@ -30,7 +30,8 @@
   "about": "The endpoint that can be used to communicate with the voter", "fields"],
   "commonStructs": [
      { "name": "NameVoter", "type": "string", "versions": "0+", "mapKeyfields": true,
        "about": "The name of the endpoint" },
      { [
-      {"name": "HostVoterId", "type": "stringint32", "versions": "0+",
        "about": "The hostname" },
+      { "name": "PortVoterId", "type": "uint16int32", "versions": "0+",
        "about": "The port" },
+      { "name": "SecurityProtocolVoterUuid", "type": "int16uuid", "versions": "01+", }
     ]}
   ]
 } 

VotersRecord

A control record for specifying the latest and entire voters set. This record will be written to the generated snapshot and will specify the voters set at the snapshot id's end offset.

This record will also be written to the log by the leader when handling the AddVoter, RemoveVoter and UpdateVoter RPCs. When handling these RPCs the leader will make sure that the difference in voters is at most one addition or one removal. This is needed to satisfy the invariant that voter changes are committed by the old voters set and new voters set.

This record will also be written by the leader to the log if it has never been written to the log in the past. This semantic is nice to have to consistently replicate the bootstrapping snapshot, at 00000000000000000000-0000000000.checkpoint, at the leader to all of the voters.

The ControlRecordType is TBD and will be updated when the code is committed to Kafka.

{
  "type": "data",
  "about": "The security protocol" }
    ]},
    { "name": "KRaftVersionFeature", "type": "KRaftVersionFeature", "versions": "0+",
      "about": "The range of versions of the protocol that the replica supports", "fields": [
      { "name": "MinSupportedVersionVotersRecord",
  "typevalidVersions": "int160",
  "versionsflexibleVersions": "0+",
        "aboutfields": "The minimum supported KRaft protocol version" },[
      { "name": "MaxSupportedVersionVersion", "type": "int16", "versions": "0+",
        "about": "The maximumversion supportedof KRaftthe protocolvoters versionrecord" },
    ]}
  ]
}

Handling

KRaft replicas will read all of the control records in the snapshot and the log irrespective of the commit state and HWM. When a replica encounters an AddVoterRecord it will add the replica ID and UUID to its voter set. If the replica getting added is itself then it will allow the transition to prospective candidate when the fetch timer expires. The fetch timer is reset whenever it receives a successful Fetch or FetchSnapshot response.

RemoveVoterRecord

A control record for instructing the voters to remove a voter from the topic partition. This record can exist in the log but not the snapshot of a topic partition.

The ControlRecordType is TBD and will be updated when the code is committed to Kafka.

{
  "typename": "dataVoters",
  "nametype": "RemoveVoterRecord[]Voter",
  "validVersionsversions": "0",
  "flexibleVersions": "0+",
  "fields": [
      { "name": "VersionVoterId", "type": "int16int32", "versions": "0+", "entityType": "brokerId",
        "about": "The replica versionid of the remove voter record"voter in the topic partition" },
      { "name": "VoterIdVoterUuid", "type": "int32uuid", "versions": "0+",
 "entityType": "brokerId",
      "about": "The replicadirectory id of the voter getting removedin from the topic partition" },
      { "name": "VoterUuidEndPoints", "type": "uuid[]Endpoint", "versions": "0+",
        "about": "The directory id ofendpoint that can be used to communicate with the voter getting removed from the topic partition"}
  ]
}

Handling

KRaft replicas will read all of the control records in the snapshot and the log irrespective of the commit state and HWM. When a replica encounters a RemoveVoterRecord it will remove the replica ID and UUID from its voter set. If the replica getting removed is the leader and is the local replica then the replica will stay leader until the RemoveVoterRecord gets committed or the epoch advances losing leadership of the latest epoch.

Quorum State

Each KRaft topic partition has a quorum state (QuorumStateData) that gets persisted in the quorum-state file in the directory for the topic partition. The following changes will be made to this state:

1. ClusterId will get removed in version 1. This field is not used because the cluster id is persisted in <metadata.log.dir>/meta.properties.
2. AppliedOffset will get removed in version 1. This field is not used in version 0. The field will get removed because it implied an inclusive offset. This is not correct because the default value is 0.
3. VoterUuid will get added in version 1. The voting replica will persist both the voter ID and UUID of the candidate for which it voted.
4. AppliedRecord will get added in version 1. This is the newest record that the KRaft client has read and applied to update the current set of voters. EndOffset is exclusive and represents the next offset that the KRaft client should read if available.
5. CurrentVoters will get extended in version 1 to include the voter's directory id or UUID.

Version 0 of this data will get written if the kraft.version is 0. Version 1 of this data will get written if the kraft.version is 1.

git diff upstream/trunk raft/src/main/resources/common/message/QuorumStateData.json
diff --git a/raft/src/main/resources/common/message/QuorumStateData.json b/raft/src/main/resources/common/message/QuorumStateData.json
index d71a32c75d..34881f05ff 100644
--- a/raft/src/main/resources/common/message/QuorumStateData.json
+++ b/raft/src/main/resources/common/message/QuorumStateData.json
@@ -16,19 +16,25 @@
 {
   "type": "data",
   "name": "QuorumStateData",
-  "validVersions": "0",
+  "validVersions": "0-1",
   "flexibleVersions": "0+",
   "fields": [
-    {"name": "ClusterId", "fields": [
        { "name": "Name", "type": "string", "versions": "0+", "mapKey": true,
          "about": "The name of the endpoint" },
        { "name": "Host", "type": "string", "versions": "0+",
          "about": "The hostname" },
        { "name": "Port", "type": "uint16", "versions": "0+",
          "about": "The port" }
      ]},
      { "name": "KRaftVersionFeature", "type": "KRaftVersionFeature", "versions": "0+",
        "about": "The range of versions of the protocol that the replica supports", "fields": [
        { "name": "MinSupportedVersion", "type": "stringint16", "versions": "0+"},
-          {"nameabout": "LeaderId", "type": "int32", "versions": "0+", "default": "-1"},
-The minimum supported KRaft protocol version" },
        { "name": "LeaderEpochMaxSupportedVersion", "type": "int32int16", "versions": "0+", "default": "-1"},
-    {"name": "VotedId", "type": "int32", "versions": "0+", "defaultabout": "The maximum supported KRaft protocol version" }
      ]}
    ]}
  ]
}

Handling

KRaft replicas will read all of the control records in the snapshot and the log irrespective of the commit state and HWM. When a replica encounters a VotersRecord it will replace the current voters set.

If the local replica is the leader and it is getting removed, the replica will stay leader until the VotersRecord gets committed or the epoch advances (which forces it to lose leadership).

If the local replica is getting added to the voters set, it will allow the transition to prospective candidate when the fetch timer expires. The fetch timer is reset whenever it receives a successful Fetch or FetchSnapshot response.

Quorum state

Each KRaft topic partition has a quorum state (QuorumStateData) that gets persisted in the quorum-state file in the directory for the topic partition. The following changes will be made to this state:

1. ClusterId will get removed in version 1. This field is not used because the cluster id is persisted in <metadata.log.dir>/meta.properties.
2. AppliedOffset will get removed in version 1. This field is not used in version 0.
3. VotedUuid will get added in version 1. The voting replica will persist both the voter ID and UUID of the candidate for which it voted.
4. CurrentVoters will get removed in version 1. This field was only used to validate that the controller.quorum.voters value didn't change. In kraft.version 1 the set of voters will be stored in the snapshot and log instead.

Version 0 of this data will get written if the kraft.version is 0. Version 1 of this data will get written if the kraft.version is 1.

If a candidate sends a vote request with a replica UUID (version 2 of the RPC) but the voter's kraft.version is at 0, the voter will not persist the voted UUID and use version 0 of quorum data. The voter will continue to track the voted UUID in memory. This mean that after a restart the voter can vote for a different UUID but this should be rare and can only happing while the protocol is getting upgraded.

% git diff upstream/trunk raft/src/main/resources/common/message/QuorumStateData.json
diff --git a/raft/src/main/resources/common/message/QuorumStateData.json b/raft/src/main/resources/common/message/QuorumStateData.json
index d71a32c75d..1ae14ec843 100644
--- a/raft/src/main/resources/common/message/QuorumStateData.json
+++ b/raft/src/main/resources/common/message/QuorumStateData.json
@@ -16,19 +16,17 @@
 {
   "type": "data",
   "name": "QuorumStateData",
-  "validVersions": "0",
+  "validVersions": "0-1",
   "flexibleVersions": "0+",
   "fields": [
-    {"name": "ClusterId-1"},
-    {"name": "AppliedOffset", "type": "int64", "versions": "0+"},
-    {"name": "CurrentVoters", "type": "[]Voter", "versions": "0+", "nullableVersions": "0+"}
+    { "name": "ClusterId", "type": "string", "versions": "0" },
+    { "name": "LeaderId", "type": "int32", "versions": "0+", "default": "-1" },
+    { "name": "LeaderEpoch", "type": "int32", "versions": "0+", "default": "-1" },
+    { "name": "VotedId", "type": "int32", "versions": "0+", "default": "-1" },
+    { "name": "VotedUuid", "type": "uuid", "versions": "1+" },
+    { "name": "AppliedOffset", "type": "int64string", "versions": "0+" },
+-    { "name": "AppliedRecordLeaderId", "type": "AppliedRecordint32", "versions": "10+", "fieldsdefault": [
+"-1"},
-      { "name": "EndOffsetLeaderEpoch", "type": "int64int32", "versions": "10+", "default": "-1"},
+  -    { "name": "EpochVotedId", "type": "int32", "versions": "10+", "default": "-1"}
+    ],
-    {"name": "AppliedOffset", "type": "int64", "versions": "0+"},
+-    { "name": "CurrentVoters", "type": "[]Voter", "versions": "0+", "nullableVersions": "0+" }
-   ],
-   "commonStructs": [
-     { "name": "Voter", "versions": "0+", "fields": [
-      {"name": "VoterId", "type": "int32", "versions": "0+"}
+    { "name": "ClusterId", "type": "string", "versions": "0" },
+    { "name": "VoterIdLeaderId", "type": "int32", "versions": "0+", "default": "-1" },
+      { "name": "VoterUuidLeaderEpoch", "type": "uuidint32", "versions": "1+0+", "default": "-1" },
+     ]}
   ]
 }

RPCs

AddVoter

This RPC can be sent by an administrative client to add a voter to the set of voters. This RPC can be sent to a broker or controller, when sent to a broker, the broker will forward the request to the controller.

Handling

When the leader receives an AddVoter request it will do the following:

1. Wait for the fetch offset of the replica (ID, UUID) to catch up to the log end offset of the leader.
   
2. Wait until there are no uncommitted add or remove voter records.
   
3. Wait for the LeaderChangeMessage control record from the current epoch to get committed.
4. Send an ApiVersions RPC to the first listener to discover the supported kraft.version of the new voter.
5. Check that the new voter supports the current kraft.version.
6. Append the AddVoterRecord to the log.
7. The KRaft internal listener will read this record from the log and add the voter to the voter set.
8. Wait for the AddVoterRecord to commit using the majority of new voter set.
9. Send the AddVoter response to the client.

