THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
Status
Current state: Under Discussion
Discussion thread: https://lists.apache.org/thread/zb5l1fsqw9vj25zkmtnrk6xm7q3dkm1v
JIRA:
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
Motivation
KIP-595 introduced KRaft topic partitions. These are partitions 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) 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 (Controllers). If the operator of a KRaft cluster wanted to make changes to the set of voters, they would have 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 can programmatically update the voter set in a way that is safe 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.
Key Terms
These are the definition of some important terms that are used through the document.
Voters: A voter is any replica that can transition to the candidate state and to the leader state. Voters are required to have an ID and UUID. 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 set. A candidate needs to get votes from the majority of its own voter set before is can become the leader of an epoch. When a voter becomes a leader it will use its voter set to determine when an offset has been committed.
Observers: An observer is any replica that is not in the voter set. This is because they have an ID and UUID which is not in the voter set or they don't have an ID or UUID.
Proposed Changes
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.
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 as part of the quorum state for the KRaft topic partition. A replica (controller and broker) may host multiple topic partitions in multiple log directories. The replica UUID will be generated per topic partition hosted by the replica.
There are two cases when a replica UUID will be generate:
- The topic partition is a new topic partition and doesn't contain a quorum state file persisted on-disk.
- The topic partition has a version 0 of the quorum state file. Version 0 of the quorum state file doesn't contain a replica UUID field. The quorum state file will be upgraded to version 1 with the replica UUID persisted.
Replicas in a KRaft topic partition that are voter or can become voters will be identified by their ID and UUID.
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 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 or the new property called controller.quorum.bootstrap.servers. These two configurations are mutually exclusive the KRaft cluster is expected to use one or the other but not both.
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 .
When this option is used the leader of the KRaft topic partition will not allow the AddVoter RPCs to add replica IDs that are not describe in the configuration and it would not allow the RemoveVoter RPCs to completely remove replica IDs that are described in the configuration. This means that the replica UUID can change by using sequence of AddVoter and RemoveVoter but the client cannot add new replica id or remove replica id from the set of voters.
When the leader of the topic partition first discovers the UUID of all the voters through the Fetch and FetchSnapshot RPCs it will write a AddVoterRecord to the log for each of the voters described in the configuration.
The KRaft leader will not perform writes from the state machine (active controller) or client until is has written to the log an AddVoterRecord for every replica id in the controller.quorum.voters configuration.
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 the quorum will be started with only one voter. This voter is bootstrapped by running the kafka-storage.sh tool. 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
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 set. This check will be removed and replicas will reply to votes request when the candidate is not in the voter set or the voting replica is not in the voter set. The candidate must still have a longer log offset and epoch before the voter will grant a vote to it.
High Watermark
As describe 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-watermakr 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 remove replicas.
With this KIP it is possible for the leader to not be part of the voter 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.
Snapshots
The snapshot generation code needs to be extended to include these new KRaft specific control records for AddVoterRecord and RemoveVoterRecord. Before this KIP the snapshot didn't include any KRaft generated control records.
Internal Listener
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 uncommitted data from log and snapshot to discover the voter set. This also means that the KRaft implementation needs to handle this uncommitted state getting truncated and reverted.
Common Scenarios
To better illustrate this feature this section describes two common scenarios that the Kafka administrator may perform.
Disk Failure Recovery
If one of the replicas encounter a disk failure the operation can replace this disk with a new disk and start the replica.
Let's assume that the cluster is configured to use controller.quorum.voters and the value is 1@host1:9092,2@host2:9092,3@host3:9094. The voter set is defined by the tuples (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 is automatically generated once by the replica when persisting the quorum state for the first time.
At this point the disk for replica 3 is replaced. This means that when replica 3 starts it will generate a new replica uuid (UUID3') and quorum state. Replica 3 will discover the partition leader using controller.quorum.voters and send UUID3' in the Fetch and FetchSnapshot request. The leader state for voters will be (1, UUID1), (2, UUID2) and (3, UUID3) and for observers will be (3, UUID3').
This state can be discovered by a client by using the DescribeQuorum RPC, the Admin client or the kafka-metadata-quorum CLI. The client can now decide to add replica (3, UUID3') to the set of voters using the AddVoter RPC, the Admin client or the kafka-metadataa-quorum CLI.
When the AddVoter RPC succeeds the voters will be (1, UUID1), (2, UUID2), (3, UUID3) and (3, UUID3'). The client can remove the failed disk from the voter set by using the RemoveVoter RPC, 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.
Node Failure Recovery
To support this scenario the Kafka cluster must be configured to use controller.quorum.bootstrap.servers. Let's assume that the voter set is (1, UUID1), (2, UUID2) and (3, UUID3).