In 1., the leader needs to wait for the replica to catch up because when the AddVoterRecord is appended to the log, the set of voter changes. If the new voter is too far behind then it can take some time for it to reach the HWM. During this time the leader cannot commit data and the quorum will be unavailable from the perspective of the state machine. We mitigate this by waiting for the new replica to catch up before adding it to the set of voters.

In 4., the new replica will be part of the quorum so the leader will start sending BeginQuorumEpoch requests to this replica. It is possible that the new replica has not yet replicated and applied this AddVoterRecord so it doesn't know that it is a voter for this topic partition. The new replica will accept the BeginQuorumEpoch RPC even if it is doesn't believe it is a member of the voter set. 

The replica will return the following errors:

1. NOT_LEADER_FOR_PARTITION - when the request is sent to a replica that is not the leader.
2. VOTER_ALREADY_ADDED - when the request contains a replica ID and UUID that is already in the committed voter set.
3. UNSUPPORTED_VERSION - when the kraft.version is not greater than 1.
4. INVALID_REQUEST - when the new voter doesn't support the current kraft.version.

Request

{ "name": "VotedId", "type": "int32", "versions": "0+", "default": "-1" },
+    { "name": "VotedUuid", "type": "uuid", "versions": "1+" },
+    { "name": "AppliedOffset", "type": "int64", "versions": "0" },
+    { "name": "CurrentVoters", "type": "[]Voter", "versions": "0", "nullableVersions": "0", "fields": [
+      { "name": "VoterId", "type": "int32", "versions": "0" }
     ]}
   ]
 }

RPCs

UpdateFeatures

The schema and version for this API will not change but the handling of the RPC will be updated to handle the kraft.version feature.

Handling

The controller node will forward any changes to the kraft.version to the RaftClient.

When upgrading the version, the RaftClient will:

1. Verify that the current set of voters support the new version. The leader will learn the range of supported version from the UpdateVoter RPC. See that section for more details.
2. Verify that all of the observers (brokers) support the new version. The leader will learn range of supported version from the QuorumController. The quorum controller will update the RaftClient whenever the broker registration changes. 
3. If supported, the RaftClient will atomically write a control record batch to the log with all of the KRaftVersion and the VotersRecord control records. The VotersRecord controller records only need to get written when updating from version 0 to version 1.
4. Reply to the RPC when the control record batch is committed.

Downgrading to the kraft.version from 1 to 0 will be rejected with INVALID_UPDATE_VERSION.

AddVoter

This RPC can be sent by an administrative client to add a voter to the set of voters. This RPC can be sent to a broker or controller, when sent to a broker, the broker will forward the request to the active controller.

Handling

When the leader receives an AddVoter request it will do the following:

1. Wait for the fetch offset of the replica (ID, UUID) to catch up to the log end offset of the leader.
   
2. Wait until there are no uncommitted VotersRecord. Note that the implementation may just return a REQUEST_TIMED_OUT error if there are pending operations.
   
3. Wait for the LeaderChangeMessage control record from the current epoch to get committed. Note that the implementation may just return a REQUEST_TIMED_OUT error if there are pending operations.
4. Send an ApiVersions RPC to the first listener to discover the supported kraft.version of the new voter.
5. Check that the new voter supports the current kraft.version.
6. Append the updated VotersRecord to the log.
7. The KRaft internal listener will read this record from the log and add the voter to the voters set.
8. Wait for the VotersRecord to commit using the majority of new voters set. Return a REQUEST_TIMED_OUT error if it doesn't succeed in time.
9. Send the AddVoter response to the client.

In 1., the leader needs to wait for the replica to catch up because when the VotersRecord is appended to the log, the set of voter changes. If the new voter is too far behind then it can take some time for it to reach the HWM. During this time the leader cannot commit data and the quorum will be unavailable from the perspective of the state machine. We mitigate this by waiting for the new replica to catch up before adding it to the set of voters.

Here is an example, where waiting for the new voter to catch up to the LEO is helpful. Let's assume that there are 3 voters with only two of them at the LEO or HWM.

Voter 1: LEO = 200
Voter 2: LEO = 200
Voter 3: LEO = 100

In this example the HWM is 200. To advance the HWM past 200 only voter 1 and 2 need to Fetch the new records. For voter 3 to be counted towards advancing the HWM it needs to Fetch to or past offset 200 (100 records in this example).

At this point if the user decides to add a new voter (4) to the voters set and the leader doesn't wait for voter (4) to catch up to the LEO, the voters set may become:

Voter 1: LEO = 200
Voter 2: LEO = 200
Voter 3: LEO = 100
Voter 4: LEO = 0

In this state the majority must include either voter 3, voter 4 or both voters 3 and 4. This means that leader cannot advance the HWM until at least either voter 3 or voter 4 have caught up to the HWM. In other words, the KRaft topic partition won't be able to commit new records until at least one of those two voters (3 or 4) are caught up to the HWM/LEO.

In 3., the leader will wait for its current epoch to commit by waiting for the LeaderChangeMessage to commit. This is required to guarantee that the two competing voters sets, the one from a previous leader and the one from the current leader, only differ by at most one voter. Waiting for the current epoch to commit means that there cannot be some other competing voters set from another leader that can later override this leader's new voters set. See bug in single-server membership changes for more details on this.

In 7., the new replica will be part of the quorum so the leader will start sending BeginQuorumEpoch requests to this replica. It is possible that the new replica has not yet replicated and applied this VotersRecord so it doesn't know that it is a voter for this topic partition. The new replica will accept the BeginQuorumEpoch RPC even if it is doesn't believe it is a member of the voters set. 

The replica will return the following errors:

1. NOT_LEADER_FOR_PARTITION - when the request is sent to a replica that is not the leader.
2. DUPLICATE_VOTER - when the request contains a replica id is already in the committed voters set. Note that this means that duplicate replica ids are not allowed. This is useful to make automatic voter addition safer.
3. UNSUPPORTED_VERSION - when the kraft.version is not greater than 1.
4. INVALID_REQUEST - when the new voter doesn't support the current kraft.version.
5. REQUEST_TIMED_OUT - when the new voter didn't catch-up to the LEO in the time specified in the request.

Request

{
  "apiKey": 76,
  "type": "request",
  "listeners": ["controller", "broker"],
  "name": "AddVoterRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [

{
  "apiKey": 75,
  "type": "request",
  "listeners": ["controller", "broker"],
  "name": "AddVoterRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ClusterId", "type": "string", "versions": "0+" },
    { "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
      "about": "The name of the topic" },
    { "name": "TopicId", "type": "uuid", "versions": "0+",
      "about": "The unique topic ID" },
    { "name": "Partition", "type": "int32", "versions": "0+",
      "about": "The partition index" },
    { "name": "VoterId", "type": "int32", "versions": "0+",
      "about": "The replica id of the voter getting added to the topic partition" },
    { "name": "VoterUuid", "type": "uuid", "versions": "0+",
      "about": "The directory id of the voter getting added to the topic partition" },
    { "name": "ListenersClusterId", "type": "[]Listenerstring", "versions": "0+" },
    {  "aboutname": "The endpoints that can be used to communicate with the voterTimeoutMs", "type": "int32", "fieldsversions": ["0+" },
      { "name": "NameTopicName", "type": "string", "versions": "0+", "mapKeyentityType": true"topicName",
        "about": "The name of the endpointtopic" },
      { "name": "HostTopicId", "type": "stringuuid", "versions": "0+",
        "about": "The hostnameunique topic ID" },
      { "name": "PortPartition", "type": "uint16int32", "versions": "0+",
        "about": "The portpartition index" },
      { "name": "SecurityProtocolVoterId", "type": "int16int32", "versions": "0+",
        "about": "The securityreplica protocol"id }
of the voter getting ]}
added  ]
}

Response

{
  "apiKey": 75,
  "typeto the topic partition" },
    { "name": "responseVoterUuid",
  "nametype": "AddVoterResponseuuid",
  "validVersionsversions": "0+",
      "flexibleVersionsabout": "0+",
  "fields": [The directory id of the voter getting added to the topic partition" },
    { "name": "ErrorCodeListeners", "type": "int16[]Listener", "versions": "0+",
      "about": "The endpoints that errorcan code,be orused 0to ifcommunicate therewith wasthe no error" },voter", "fields": [
      { "name": "CurrentLeaderName", "type": "LeaderIdAndEpochstring", "versions": "0+", "taggedVersionsmapKey": "0+", "tag": 0, "fields": [true,
        "about": "The name of the endpoint" },
      { "name": "LeaderIdHost", "type": "int32string", "versions": "0+", "default": "-1", "entityType" : "brokerId",
        
        "about": "The replica id of the current leader or -1 if the leader is unknownhostname" },
      { "name": "LeaderEpochPort", "type": "int32uint16", "versions": "0+", "default": "-1",
        "about": "The latest known leader epochport" }
    ]},
  ]
}

Response

{
  "apiKey  { "name": "NodeEndpoint"76,
  "type": "NodeEndpointresponse",
  "versionsname": "0+AddVoterResponse",
  "taggedVersionsvalidVersions": "0+", "tag": 1,
      "aboutflexibleVersions": "Endpoint for current leader of the topic partition0+",
  "fields": [
      { "name": "HostErrorCode", "type": "stringint16", "versions": "0+",
      "about": "The node's hostname" },
      error code, or 0 if there was no error" },
    { "name": "PortErrorMessage", "type": "int32string", "versions": "0+", "nullableVersions": "0+", "ignorable": true,
      "about": "The node's port error message, or null if there was no error." },
    { "name": "CurrentLeader", "type": "LeaderIdAndEpoch", "versions": "0+", "taggedVersions": "0+", "tag": 0,  ]}
  ]
}

RemoveVoter

This RPC can be sent by an administrative client to remove a voter from the set of voters. This RPC can be sent to a broker or controller. The broker will forward the request to the controller.

Handling

When the leader receives a RemoveVoter request it will do the following:

1. Wait until there are no uncommitted add or remove voter records.
   
2. Wait for the LeaderChangeMessage control record from the current epoch to get committed.
3. Append the RemoveVoterRecord to the log.
4. The KRaft internal listener will read this record from the log and remove the voter from the voter set.
5. Wait for the RemoveVoterRecord to commit using the majority of new configuration.
6. Send the RemoveVoter response to the client.
7. Resign by sending EndQuorumEpoch RPCs if the removed replica is the leader.

In 3. and 4. it is possible for the RemoveVoterRecord would remove the current leader from the voter set. In this case the leader needs to allow Fetch and FetchSnapshot requests from replicas. The leader should not count itself when determining the majority and determining if records have been committed.

The replica will return the following errors:

1. NOT_LEADER_FOR_PARTITION - when the request is sent to a replica that is not the leader.
2. VOTER_ALREADY_REMOVED - when the request contains a replica ID and UUID that is already not in the committed voter set.
3. UNSUPPORTED_VERSION - when the kraft.version is not greater than 1.

Request

{
  "apiKey": 76,
  "type": "request",
  "listeners": ["controller", "broker"],
  "name": "RemoveVoterRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
      "about": "The name of the topic" },
"fields": [
      { "name": "LeaderId", "type": "int32", "versions": "0+", "default": "-1", "entityType" : "brokerId",
        "about": "The replica id of the current leader or -1 if the leader is unknown" },
      { "name": "LeaderEpoch", "type": "int32", "versions": "0+", "default": "-1",
        "about": "The latest known leader epoch" }
    ]},
    { "name": "NodeEndpoint", "type": "NodeEndpoint", "versions": "0+", "taggedVersions": "0+", "tag": 1,
      "about": "Endpoint for current leader of the topic partition", "fields": [
      { "name": "TopicIdHost", "type": "uuidstring", "versions": "0+",
      "about": "The unique topic IDnode's hostname" },
      { "name": "PartitionPort", "type": "int32", "versions": "0+",
      "about": "The partitionnode's indexport" },
    { "name": "VoterId", "type": "int32", "versions": "0+",
      "about": "The replica id of the voter getting removed from the topic partition" },
    { "name": "VoterUuid", "type": "uuid", "versions": "0+",
      "about": "The directory id of the voter getting removed from the topic partition" }
  ]
}

Response

]}
  ]
}

RemoveVoter

This RPC can be sent by an administrative client to remove a voter from the set of voters. This RPC can be sent to a broker or controller. The broker will forward the request to the controller.

Handling

When the leader receives a RemoveVoter request it will do the following:

1. Wait until there are no uncommitted VotersRecord. Note that the implementation may just return a REQUEST_TIMED_OUT error if there are pending operations.
   
2. Wait for the LeaderChangeMessage control record from the current epoch to get committed. Note that the implementation may just return a REQUEST_TIMED_OUT error if there are pending operations.
3. Append the VotersRecord to the log with the updated voters set.
4. The KRaft internal listener will read this record from the log and remove the voter from the voters set.
5. Wait for the VotersRecord to commit using the majority of new configuration. Return a REQUEST_TIMED_OUT error if it doesn't succeed in time.
6. Send the RemoveVoter response to the client.
7. Resign by sending EndQuorumEpoch RPCs if the removed replica is the leader.

In 3. and 4. it is possible for the VotersRecord would remove the current leader from the voters set. In this case the leader needs to allow Fetch and FetchSnapshot requests from replicas. The leader should not count itself when determining the majority and determining if records have been committed.

The replica will return the following errors:

1. NOT_LEADER_FOR_PARTITION - when the request is sent to a replica that is not the leader.
2. VOTER_NOT_FOUND - when the request contains a replica ID and UUID that is already not in the committed voters set.
3. UNSUPPORTED_VERSION - when the kraft.version is not greater than 1.
4. REQUEST_TIMED_OUT

Request

{
  "apiKey": 77,
  "type": "request",
  "listeners": ["controller", "broker"],
  "name": "RemoveVoterRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ClusterId

{
  "apiKey": 76,
  "type": "response",
  "name": "RemoveVoterResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The error code, or 0 if there was no error" },
    { "name": "CurrentLeader", "type": "LeaderIdAndEpoch", "versions": "0+", "taggedVersions": "0+", "tag": 0, "fields": [
      { "name": "LeaderId", "type": "int32", "versions": "0+", "default": "-1", "entityType" : "brokerId",
        "about": "The replica id of the current leader or -1 if the leader is unknown" },
      { "name": "LeaderEpoch", "type": "int32", "versions": "0+", "default": "-1",
        "about": "The latest known leader epoch" }
    ]},
    { "name": "NodeEndpoint", "type": "NodeEndpointstring", "versions": "0+" },
    "taggedVersions{ "name": "TopicName", "type": "string", "versions": "0+", "tagentityType": 1"topicName",
      "about": "EndpointThe forname current leader of the topic" partition", "fields": [},
      { "name": "HostTopicId", "type": "stringuuid", "versions": "0+",
      "about": "The unique node'stopic hostnameID" },
      { "name": "PortPartition", "type": "int32", "versions": "0+",
      "about": "The node'spartition portindex" },
    ]}
  ]
}

UpdateVoter

This RPC is different from AddVoter in two ways. It will be sent only by voters to the latest known leader. It include both listener endpoints and kraft.version information. This is useful to automatically update that information without additional operator intervention.

Handling

When the leader receives an UpdateVoter request it will do the following:

1. Wait until there are no uncommitted add or remove voter records.
   
2. Wait for the LeaderChangeMessage control record from the current epoch to get committed.
3. Check that the updated voter supports the current kraft.version.
4. Append the updated AddVoterRecord to the log.
5. The KRaft internal listener will read this record from the log and update the voter's information. This include updating the endpoint used by the KRaft NetworkClient.
6. Wait for the AddVoterRecord to commit using the majority of new voter set.
7. Send the UpdateVoter response to the client.

Request

{ "name": "VoterId", "type": "int32", "versions": "0+",
      "about": "The replica id of the voter getting removed from the topic partition" },
    { "name": "VoterUuid", "type": "uuid", "versions": "0+",
      "about": "The directory id of the voter getting removed from the topic partition" }
  ]
}

Response

{
  "apiKey": 77,
  "type": "requestresponse",
  "listeners": ["controller"],
  "name": "UpdateVoterRequestRemoveVoterResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ClusterIdErrorCode", "type": "stringint16", "versions": "0+" },
    {  "nameabout": "TopicName", "The error code, or 0 if there was no error" },
    { "name": "ErrorMessage", "type": "string", "versions": "0+", "entityTypenullableVersions": "0+", "topicNameignorable": true,
      "about": "The name of the topicerror message, or null if there was no error." },
     { "name": "TopicIdCurrentLeader", "type": "uuidLeaderIdAndEpoch", "versions": "0+",
      "abouttaggedVersions": "0+"The unique topic ID" },, "tag": 0, "fields": [
      { "name": "PartitionLeaderId", "type": "int32", "versions": "0+", "default": "-1", "entityType" : "brokerId",
        "about": "The partition index replica id of the current leader or -1 if the leader is unknown" },
      { "name": "VoterIdLeaderEpoch", "type": "int32", "versions": "0+",
      "default": "-1",
        "about": "The replicalatest idknown ofleader theepoch" voter}
 getting updated in the topic partition"  ]},
    { "name": "VoterUuidNodeEndpoint", "type": "uuidNodeEndpoint", "versions": "0+",
      "abouttaggedVersions": "The directory id of the voter getting updated in the topic partition" },
    { "name": "Listeners", "type": "[]Listener", "versions": "0+"0+", "tag": 1,
      "about": "TheEndpoint endpointfor thatcurrent canleader beof usedthe to communicate with the leadertopic partition", "fields": [
      { "name": "NameHost", "type": "string", "versions": "0+", "mapKey": true,
        "about": "The name of the endpointnode's hostname" },
      { "name": "HostPort", "type": "stringint32", "versions": "0+",
        " "about": "The hostname" },
      { "name": "Port", "type": "uint16", "versions": "0+",
        "about": "The node's port" },
      { "name": "SecurityProtocol", "type": "int16", "versions": "0+",
        "about": "The security protocol" }
    ]},
    { "name": "KRaftVersionFeature", "type": "KRaftVersionFeature", "versions": "0+",
      "about": "The range of versions of the protocol that the replica supports", "fields": [
      { "name": "MinSupportedVersion", "type": "int16", "versions": "0+",
        "about": "The minimum supported KRaft protocol version" },
      { "name": "MaxSupportedVersion", "type": "int16", "versions": "0+",
        "about": "The maximum supported KRaft protocol version" }
    ]}
  ]
}

Response

]}
  ]
}

UpdateVoter

This RPC is different from AddVoter in two ways: 1. it will be sent only by voters to the latest known leader and 2. it includes both listener endpoints and kraft.version information. This RPC is useful to automatically update a voter's endpoints and kraft.version information without additional operator intervention.

The voter will always send the UpdateVoter RPC whenever it starts and whenever the leader changes. The voter will continue to send the UpdateVoter RPC until the call has been acknowledge by the current leader. 

Handling

When the leader receives an UpdateVoter request it will do the following:

1. Wait until there are no uncommitted add or remove voter records. Note that the implementation may just return a REQUEST_TIMED_OUT error if there are pending operations.
   
2. Wait for the LeaderChangeMessage control record from the current epoch to get committed. Note that the implementation may just return a REQUEST_TIMED_OUT error if there are pending operations.
3. Check that the updated voter supports the current kraft.version.
4. Append the updated VotersRecord to the log.
5. The KRaft internal listener will read this record from the log and update the voter's information. This include updating the endpoint used by the KRaft NetworkClient.
6. Wait for the VotersRecord to commit using the majority of new voters set.
7. Send the UpdateVoter response to the client.

The replica will return the following errors:

1. NOT_LEADER_FOR_PARTITION - when the request is sent to a replica that is not the leader.
2. VOTER_NOT_FOUND - when the request contains a replica ID and UUID that is not in the committed voters set.
3. INVALID_UPDATE - when the minimum and maximum supported kraft.version in the request  doesn't include the finalized kraft.version.
4. REQUEST_TIMED_OUT

Request

{
  "apiKey": 78,
  "type": "request",
  "listeners": ["controller"],
  "name": "UpdateVoterRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ClusterId", "type": "string", "versions": "0+" },

{
  "apiKey": 77,
  "type": "response",
  "name": "UpdateVoterResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ErrorCodeTopicName", "type": "int16string", "versions": "0+", "entityType": "topicName",
      "about": "The errorname code,of or 0 if there was no errorthe topic" },
    { "name": "CurrentLeaderTopicId", "type": "LeaderIdAndEpochuuid", "versions": "0+",
      "taggedVersionsabout": "0+", "tag": 0, "fields": [The unique topic ID" },
      { "name": "LeaderIdPartition", "type": "int32", "versions": "0+",
      "defaultabout": "-1", "entityType" The partition index" },
    { "name": "VoterId", "type": "brokerIdint32",
 "versions": "0+",
      "about": "The replica id of the currentvoter leadergetting orupdated -1in if the leadertopic is unknownpartition" },
      { "name": "LeaderEpochVoterUuid", "type": "int32uuid", "versions": "0+", "default": "-1",
        "about": "The latest known leader epoch" }
    ] directory id of the voter getting updated in the topic partition" },
    { "name": "NodeEndpointListeners", "type": "NodeEndpoint[]Listener", "versions": "0+", "taggedVersions": "0+", "tag": 1,
      "about": "Endpoint for current leader ofThe endpoint that can be used to communicate with the topic partitionleader", "fields": [
      { "name": "HostName", "type": "string", "versions": "0+", "mapKey": true,
        "about": "The node's hostname name of the endpoint" },
      { "name": "PortHost", "type": "int32string", "versions": "0+",
        "about": "The node's porthostname" },
    ]}
  ]
}

UpdateFeatures

TODO: Fill this out

Handling

Request

Response

Fetch

Request

Version 14 adds the field ReplicaUuid to the FetchPartition. This field is populated with the replica generated UUID. If the ReplicaUuid and the ReplicaId fields are populated, the topic partition leader can assume that the replica supports becoming a voter.