At this replica 3 has failed and the Kafka operation would like to replica it. The operator would start a new controller with replica id 4. This replica will generate a UUID4 and persisted to the quorum state. Replica 4 will discover the leader by sending Fetch and FetchSnapshot request to the servers enumerated in controller.quorum.bootstrap.servers. Until it discovers the leader is able to fetch from the leader. The leader state for voters will be (1, UUID1), (2, UUID2), (3, UUID3) and for observers will be (4, UUID4).
The client 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 client can now decided to remove replica (3, UUID3) from the set of voters.
Public Interfaces
Configuration
These two configurations are mutually exclusive the KRaft cluster is expected to use one or the other but not both.
controller.quorum.voters
This is an existing configuration. If the cluster uses this configuration to configure the quorum, adding new replica ids will not be supported. The cluster will only support changing the UUID for an existing replica id.
controller.quorum.bootstrap.servers
This configuration can be used instead of controller.quorum.voters. 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.
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.
{
"type": "data",
"name": "LeaderChangeMessage",
"validVersions": "0-1",
"flexibleVersions": "0+",
"fields": [
{ "name": "Version", "type": "int16", "versions": "0+",
"about": "The version of the leader change message" },
{ "name": "LeaderId", "type": "int32", "versions": "0+", "entityType": "brokerId",
"about": "The ID of the newly elected leader" },
{ "name": "Voters", "type": "[]Voter", "versions": "0+",
"about": "The set of voters in the quorum for this epoch" },
{ "name": "GrantingVoters", "type": "[]Voter", "versions": "0+",
"about": "The voters who voted for the leader at the time of election" }
],
"commonStructs": [
{ "name": "Voter", "versions": "0+", "fields": [
{ "name": "VoterId", "type": "int32", "versions": "0+" },
{ "name": "VoterUuid", "type": "uuid", "versions": "1+" }
]}
]
}
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.
{
"type": "data",
"name": "AddVoterRecord",
"validVersions": "0",
"flexibleVersions": "0+",
"fields": [
{ "name": "Version", "type": "int16", "versions": "0+",
"about": "The version of the add voter record"},
{ "name": "VoterId", "type": "int32", "versions": "0+", "entityType": "brokerId",
"about": "The ID of the voter getting added to the topic partition"},
{ "name": "VoterUuid", "type": "uuid", "versions": "0+",
"about": "The replica generated UUID of the replica getting added as a voter to the topic partition"},
{ "name": "EndPoints", "type": "[]VoterEndpoint", "versions": "0+",
"about": "The endpoints that can be used to communicate with the voter", "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": "SecurityProtocol", "type": "int16", "versions": "0+",
"about": "The security protocol." }
]}
]
}
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 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 new voter to 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.
{
"type": "data",
"name": "RemoveVoterRecord",
"validVersions": "0",
"flexibleVersions": "0+",
"fields": [
{ "name": "Version", "type": "int16", "versions": "0+",
"about": "The version of the add voter record"},
{ "name": "VoterId", "type": "int32", "versions": "0+", "entityType": "brokerId",
"about": "The ID of the voter getting removed from the topic partition"},
{ "name": "VoterUuid", "type": "uuid", "versions": "0+",
"about": "The replica generated UUID of the replica getting removed as a voter 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 qurorum-state.json file in the directory for the topic partition.
A new field called ReplicaUuid will get added to QuorumStateData . Each KRaft replica will store a locally generated UUID in this field. The replica will generate and persist an UUID when it reads a version 0 of the QuorumStateData or if the QuorumStateData hasn't been persistent in the past.
Remove the CurrentVoters field from QuorumStateData . This information will instead be persisted in the log and snapshot using the AddVoterRecord and RemoveVoterRecord.
{
"type": "data",
"name": "QuorumStateData",
"validVersions": "0-1",
"flexibleVersions": "0+",
"fields": [
{ "name": "ClusterId", "type": "string", "versions": "0+"},
{ "name": "ReplicaUuid", "type" "uuid", "versions": "1+",
"about": "The locally generated UUID for this replica.",
{ "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+", "nullableVersions": "1+", "default": "null" },
{ "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"}
]}
]
}
RPCs
AddVoter
Request
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. The broker will forward the request to the controller.