{
  "apiKeyname": 1"Port",
  "type": "requestuint16",
  "listenersversions": ["zkBroker0+",
 "broker", "controller"],
        "about": "The port" }
    ]},
    { "name": "FetchRequestKRaftVersionFeature",
  "validVersionstype": "0-14KRaftVersionFeature",
  "flexibleVersionsversions": "120+",
  "fields": [
    { "nameabout": "ClusterId", "type": "string", "versions": "12+", "nullableVersions": "12+", "default": "null", "taggedVersions": "12+", "tag": 0, "ignorable": true,
The range of versions of the protocol that the replica supports", "fields": [
      { "name": "MinSupportedVersion", "type": "int16", "versions": "0+",
        "about": "The clusterIdminimum ifsupported known.KRaft This is used to validate metadata fetches prior to broker registration.protocol version" },
      { "name": "ReplicaIdMaxSupportedVersion", "type": "int32int16", "versions": "0+",
 "entityType": "brokerId",
      "about": "The replicamaximum IDsupported ofKRaft theprotocol version" }
    ]}
  ]
}

Response

{
  "apiKey": 78follower, of -1 if this request is from a consumer." },
    ..."type": "response",
    { "name": "TopicsUpdateVoterResponse",
  "typevalidVersions": "[]FetchTopic0",
  "versionsflexibleVersions": "0+",
      "about": "The topics to fetch.", "fields": [
      { "name": "TopicErrorCode", "type": "stringint16", "versions": "0-12+", "entityType": "topicName", "ignorable": true,
        "about": "The name of the topic to fetch. error code, or 0 if there was no error" },
      { "name": "TopicIdCurrentLeader", "type": "uuidLeaderIdAndEpoch", "versions": "130+", "ignorabletaggedVersions": true"0+",
        "about": "The unique topic ID"},"tag": 0, "fields": [
      { "name": "PartitionsLeaderId", "type": "[]FetchPartitionint32", "versions": "0+", "default": "-1", "entityType" : "brokerId",
        "about": "The partitions to fetch.", "fields": [replica id of the current leader or -1 if the leader is unknown" },
        { "name": "PartitionLeaderEpoch", "type": "int32", "versions": "0+",
 "default": "-1",
        "about": "The partition index.latest known leader epoch" },
     ]},
    { "name": "ReplicaUuidNodeEndpoint", "type": "uuidNodeEndpoint", "versions": "140+", "nullableVersionstaggedVersions": "140+", "defaulttag": "null"1,
          "about": "The replica generated UUID. null otherwise." },
        ...Endpoint for current leader of the topic partition", "fields": [
      ]}
    ]}
  ]
}

Response

Version 14 rename LeaderIdAndEpoch to CurrentLeader and adds Endpoint to CurrentLeader.

{
  "apiKeyname": 1"Host",
  "type": "responsestring",
  "name": "FetchResponse",
  "validVersions": "0-14",
  "flexibleVersions": "12+",
  "fields": [
    ...
    { "name": "Responses", "type": "[]FetchableTopicResponse", "versions": "0+",
      "about": "The response topics.", "fields": [node's hostname" },
      { "name": "TopicPort", "type": "stringint32", "versions": "0-12+", "ignorable": true, "entityType": "topicName",
        "about": "The topic name.node's port" },
      { "name": "TopicId", "type": "uuid", "versions": "13+", "ignorable": true, "about": "The unique topic ID"},
      { "name": "Partitions", "type": "[]PartitionData", "versions": "0+",
        "about": "The topic partitions.", "fields": [
        ...
        { "name": "CurrentLeader", "type": "CurrentLeader",
          "versions": "12+", "taggedVersions": "12+", "tag": 1, "fields": [
           { "name": "LeaderId", "type": "int32", "versions": "12+", "default": "-1", "entityType": "brokerId",
            "about": "The ID of the current leader or -1 if the leader is unknown."},
          { "name": "LeaderEpoch", "type": "int32", "versions": "12+", "default": "-1",
            "about": "The latest known leader epoch"},
          ]}
  ]
}

Vote

Handling

Since the set of voters can change and not all replicas know the latest voters set, handling of Vote request needs to be relaxed from what was defined and implemented for KIP-595. KRaft replicas will accept Vote requests from all replicas. Candidate replicas don't need to be in the voters' voters set to receive a vote. This is needed to be able to elect a leader from the new voters set even though the new voters set hasn't been replicated to all of its voters.

When the leader removes a voter from the voters set it is possible for the removed voter's Fetch timeout to expire before the replica has replicated the latest VotersRecord. If this happens removed replica will become a candidate, increase its epoch and eventually force the leader to change. To avoid this scenario this KIP is going to rely on KIP-996: Pre-Vote to fenced the removed replica from increasing its epoch:

When servers receive VoteRequests with the PreVote field set to true, they will respond with VoteGranted set to

• true if they are not a Follower and the epoch and offsets in the Pre-Vote request satisfy the same requirements as a standard vote
• false if otherwise

The voter will persist both the candidate ID and UUID in the quorum state so that it only votes for at most one candidate for a given epoch.

The replica will return the following new errors:

1. INVALID_REQUEST - when the voter ID and UUID doesn't match the local ID and UUID.
2. UNSUPPORTED_VERSION - when a non-empty candidate UUID is specified but the voter doesn't support kraft.version 1.

The replica will not return the following errors anymore:

1. INCONSISTENT_VOTER_SET - because the voting replica will not validate that either the candidate or the voter are voters.

Request

git diff upstream/trunk clients/src/main/resources/common/message/VoteRequest.json
diff --git a/clients/src/main/resources/common/message/VoteRequest.json b/clients/src/main/resources/common/message/VoteRequest.json
index 35583a790b..ff808cbafe 100644
--- a/clients/src/main/resources/common/message/VoteRequest.json
+++ b/clients/src/main/resources/common/message/VoteRequest.json
@@ -18,11 +18,13 @@
   "type": "request",
   "listeners": ["controller"],
   "name": "VoteRequest",
-  "validVersions": "0",
+  "validVersions": "0-1",
   "flexibleVersions": "0+",
   "fields": [
     { "name": "EndPointClusterId", "type": "Endpointstring", "versions": "140+",
            "aboutnullableVersions": "The endpoint that can be used to communicate with the leader0+", "fieldsdefault": [
       "null"},
+     { "name": "HostVoterId", "type": "stringint32", "versions": "141+",
 "ignorable": true, "default": "-1",   "entityType": "brokerId",
+       "about": "The hostname." },
       replica id of the voter receiving the request" },
     { "name": "PortTopics", "type": "uint16[]TopicData",
       "versions": "140+",
 "fields": [
         {   "aboutname": "TopicName"The, port."type" }: "string", "versions": "0+", "entityType": "topicName",
@@ -34,14 +36,16 @@
          ]}
        ]{ "name": "CandidateEpoch", "type": "int32", "versions": "0+",
           "about": "The bumped epoch of the candidate sending the request"},
        ...
 { "name": "CandidateId", "type":  ]}
"int32", "versions": "0+", "entityType": "brokerId",
-    ]}
  ]
}

Handling

Replica that support becoming voters will send both the replica ID and UUID in the Fetch request. The leader will assume that replicas that report both fields are voters or are able to become voters.

There are a few changes to the leader request handling described in KIP-595. The leaders will track the fetch offset for the replica tuple (ID, UUID). This means that replica are unique identified by their ID and UUID. So their state will be tracking using the ID and UUID.

When removing the leader from the voter set, it will remain the leader for that epoch until the RemoveVoterRecord gets committed. This means that the leader needs to allow replicas (voters and observers) to fetch from the leader even if it is not part of the voter set. This also means that if the leader is not part of the voter set it should not include itself when computing the committed offset (also known as the high-watermark).

FetchSnapshot

Request

Version 1 adds the field ReplicaUuid to PartitionSnapshot. If the ReplicaUuid and the ReplicaId fields are populated, the topic partition leader can assume that the replica supports becoming a voter.

{
  "apiKey": 59,
        "about": "The ID of the voter sending the request"},
+          "about": "The replica id of the voter sending the request"},
+        { "name": "CandidateUuid", "type": "requestuuid",
  "listenersversions": ["controller1+"],
+  "name": "FetchSnapshotRequest",
  "validVersions": "0-1",
        "flexibleVersionsabout": "0+",
The directory "fields": [
id of the voter sending the request" },
+        { "name": "ClusterIdVoterUuid", "type": "stringuuid", "versions": "01+", "nullableVersions": "0+", "default": "null", "taggedVersions": "0+", "tag": 0,

+           "about": "The directory id clusterIdof ifthe known,voter thisreceiving isthe usedrequest to validatevote, metadataempty fetchesuuid priorif tounknown" broker},
   registration"  },
    { "name": "ReplicaIdLastOffsetEpoch", "type": "int32", "versions": "0+",
 "default": "-1", "entityType": "brokerId",
          "about": "The replica ID ofepoch of the last record written to the followermetadata log" },
         { "name": "MaxBytesLastOffset", "type": "int32int64", "versions": "0+", "default": "0x7fffffff",

           "about": "The maximumoffset bytesof tothe fetchlast fromrecord allwritten ofto the snapshotsmetadata log" },
    { "name": "Topics", "type": "[]TopicSnapshot", "versions": "0+",]}
     ]}
   ]
 }

Response

git diff upstream/trunk clients/src/main/resources/common/message/VoteResponse.json
diff --git a/clients/src/main/resources/common/message/VoteResponse.json b/clients/src/main/resources/common/message/VoteResponse.json
index b92d0070c1..21d0aa5312 100644
--- a/clients/src/main/resources/common/message/VoteResponse.json
+++ b/clients/src/main/resources/common/message/VoteResponse.json
@@ -17,7 +17,7 @@
   "apiKey": 52,
   "about": "The topics to fetch", "fields": [
      { "name": "Name", "type": "stringresponse",
   "versionsname": "0+VoteResponse",
-  "entityTypevalidVersions": "topicName0",
+  "validVersions": "0-1",
     "aboutflexibleVersions": "The name of the topic to fetch" },0+",
   "fields": [
      { "name": "PartitionsErrorCode", "type": "[]PartitionSnapshotint16", "versions": "0+",
@@ -37,9 +37,14 @@
           "about": "The partitionslatest known toleader fetchepoch"},
 "fields": [
        { "name": "PartitionVoteGranted", "type": "int32bool", "versions": "0+",
           "about": "TheTrue partitionif index" },the vote was granted and false otherwise"}
       ]}
     ]},
+    { "name": "ReplicaUuidNodeEndpoints", "type": "uuid[]NodeEndpoint", "versions": "1+", "defaulttaggedVersions": "null1+",
 "tag": 0,
+         "about": "TheEndpoints for replicaall UUIDcurrent-leaders ofenumerated thein followerPartitionData" }, 
  , "fields": [
+      { "name": "CurrentLeaderEpochNodeId", "type": "int32", "versions": "01+",
+        "mapKey": true, "entityType": "brokerId", "about": "The current leader epochID of the partition, -1 for unknown leader epoch" associated node"},
+        { "name": "SnapshotIdHost", "type": "SnapshotIdstring", "versions": "01+",
 "about": "The node's hostname" },
+      { "aboutname": "The snapshot endOffset and epoch to fetchPort", "fieldstype": [
          { "name": "EndOffset", "type": "int64", "versions": "0+" },
          { "name": "Epoch", "type": "int32", "versions": "0+" }
        ]},
        { "name": "Position", "type": "int64", "versions": "01+",
          "about": "The byte position within the snapshot to start fetching fromnode's port" }
 +     ]}
    ]}
  ]
}

Response

Version 1 renames LeaderIdAndEpoch to CurrentLeader and adds Endpoint to CurrentLeader.

}

BeginQuorumEpoch

Handling

When handling the BeginQuorumEpoch request the replica will accept the request if the LeaderEpoch is equal or greater than their epoch. The receiving replica will not check if the new leader or itself is in the voters set. This change is required because the receiving replica may not have fetched the latest voters set.

The BeginQuorumEpoch RPC will be used by the leader to propagate the leader endpoint to all of the voters. This is needed so that all voters have the necessary information to send Fetch and FetchSnaphsot requests.

The check quorum algorithm implemented by the leader needs to be extended so that BeginQuorumEpoch request are sent to any voter that is not actively fetching (Fetch and FetchSnapshot) from the leader.

The replica will return the following new errors:

1. INVALID_REQUEST - when the voter ID and UUID doesn't match the local ID and UUID.

The replica will not return the following errors anymore:

1. INCONSISTENT_VOTER_SET - because the voting replica will not validate that either the candidate or the voter are voters.

Request

git diff upstream/trunk clients/src/main/resources/common/message/BeginQuorumEpochRequest.json
diff --git a/clients/src/main/resources/common/message/BeginQuorumEpochRequest.json b/clients/src/main/resources/common/message/BeginQuorumEpochRequest.json
index d9d6d92c88..edd128fb8c 100644
--- a/clients/src/main/resources/common/message/BeginQuorumEpochRequest.json
+++ b/clients/src/main/resources/common/message/BeginQuorumEpochRequest.json
@@ -18,24 +18,35 @@
   "type": "request",
   "listeners": ["controller"],
  

{
  "apiKey": 59,
  "type": "response",
  "name": "FetchSnapshotResponse",
  "validVersions": "0-1",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ThrottleTimeMsBeginQuorumEpochRequest",
-  "typevalidVersions": "int320",
-  "versionsflexibleVersions": "0+none",
+  "ignorablevalidVersions": true"0-1",
 +     "aboutflexibleVersions": "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." },"1+",
   "fields": [
     { "name": "ErrorCodeClusterId", "type": "int16string", "versions": "0+",
       "nullableVersions": "0+", "default": "null"},
+ "ignorable   { "name": "VoterId", "type": "int32", "versions": "1+", "entityType": false"brokerId",
+      "about": "The topvoter ID levelof responsethe errorreceiving code.replica" },
     { "name": "Topics", "type": "[]TopicSnapshotTopicData", "versions": "0+",

       "aboutversions": "The topics to fetch.0+", "fields": [
       { "name": "NameTopicName", "type": "string", "versions": "0+", "entityType": "topicName",
         "about": "The name of the topic to fetch.name" },
       { "name": "Partitions", "type": "[]PartitionSnapshotPartitionData", "versions": "0+",

         "aboutversions": "The partitions to fetch.0+", "fields": [
         { "name": "IndexPartitionIndex", "type": "int32", "versions": "0+",
           "about": "The partition index." },
+        { "name": "ErrorCodeVoterUuid", "type": "int16uuid", "versions": "01+",
+          "about": "The errordirectory code,id orof 0the if there was no fetch error.receiving replica" },
         { "name": "SnapshotIdLeaderId", "type": "SnapshotIdint32", "versions": "0+", "entityType": "brokerId",
           "about": "The snapshotID of endOffsetthe andnewly epochelected fetchedleader"},
          "fields": [
          { "name": "EndOffsetLeaderEpoch", "type": "int64int32", "versions": "0+" },
          { "nameabout": "Epoch", "type": "int32", "versions": "0+" The epoch of the newly elected leader"}
        ]},
+    ]},
+    { "name": "CurrentLeaderNodeEndpoint", "type": "CurrentLeaderNodeEndpoint",
          "versions": "01+", "taggedVersions": "01+", "tag": 0,
+      "fieldsabout": [
    "Endpoint for the leader", "fields": [
+      { "name": "LeaderIdNodeId", "type": "int32", "versions": "01+", "entityType": "brokerId",

+        "mapKey": true, "entityType": "brokerId", "about": "The ID of the currentassociated leader or -1 if the leader is unknown."node" },
+          { "name": "LeaderEpochHost", "type": "int32string", "versions": "01+",
            "about": "The latest known leader epoch"node's hostname" },
+          { "name": "EndPointPort", "type": "Endpointint32", "versions": "1+",
            "about": "The endpoint that can be used to communicate with the leader", "fields": [
            { "name": "Host",node's port" }
     ]}
   ]
 }

Response

git diff upstream/trunk clients/src/main/resources/common/message/BeginQuorumEpochResponse.json
diff --git a/clients/src/main/resources/common/message/BeginQuorumEpochResponse.json b/clients/src/main/resources/common/message/BeginQuorumEpochResponse.json
index 4b7d7f5a95..12639bba2f 100644
--- a/clients/src/main/resources/common/message/BeginQuorumEpochResponse.json
+++ b/clients/src/main/resources/common/message/BeginQuorumEpochResponse.json
@@ -17,25 +17,32 @@
   "apiKey": 53,
   "type": "stringresponse",
   "versionsname": "1+BeginQuorumEpochResponse",
  -            "about"validVersions": "The hostname." }0",
 -           { "name"flexibleVersions": "Portnone",
+  "typevalidVersions": "uint160-1",
+  "versionsflexibleVersions": "1+",
   "fields": [
     { "name": "ErrorCode", "type": "int16", "aboutversions": "The port." }0+",
       "about": "The top level ]error code"},
        { "name": "SizeTopics", "type": "int64[]TopicData",
       "versions": "0+",
          "aboutfields": "The total size of the snapshot." },
 [
       { "name": "PositionTopicName", "type": "int64string", "versions": "0+", "entityType": "topicName",
          "about": "The starting byte position within the snapshot included in the Bytes field."   topic name" },
        { "name": "UnalignedRecordsPartitions", "type": "records[]PartitionData",
         "versions": "0+", "fields": [
         { "aboutname": "Snapshot data in records format which may not be aligned on an offset boundary" }PartitionIndex", "type": "int32", "versions": "0+",
           "about": "The partition index" },
      ]}
    ]}
  ]
}

Handling

Similar to Fetch, replica that support becoming voters will send both the replica ID and UUID in the FetchSnapshot request. The leader will assume that replicas that report both fields are voters or are able to become voters.

There are a few changes to the leader request handling described in KIP-630. The leaders will track the fetch offset for the replica tuple (ID, UUID). This means that replica are unique identified by their ID and UUID. So their state will be tracking using the ID and UUID.

When removing the leader from the voter set, it will remain the leader for that epoch until the RemoveVoterRecord gets committed. This means that the leader needs to allow replicas (voters and observers) to fetch snapshots from the leader even if it is not part of the voter set.

Vote

Request

Changes:

1. Candidate Id was moved out of the topic partition maps
2. Candidate Uuid was added to the PartitionData
3. VoterId was added to the top level
4. VoterUuId was added to PartitionData

{
  "apiKey": 52,
  "type": "request",
  "listeners": ["controller"],
  "name": "VoteRequest",
  "validVersions": "0-1",
  "flexibleVersions": "0+",
  "fields": [
   { "name": "ErrorCode", "type": "int16", "versions": "0+"},
         { "name": "LeaderId", "type": "int32", "versions": "0+", "entityType": "brokerId",
           "about": "The ID of the current leader or -1 if the leader is unknown"},
         { "name": "LeaderEpoch", "type": "int32", "versions": "0+",
           "about": "The latest known leader epoch"}
       ]}
+    ]},
+    { "name": "ClusterIdNodeEndpoints", "type": "string[]NodeEndpoint", "versions": "01+", "nullableVersionstaggedVersions": "01+", "defaulttag": "null" }0,
+    {  "nameabout": "CandidateIdEndpoints for all leaders enumerated in PartitionData", "typefields": "int32", "versions [
+      { "name": "1+NodeId", "entityTypetype": "brokerIdint32",
      "aboutversions": "The ID of the voter sending the request" },
1+",
+    { "name": "VoterId", "type": "int32", "versionsmapKey": "1+"true, "entityType": "brokerId",
      "about": "The ID of the replica receiving the request to vote.associated node" },
+      { "name": "TopicsHost", "type": "[]TopicDatastring", "versions": "01+", "fieldsabout": [
"The node's hostname" },
+      { "name": "TopicNamePort", "type": "stringint32", "versions": "01+", "entityType": "topicName",
        "about": "The topic name.node's port" },
     ]}
 { "name": "Partitions", "type": "[]PartitionData",
        "versions": "0+", "fields": [
        { "name": "PartitionIndex", "type": "int32", "versions": "0 ]
 }

EndQuorumEpoch

In addition to the current cases where the leader resigns and sends an EndQuorumEpoch RPC, the leader will also resign and send this RPC once a VotersRecord has committed and the leader is not part of the latest voters set.

Handling

This request will be handle similar to how it is described in KIP-595. The receiving replica will take the directory id (candidate uuid) into account when computing the fetch timeout exponential backoff.

Request

PreferredSuccessor which is an array is replica ids, will be replaced by PreferredCandidates which is an array of the tuple candidate id and candidate uuid.

git diff upstream/trunk clients/src/main/resources/common/message/EndQuorumEpochRequest.json
diff --git a/clients/src/main/resources/common/message/EndQuorumEpochRequest.json b/clients/src/main/resources/common/message/EndQuorumEpochRequest.json
index a6e4076412..d9122fa930 100644
--- a/clients/src/main/resources/common/message/EndQuorumEpochRequest.json
+++ b/clients/src/main/resources/common/message/EndQuorumEpochRequest.json
@@ -18,8 +18,8 @@
   "type": "request",
   "listeners": ["controller"],
   "name": "EndQuorumEpochRequest",
-  "validVersions": "0",
-  "flexibleVersions": "none",
+  "validVersions": "0-1",
+  "flexibleVersions": "1+",
   "fields": [
     { "aboutname": "ClusterId", "The partition index." }type": "string", "versions": "0+",
        { "namenullableVersions": "CandidateEpoch0+", "typedefault": "int32null"}, "versions": "0+",
@@ -35,8 +35,13 @@
           "about": "The bumpedcurrent epochleader ofID thethat candidate sending the requestis resigning"},
         { "name": "CandidateIdLeaderEpoch", "type": "int32", "versions": "0+", "entityType": "brokerId",

           "about": "The ID of the voter sending the requestcurrent epoch"},
-        { "name": "CandidateUuidPreferredSuccessors", "type": "uuid[]int32", "versions": "10+",
-          "about": "The candidate generated UUID, null otherwise" },
A sorted list of preferred successors to start the election"}
+        { "name": "VoterUuidPreferredSuccessors", "type": "uuid[]int32", "versions": "1+0", "nullableVersions": "1+", "default": "null" }

+            ""about": "TheA replicasorted generatedlist UUID of the replica receiving the requestpreferred successors to vote,start nullthe otherwiseelection" }, 
+        { "name": "LastOffsetEpochPreferredCandidates", "type": "int32[]ReplicaInfo", "versions": "01+",
+          "about": "TheA sorted epochlist of thepreferred lastsuccessors recordto writtenstart to the metadata log"},
election", "fields": [
+          { "name": "LastOffsetCandidateId", "type": "int64int32", "versions": "0+"1+", "entityType": "brokerId" },
+          "about": "The offset of the last record written to the metadata log"{ "name": "CandidateUuid", "type": "uuid", "versions": "1+" }
+        ]}
       ]}
     ]}
   ]
}

Response

Version 1 addes the tagged field LeaderEndpoint to PartitionData.

{
git  "apiKey": 52,
  "diff upstream/trunk clients/src/main/resources/common/message/EndQuorumEpochResponse.json
diff --git a/clients/src/main/resources/common/message/EndQuorumEpochResponse.json b/clients/src/main/resources/common/message/EndQuorumEpochResponse.json
index cd23247045..0d5d61b7e7 100644
--- a/clients/src/main/resources/common/message/EndQuorumEpochResponse.json
+++ b/clients/src/main/resources/common/message/EndQuorumEpochResponse.json
@@ -17,8 +17,8 @@
   "apiKey": 54,
   "type": "response",
   "name": "VoteResponseEndQuorumEpochResponse",
-  "validVersions": "0-1",
-  "flexibleVersions": "0+none",
+  "fieldsvalidVersions": [
  "0-1",
+  { "nameflexibleVersions": "ErrorCode1+",
   "fields": [
     { "name": "ErrorCode", "type": "int16", "versions": "0+",
       "about": "The top level error code."},
@@ -36,6 +36,13 @@
         { "name": "TopicsLeaderEpoch", "type": "[]TopicDataint32",
      "versions": "0+",
  "fields": [
         "about": "The latest known leader epoch"}
       ]}
+    ]},
+    { "name": "TopicNameNodeEndpoints", "type": "string[]NodeEndpoint", "versions": "01+", "entityTypetaggedVersions": "topicName1+",
 "tag": 0,
+      "about": "The topic name." },
Endpoints for all leaders enumerated in PartitionData", "fields": [
+      { "name": "PartitionsNodeId", "type": "[]PartitionDataint32",
        "versions": "01+",
+ "fields": [
               "mapKey": true, "entityType": "brokerId", "about": "The ID of the associated node" },
+      { "name": "Host", "type": "string", "versions": "1+", "about": "The node's hostname" },
+      { "name": "PartitionIndexPort", "type": "int32", "versions": "01+", "about": "The node's port" }
     ]}
   ]
 }

Fetch

Handling

The leader will track the fetched offset for the replica tuple (ID and UUID). Replicas are uniquely identified by their ID and UUID so their state will be tracked using their ID and UUID.

When removing the leader from the voters set, it will remain the leader for that epoch until the VotersRecord gets committed. This means that the leader needs to allow replicas (voters and observers) to fetch from the leader even if it is not part of the voters set. This also means that if the leader is not part of the voters set it should not include itself when computing the committed offset (also known as the high-watermark) and when checking that the quorum is alive.

Request

Add the replica UUID (directory id) to the Fetch request. This is needed so that the leader can correctly track persisted offsets all of the voters.

git diff upstream/trunk  "about": "The partition index." },
        { "name": "ErrorCode", "type": "int16", "versions": "0+"},
        { "name": "LeaderId", "type": "int32", "versions": "0+", "entityType": "brokerId",
          "about": "The ID of the current leader or -1 if the leader is unknown."},
        { "name": "LeaderEpoch", "type": "int32", "versions": "0+",
          "about": "The latest known leader epoch"},
        { "name": "VoteGranted", "type": "bool", "versions": "0+",
          "about": "True if the vote was granted and false otherwise"},
clients/src/main/resources/common/message/FetchRequest.json
diff --git a/clients/src/main/resources/common/message/FetchRequest.json b/clients/src/main/resources/common/message/FetchRequest.json
index 235357d004..ff86469831 100644
--- a/clients/src/main/resources/common/message/FetchRequest.json
+++ b/clients/src/main/resources/common/message/FetchRequest.json
@@ -55,7 +55,9 @@
   // deprecate the old ReplicaId field and set its default value to -1. (KIP-903)
   //
   // Version 16 is the same as version 15 (KIP-951).
-  "validVersions": "0-16",
+  //
+  // Version 17 adds directory id support from KIP-853
+  "validVersions": "0-17",
   "deprecatedVersions": "0-3",
   "flexibleVersions": "12+",
   "fields": [
@@ -100,7 +102,9 @@
         { "name": "VoterUuidLogStartOffset", "type": "boolint64", "versions": "5+", "default": "-1+"", "ignorable": true,
           "about": "The earliest available offset of the follower replica generated uuid for the replica casting a vote." .  The field is only used when the request is sent by the follower."},
        { "name": "LeaderEndPoint", "type": "Endpoint", { "versionsname": "1+PartitionMaxBytes", "taggedVersionstype": "+1int32", "tagversions": "0+",
-          "about": "The endpoint that can be used to communicate with the leader", "fields": [
 maximum bytes to fetch from this partition.  See KIP-74 for cases where this limit may not be honored." }
+          { "nameabout": "Host", "type": "string", "versions": "1+",
            "about": "The hostnameThe maximum bytes to fetch from this partition.  See KIP-74 for cases where this limit may not be honored." },
+          { "name": "PortReplicaUuid", "type": "uint16uuid", "versions": "117+", "taggedVersions": "17+",
 "tag": 0,
+          "about": "The port.directory id of the follower fetching" }
       ]}
     ]},
     {  ]}
    ]}
  ]
}

Handling

The handling of the Vote request will from that described in KIP-595 in that all replicas are allowed to vote event if they are not voters for the quorum. The voter will persist in the quorum state both the candidate ID and UUID so that it only votes for at most one candidate for a given epoch.

The replica will return the following errors:

1. INVALID_REQUEST - when the voter ID and UUID doesn't match the local ID and UUID.

BeginQuorumEpoch

The version of the response is increase and the fields remain unchanged.

TODO: Talk about the leader sending this request as part of quorum check. This is needed because the voter could come back with a new replica uuid. In that case voter can only discover the leader using Fetch unless the leader sends the begin quorum epoch request.

TODO: The receiver of the RPC always applies the request even if it is not a member of the voter set.

Request

1. LeaderId was moved out of the topic partition maps
2. VoterId was added to the top level
3. VoterUuId was added to PartitionData
4. Allow tagged fields for version greater than or equal to 1.

"name": "ForgottenTopicsData", "type": "[]ForgottenTopic", "versions": "7+", "ignorable": false,

Response

git diff upstream/trunk clients/src/main/resources/common/message/FetchResponse.json
diff --git a/clients/src/main/resources/common/message/FetchResponse.json b/clients/src/main/resources/common/message/FetchResponse.json
index e5f49ba6fd..b432a79719 100644
--- a/clients/src/main/resources/common/message/FetchResponse.json
+++ b/clients/src/main/resources/common/message/FetchResponse.json
@@ -47,7 +47,7 @@
   // Version 15 is the same as version 14 (KIP-903).
   //
   // Version 16 adds the 'NodeEndpoints' field (KIP-951).
-  "validVersions": "0-16",
+  "validVersions": "0-17",
   "flexibleVersions": "12+",
   "fields": [
     

{
  "apiKey": 53,
  "type": "request",
  "listeners": ["controller"],
  "name": "BeginQuorumEpochRequest",
  "validVersions": "0-1",
  "flexibleVersions": "1+",
  "fields": [
    { "name": "ClusterId", "type": "string", "versions": "0+",
      "nullableVersions": "0+", "default": "null"},
    { "name": "LeaderIdThrottleTimeMs", "type": "int32", "versions": "1+", "entityTypeignorable": "brokerId",
      "about": "The ID of the newly elected leader"}, 
    { "name": "VoterId", "type": "int32", "versions": "1+", "entityType": "brokerId",
      "about": "The voter ID of the receiving replica." },
    { "name": "Topics", "type": "[]TopicData", "versions": "0+", "fields": [
      { "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
        "about": "The topic name." },
      { "name": "Partitions", "type": "[]PartitionData", "versions": "0+",true,

FetchSnapshot

Handling

No changes are needed to the handling of this RPC. The only thing to keep in mind is that replicas will accept FetchSnapshot request as long as they are the latest leader even if they are not in the current voters set.

Request

Add the replica UUID (directory id) to the Fetch request. This is not needed for correctness because leaders only track fetched offsets using the Fetch RPC but it is good add for consistency and debugability.

git diff upstream/trunk clients/src/main/resources/common/message/FetchSnapshotRequest.json
diff --git a/clients/src/main/resources/common/message/FetchSnapshotRequest.json b/clients/src/main/resources/common/message/FetchSnapshotRequest.json
index 358ef2e322..9a577d6289 100644
--- a/clients/src/main/resources/common/message/FetchSnapshotRequest.json
+++ b/clients/src/main/resources/common/message/FetchSnapshotRequest.json
@@ -18,11 +18,11 @@
   "type": "request",
   "listeners": ["controller"],
   "name": "FetchSnapshotRequest",
-  "validVersions": "0",
+  "validVersions": "0-1",
   "flexibleVersions": "0+",
   "fields": [
        { "name": "PartitionIndexClusterId", "type": "int32string", "versions": "0+",
          "about": "The partition index." },
        { "name", "nullableVersions": "VoterUuid0+", "typedefault": "uuidnull", "versionstaggedVersions": "10+",
 "tag": 0,
        "about": "The replicacluster UUIDID of the receiving replica.if known" },
        { "name": "LeaderIdReplicaId", "type": "int32", "versions": "0+", "default": "-1", "entityType": "brokerId",
          "about": "The broker ID of the newly elected leader"follower" },
        { "name": "LeaderEpochMaxBytes", "type": "int32", "versions": "0+",
          "aboutdefault": "The epoch of the newly elected leader"}
      ]}0x7fffffff",
@@ -44,7 +44,9 @@
    ]}
  ]
}

Response

Version 1 is a flexible version and add the tagged field LeaderEndpoint to PartitionData.

{
  "apiKey": 53,
  "type": "response",
    { "name": "BeginQuorumEpochResponseEpoch",
  "validVersionstype": "0-1int32",
  "flexibleVersionsversions": "10+" }
         ]},
  "fields": [
       { "name": "ErrorCodePosition", "type": "int16int64", "versions": "0+",
-          "about": "The top level error code."},
byte position within the snapshot to start fetching from" }
+         { "nameabout": "Topics", "type": "[]TopicData",
      "versions": "0+", "fields": [
The byte position within the snapshot to start fetching from" },
+        { "name": "TopicNameReplicaUuid", "type": "stringuuid", "versions": "01+", "entityTypetaggedVersions": "topicName"1+", "tag": 0,
+          "about": "The topic name." },directory id of the follower fetching" }
       ]}
     ]}
 { "name": "Partitions", "type": "[]PartitionData",
        "versions": "0+", "fields]

Response

Version 1 renames LeaderIdAndEpoch to CurrentLeader and adds Endpoint to CurrentLeader.

git diff upstream/trunk clients/src/main/resources/common/message/FetchSnapshotResponse.json
diff --git a/clients/src/main/resources/common/message/FetchSnapshotResponse.json b/clients/src/main/resources/common/message/FetchSnapshotResponse.json
index 887a5e4401..2d9d269930 100644
--- a/clients/src/main/resources/common/message/FetchSnapshotResponse.json
+++ b/clients/src/main/resources/common/message/FetchSnapshotResponse.json
@@ -17,23 +17,23 @@
   "apiKey": 59,
   "type": "response",
   ": [
        { "name": "PartitionIndex", "type": "int32", "versions": "0+",
          "about": "The partition index." },
        { "name": "ErrorCodeFetchSnapshotResponse",
- "type": "int16", "versionsvalidVersions": "0+"},
+        { "name"validVersions": "LeaderId0-1",
 "type": "int32", "versionsflexibleVersions": "0+", "entityType": "brokerId",
          "about": "The ID of the current leader or -1 if the leader is unknown."},
   "fields": [
     { "name": "LeaderEpochThrottleTimeMs", "type": "int32", "versions": "0+",
 "ignorable": true,
        "about": "The latest known leader epoch" 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": "LeaderEndPointErrorCode", "type": "Endpointint16", "versions": "10+", "taggedVersionsignorable": "+1", "tag": 0false,
          "about": "The endpointtop thatlevel canresponse beerror used to communicate with the leader", "fields": [
   code" },
       { "name": "HostTopics", "type": "string[]TopicSnapshot", "versions": "10+",
            "about": "The hostname." },
  topics to fetch", "fields": [
        { "name": "Name", "type": "Portstring", "typeversions": "uint160+", "versionsentityType": "1+topicName",
            "about": "The port." }
        ]}name of the topic to fetch" },
      ]}
 { "name": "Partitions", ]}
  ]
}

Handling

This request will be handle as described in KIP-595 with the following additional errors:

1. INVALID_REQUEST - when the voter ID and UUID doesn't match.
2. NOT_VOTER - when the the replica is not a voter in the topic partition. This error should be retry by the leader of the topic partition.

EndQuorumEpoch

Request

1. LeaderId was moved out of the topic partition maps
2. VoterId was added to the request
3. VoterUuid was added to the Partitions
4. ReplicaUuid was added to PreferredSuccessors
5. Allow tagged fields for versions greater than or equal to 1.

{
  "apiKey": 54,
  "type": "request",
  "listeners": ["controller"],
 "type": "[]PartitionSnapshot", "versions": "0+",
         "about": "The partitions to fetch", "fields": [
         { "name": "Index", "type": "int32", "versions": "0+",
           "about": "The partition index" },
         { "name": "EndQuorumEpochRequestErrorCode",
  "type": "int16", "validVersionsversions": "0-1+",
     "flexibleVersions      "about": "1+",
  "fields": [
The error code, or 0 if there was no fetch error" },
         { "name": "ClusterIdSnapshotId", "type": "stringSnapshotId", "versions": "0+",
           "nullableVersionsabout": "0+The snapshot endOffset and epoch fetched",
 "default": "null"},          "fields": [
@@ -43,17 +43,24 @@
         { "name": "LeaderIdCurrentLeader", "type": "int32LeaderIdAndEpoch",
           "versions": "10+", "entityTypetaggedVersions": "brokerId0+",
     "tag": 0, "aboutfields": "The[
 current leader ID that is resigning." },
    { "name": "VoterIdLeaderId", "type": "int32", "versions": "10+", "entityType": "brokerId",
             "about": "The voter ID of the receivingcurrent replica." }, 
    { "name": "Topics", "type": "[]TopicData", "versions": "0+", "fields": [
leader or -1 if the leader is unknown" },
           { "name": "TopicNameLeaderEpoch", "type": "stringint32", "versions": "0+", "entityType": "topicName",

             "about": "The topic name.latest known leader epoch" },
      { "name": "Partitions", "type": "[]PartitionData", "versions": "0+", "fields": [
},
         { "name": "PartitionIndexSize", "type": "int32int64", "versions": "0+",
           "about": "The partition index.total size of the snapshot" },
         { "name": "VoterUuidPosition", "type": "uuidint64", "versions": "10+",
           "about": "The replica UUID ofstarting byte position within the snapshot included in the receivingBytes replica.field" },
 
        { "name": "LeaderIdUnalignedRecords", "type": "int32records", "versions": "0+",
    "entityType       "about": "brokerId",
          "about": "The current leader ID that is resigning"},
    Snapshot data in records format which may not be aligned on an offset boundary" }
       ]}
+    ]},
+    { "name": "LeaderEpochNodeEndpoints", "type": "int32[]NodeEndpoint", "versions": "01+",
 "taggedVersions": "1+", "tag": 0,
+      "about": "TheEndpoints for all current epoch"},
  -leaders enumerated in PartitionSnapshot", "fields": [
+      { "name": "PreferredSuccessorsNodeId", "type": "[]ReplicaInfoint32", "versions": "01+",
+        "mapKey": true, "entityType": "brokerId", "about": "AThe sorted listID of preferred successors to start the election",associated node"fields": [},
+          { "name": "ReplicaIdHost", "type": "int32string", "versions": "01+", "entityTypeabout": "brokerIdThe node's hostname" },
  +        { "name": "ReplicaUuidPort", "type": "int32", "versions": "uuid1+", "versionsabout": "1+The node's port" }
     ]}
   ]
 } 
      ]}
    ]}
  ]
}

Response

Version 1 is a flexible version and add the tagged field LeaderEndpoint to PartitionData.

DescribeQuorum

Handling

The DescribeQuorum RPC will get forwarded to the KRaft cluster metadata leader. The response for this RPC will be extended to include the UUID of all of the voters and observers, and the endpoint information for all of the voters.

Since the node and listener information is index by the node id, if there are multiple voters with the same replica id only the latest entry will be returned and used.

Request

git diff upstream/trunk clients/src/main/resources/common/message/DescribeQuorumRequest.json
diff --git a/clients/src/main/resources/common/message/DescribeQuorumRequest.json b/clients/src/main/resources/common/message/DescribeQuorumRequest.json
index cee8fe6982..93ab303aaf 100644
--- a/clients/src/main/resources/common/message/DescribeQuorumRequest.json
+++ b/clients/src/main/resources/common/message/DescribeQuorumRequest.json
@@ -19,7 +19,7 @@
   "listeners": ["broker", "controller"],
   "name": "DescribeQuorumRequest",
   // Version 1 adds additional fields in the response. The request is unchanged (KIP-836).
-  "validVersions": "0-1",
+  "validVersions": "0-2",
   "flexibleVersions

{
  "apiKey": 54,
  "type": "response",
  "name": "EndQuorumEpochResponse",
  "validVersions": "0-1",
  "flexibleVersions": "1+",
  "fields": [
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "aboutfields": "The[
 top level error code."},
    { "name": "Topics", "type": "[]TopicData",

Response

git diff upstream/trunk clients/src/main/resources/common/message/DescribeQuorumResponse.json
diff --git a/clients/src/main/resources/common/message/DescribeQuorumResponse.json b/clients/src/main/resources/common/message/DescribeQuorumResponse.json
index 0ea6271238..b54cd6bd50  "versions": "0+", "fields": [
      { "name": "TopicName",100644
--- a/clients/src/main/resources/common/message/DescribeQuorumResponse.json
+++ b/clients/src/main/resources/common/message/DescribeQuorumResponse.json
@@ -18,11 +18,13 @@
   "type": "stringresponse",
 "versions": "0+", "entityTypename": "topicNameDescribeQuorumResponse",
   // Version 1 adds  "about": "The topic name." },
      { "nameLastFetchTimeStamp and LastCaughtUpTimestamp in ReplicaState (KIP-836).
-  "validVersions": "Partitions0-1",
+  "typevalidVersions": "[]PartitionData0-2",
        "versionsflexibleVersions": "0+",
   "fields": [
        { "name": "PartitionIndexErrorCode", "type": "int32int16", "versions": "0+",
          "about": "The partitiontop index." },
        { "name": "ErrorCode", "type": "int16", "versions": "0+"},
    level error code."},
+    { "name": "LeaderIdErrorMessage", "type": "int32string", "versions": "02+", "entityTypenullableVersions": "brokerId2+",
   "ignorable": true,
+       "about": "The IDerror of the current leader message, or -1null if thethere leaderwas isno unknownerror." },
        { "name": "LeaderEpochTopics", "type": "int32[]TopicData",
       "versions": "0+",
          "aboutfields": "The latest known leader epoch"},
[
           { "name": "LeaderEndPointTopicName", "type": "Endpointstring", "versions": "10+", "taggedVersionsentityType": "+1topicName", "tag": 0,
          "about": "The endpoint that can be used to communicate with the leader", "fields": [
@@ -32,6 +34,8 @@
          { "name": "HostPartitionIndex", "type": "stringint32", "versions": "10+",
            "about": "The hostnamepartition index." },
          { "name": "PortErrorCode", "type": "uint16int16", "versions": "10+"},
+          {  "about": "The port." }
        ]}
      ]}
    ]}
  ]
}

Handling

This request will be handle as described in KIP-595 with the following additional errors:

1. INVALID_REQUEST - when the voter ID and UUID doesn't match.
2. NOT_VOTER - when the the replica is not a voter in the topic partition. This error could be retry by the leader of the topic partition.

DescribeQuorum

The version of the request is increase and the fields remain unchanged.

Response

1. Add ReplicaUuid to ReplicaState

{
  "apiKey": 55,
  "type": "response",
  "name": "DescribeQuorumResponse",
  "validVersions": "0-2",
  "flexibleVersions": "0+",
  "fields": [
    name": "ErrorMessage", "type": "string", "versions": "2+", "nullableVersions": "2+", "ignorable": true,
+          "about": "The error message, or null if there was no error." },
         { "name": "LeaderId", "type": "int32", "versions": "0+", "entityType": "brokerId",
           "about": "The ID of the current leader or -1 if the leader is unknown."},
         { "name": "ErrorCodeLeaderEpoch", "type": "int16int32", "versions": "0+",
      "about": "The top level error code."},
@@ -40,10 +44,25 @@
         { "name": "TopicsCurrentVoters", "type": "[]TopicDataReplicaState",
      "versions": "0+" },
 "fields": [
        { "name": "TopicNameObservers", "type": "string[]ReplicaState", "versions": "0+", "entityType": "topicName", }
        "about": "The topic name." },
  ]}
-    ]}],
+    ]},
+    { "name": "PartitionsNodes", "type": "[]PartitionDataNode",
        "versions": "02+", "fields": [
  +      { "name": "PartitionIndexNodeId", "type": "int32", "versions": "02+",
+        "mapKey": true, "entityType": "brokerId", "about": "The partition index.ID of the associated node" },
+        { "name": "ErrorCodeListeners", "type": "int16"[]Listener",
+        "about": "The listeners of this controller", "versions": "0+"}, "fields": [
+        { "name": "LeaderIdName", "type": "int32string", "versions": "0+", "entityTypemapKey": "brokerId"true,
+          "about": "The IDname of the current leader or -1 if the leader is unknown."endpoint" },
+        { "name": "LeaderEpochHost", "type": "int32string", "versions": "0+",
+          "about": "The latest known leader epoch"hostname" },
+        { "name": "HighWatermarkPort", "type": "int64uint16", "versions": "0+"},
+        { "name": "CurrentVoters", "typeabout": "[]ReplicaState", "versions": "0+" },
        { "name": "Observers", "type": "[]ReplicaState", "versions": "0+" }
The port" }
+       ]}
+    ]}
+  ],
   "commonStructs": [
     { "name": "ReplicaState", "versions": "0+", "fields": [
       { "name": "ReplicaId", "type": "int32", "versions": "0+", "entityType": "brokerId" },
+      { "name": "ReplicaUuid", "type": "uuid", "versions": "2+" },
       { "name": "LogEndOffset", "type": "int64", "versions": "0+",
         "about": "The last known log end offset of the follower or -1 if it is unknown"},
       { "name": "LastFetchTimestamp", "type": "int64", "versions": "1+", "ignorable": true, "default": -1,
        true, "aboutdefault": "The last known leader wall clock time time when a follower fetched from the leader. This is reported as -1 both for the current leader or if it is unknown for a voter" },
      { "name": "LastCaughtUpTimestamp", "type": "int64", "versions": "1+", "ignorable": true, "default": -1,
        "about": "The leader wall clock append time of the offset for which the follower made the most recent fetch request. This is reported as the current time for the leader and -1 if unknown for a voter" }
    ]}
  ]
}

Handling

The handling of the request is the same as that described in KIP-595 with just the additional fields. Clients handling the response can assume that if a ReplicaState include both the ID and the UUID that replica can become a voter if is enumerated in the Observers field.

Admin Client

The Java Admin client will be extended to support the new field in the DescribeQuorum response and the new AddVoter and RemoveVoter RPCs.

Monitoring

...

1 if there is a pending add voter operation, 0 otherwise.

...

1 if a controller node is not a voter for the KRaft cluster metadata partition, 0 otherwise.

...

Counts the number of duplicate replica id in the set of voters.

...

Number of voters with a last Fetch timestamp greater than the Fetch timeout.

...

1 if controller.quorum.voter is set and the kraft.version is greater than 0, 0 otherwise.

-1,

Admin client

The Java Admin client will be extended to support the new field in the DescribeQuorum response and the new AddVoter and RemoveVoter RPCs. When the broker bootstrap servers is used the admin client will send the request to the least loaded broker. When the controller bootstrap servers is used the admin client will send the request to the KRaft leader (active controller).

Monitoring

Command line interface

kafka-metadata-shell

A future KIP will describe how the kafka-metadata-shell tool will be extended to be able to read and display KRaft control records from the quorum, snapshot and log.

kafka-storage

The format command will get extended as follow.

--standalone

This command will 1) create a meta.properties file in metadata.log.dir with a randomly generated directory.id, 2) create a snapshot at 00000000000000000000-0000000000.checkpoint with the necessary control records (KRaftVersionRecord and VotersRecord) to make this Kafka node the only voter for the quorum.

This option is unsafe because it doesn’t use the quorum to establish the new quorum. This only should only be executed once on one of the voters. The rest of the voters should join the quorum by using the kafka-metadata-quorum add-controller command.

--controller-quorum-voters

The value to this option will have the follow schema <replica-id>[-<replica-uuid>]@<host>:<port>. Logically, this function is very similar to the controller.quorum.voters. The important difference is that it will optionally support the user directly specifying the replica's directory id.

When the format command is executed with this option it will read the node.id configured in the properties file specified by the --config option and compare it against the <replica-id> specified in --controller.quorum.voters. If there is a match, it will write the <replica-uuid> specified to the directory.id property in the meta.properties for the metadata.log.dir directory.

Similar to --standalone this command will create a snapshot at 00000000000000000000-0000000000.checkpoint with the necessary control records (KRaftVersionRecord and VotersRecord).

This option is unsafe. It is important that the operator uses the same value across all of the voters specified and only executes this command once per voter.

kafka-features

The upgrade and downgrade command will support a new configuration flag. A downgrade that results in the decrease of the kraft.version will be rejected by the KRaft leader.

--release-software

The value specified in this flag will be used to find the corresponding metadata.version and kraft.version. The --metadata version flag will get deprecated and will be a synonym for --release-software.

kafka-metadata-quorum

This tool as

Command Line Interface

kafka-metadata-shell.sh

A future KIP will describe how the kafka-metadata-shell tool will be extended to be able to read and display KRaft control records from the quorum,  snapshot and log.

kafka-storage.sh

A future KIP describe how the kafka-storage tool will be extended to be able to bootstrap the first quorum node by writing an AddVoterRecord to the cluster metadata log when the controller.quorum.bootstrap.servers configuration is used.

kafka-metadata-quorum.sh

The kafka-metadata-quorum tool described in KIP-595 and KIP-836 will be improved to support these additional commands and options:

describe --status

TODO: This command should be extended to print all of the voter endpoints that the leader knows about. It should also display if there are any uncommitted add or remove voter changes.

This command will print both the ReplicaId and ReplicaUuid for CurrentVoters. A new row called CouldBeVoters will be added which print the Replica ID and UUID of any replica that could be added to the voter set. E.g.

the new information added to the DescribeQuorum RPC. The includes the directory id for all of the replicas (voters and observers). The known endpoints for all of the voters. Any uncommitted voter changes.

> bin/kafka-metadata-quorum.sh describe --describestatus
ClusterId:                SomeClusterId
LeaderId:                 0
LeaderEpoch:              15
HighWatermark:            234130
MaxFollowerLag:           34
MaxFollowerLagTimeMs:     15
CurrentVoters:            [{"id": 0, "uuid": "UUID1", "endpoints": ["host:port"]}, {"id": 1, "uuid": "UUID2", "endpoints": ["host:port"]}, {"id": 2, "uuid": "UUID2", "endpoints": ["host:port"]}]
Observers:                [{"id": 3, "uuid": "UUID3"}]
UncommittedAddedVoter:    {"id": 2, "uuid": "UUID2", "endpoints": ["host:port"]}
UncommittedRemovedVoter:  {"id": 2, "uuid": "UUID2", "endpoints": ["host:port"]}

describe --replication

This command will print on additional column for the replica uuid after the replica id. E.g.

> bin/kafka-metadata-quorum.sh --describe replication
ReplicaId   ReplicaUuid   LogEndOffset   ...
0           uuid1         234134         ...
...

add-controller --config <server.properties>

This command is use to add controllers to the KRaft cluster metadata partition.

...

This command must be executed using the server configuration of the new controller. The command will read the server properties file to read the replica id, the endpoints, and the meta.properties for the directory id. The tool will set the TimoutMs for the AddVoterRequest to 30 seconds.

remove-controller --controller-id <controller-id> --controller-uuid <controller-uuid>

...

This command is used to add new remove voters to from the topic KRaft cluster metadata partition. The flags --replicacontroller-id and --replicacontroller-uuid must be specified. A future KIP will describe how the user can specify endpoint information for the replica.

remove-controller

This command is used to remove voters from the topic partition. The flags --replica-id and --replica-uuid must be specified.

Compatibility, Deprecation, and Migration Plan

The features in this KIP will be supported if the ApiVersions of all of the voters and observers is greater than the versions described here. If the leader has a replica UUID for all of the voters then this KIP is supported by all of the voters.

Upgrading to controller. quorum.bootstrap.servers

TODO: figure out a way to do this. The requirement is the AddVoterRecord for all of the voters in the static configuration are committed. How do we guarantee this? Is seeing a committed AddVoterRecord enough?

...

.

Compatibility, deprecation, and migration plan

RPC versions will be negotiated using the ApiVersions RPC. KRaft will use the kraft.version to determine which version of KRaftVersionRecord and VotersRecord to write to the log and which version of QuorumStateData to write to the quorum-state file.

Downgrading the kraft.version from 1 to 0 is not possible. This is mainly due to the fact that kraft.version 1 writes data to the log and snapshot that gets replicated to all of the replicas.

When to remove controller.quorum.voters

It is safe for the operator to remove the configuration for controller.quorum.voters when the kraft.version has been upgrade to version 1. All of the Kafka nodes will expose the ignored-static-voter metrics. If all of the Kafka nodes expose a 1 for this metrics, it is safe to remove controller.quorum.voters from the node configuration.

Test plan

This KIP will be tested using unittest, integration tests, system test, simulation tests and TLA+ specification.

Rejected

...

alternatives

KIP-642: Dynamic quorum reassignment

KIP-642: Dynamic quorum reassignment. KIP-642 describe how to perform dynamic reassignment will multiple voters added and removed. KIP-642 doesn't support disk failures and it would be more difficult to implement compared to this KIP-853. In a future KIP we can describe how we can add administrative operations that support the addition and removal of multiple voters.

Leverage metadata controller mechanisms

There are some common problems between the Kafka Controller and KRaft like bootstrapping cluster information, versioning and propagating node information. The Kafka controller has solved both of these problems in the metadata layer. The metadata layer is an application built using the KRaft layer. KRaft and KIP-853 should not dependent on the metadata layer for its solution to these problems. One advantage is to allow other use cases for KRaft that are not the cluster metadata partition but the main advantage is achieving a clear separation for both reliability, testability and debuggability.

In the future we can extend Kafka so that the Kafka controller relies on KRaft's solution to these problems.

References

1. Ongaro, Diego, and John Ousterhout. "In search of an understandable consensus algorithm." 2014 {USENIX} Annual Technical Conference ({USENIX}{ATC} 14). 2014.

2. Ongaro, Diego. Consensus: Bridging theory and practice. Diss. Stanford University, 2014.

3. Bug in single-server membership changes
4. KIP-595: A Raft Protocol for the Metadata Quorum
5. KIP-630: Kafka Raft Snapshot
6. KIP-631: The Quorum-based Kafka Controller