{
"apiKey": "TBD",
"type": "request",
"listeners": ["controller", "broker"],
"name": "AddVoterRequest",
"validVersions": "0",
"flexibleVersions": "0+",
"fields": [
{ "name": "ClusterId", "type": "string", "versions": "0+" }
{ "name": "Topics", "type": "[]TopicData", "versions": "0+", "fields": [
{ "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
"about": "The name of the topic." },
{ "name": "Partitions", "type": "[]PartitionData", "versions": "0+", "fields": [
{ "name": "Index", "type": "int32", "versions": "0+",
"about": "The partition index." },
{ "name": "VoterId", "type": "int32", "versions": "0+",
"about": "The ID of the voter getting added to the topic partition." },
{ "name": "VoterUuid", "type": "uuid", "versions": "0+",
"about": "The replica generated UUID of the replica getting added as a voter to the topic partition." },
{ "name": "EndPoints", "type": "[]VoterEndpoint", "versions": "0+",
"about": "The endpoints that can be used to communicate with the voter", "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": "SecurityProtocol", "type": "int16", "versions": "0+",
"about": "The security protocol." }
]}
]}
]}
]
}
Response
{
"apiKey": "TBD",
"type": "response",
"name": "AddVoterResponse",
"validVersions": "0",
"flexibleVersions": "0+",
"fields": [
{ "name": "ErrorCode", "type": "int16", "versions": "0+",
"about": "The top level error code." }
{ "name": "Topics", "type": "[]TopicData", "versions": "0+", "fields": [
{ "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
"about": "The name of the topic." },
{ "name": "Partitions", "type": "[]PartitionData", "versions": "0+", "fields": [
{ "name": "Index", "type": "int32", "versions": "0+",
"about": "The partition index." },
{ "name": "ErrorCode", "type": "int16", "versions": "0+",
"about": "The error code, or 0 if there was no fetch error." },
{ "name": "CurrentLeader", "type": "LeaderIdAndEpoch", "versions": "0+", "taggedVersions": "0+", "tag": 0, "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"}
]}
]}
]}
]
}
Handling
When the leader receives a AddVoter request it will do the following:
- Wait for the fetch offset of the replica (ID, UUID) to catch up to the log end offset of the leader.
- Wait for until there are no uncommitted add or remove voter records.
- Append the AddVoterRecord to the log.
- The KRaft internal listener will read this record from the log and add the voter to the voter set.
- Wait for the AddVoterRecord to commit using the majority of new configuration.
- 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 added voter is 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 fail this RPC until it discovers that it is in the voter set. The leader will continue to retry until the RPC succeeds.
The replica will return the following errors:
- NOT_LEADER_FOR_PARTITION - when the request is sent to a replica that is not the leader.
- VOTER_ALREADY_ADDED - when the request contains a replic ID and UUID that is already in the committed voter set.
- INVALID_REQUEST - when the request contains a replica ID and UUID that is not an observer.
- INVALID_REQUEST - when the request contains endpoints but the quorum was configured to use controller.quorum.voters.
- INVALID_REQUEST - when the request contains a replica ID that is not enumerated in the controller.quorum.voters configuration.
The broker will handle this request by forwarding the request to the active controller.
RemoveVoter
Request
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.
{
"apiKey": "TBD",
"type": "request",
"listeners": ["controller", "broker"],
"name": "RemoveVoterRequest",
"validVersions": "0",
"flexibleVersions": "0+",
"fields": [
{ "name": "Topics", "type": "[]TopicData", "versions": "0+", "fields": [
{ "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
"about": "The name of the topic." },
{ "name": "Partitions", "type": "[]PartitionData", "versions": "0+", "fields": [
{ "name": "Index", "type": "int32", "versions": "0+",
"about": "The partition index." },
{ "name": "VoterId", "type": "int32", "versions": "0+",
"about": "The ID of the voter getting removed from the topic partition." },
{ "name": "VoterUuid", "type": "uuid", "versions": "0+",
"about": "The replica generated UUID of the replica getting removed as a voter from the topic partition." },
]}
]}
]
}
Response
{
"apiKey": "TBD",
"type": "response",
"name": "RemoveVoterResponse",
"validVersions": "0",
"flexibleVersions": "0+",
"fields": [
{ "name": "ErrorCode", "type": "int16", "versions": "0+",
"about": "The top level error code." }
{ "name": "Topics", "type": "[]TopicData", "versions": "0+", "fields": [
{ "name": "TopicName", "type": "string", "versions": "0+", "entityType": "topicName",
"about": "The name of the topic." },
{ "name": "Partitions", "type": "[]PartitionData", "versions": "0+", "fields": [
{ "name": "Index", "type": "int32", "versions": "0+",
"about": "The partition index." },
{ "name": "ErrorCode", "type": "int16", "versions": "0+",
"about": "The error code, or 0 if there was no fetch error." },
{ "name": "CurrentLeader", "type": "LeaderIdAndEpoch", "versions": "0+", "taggedVersions": "0+", "tag": 0, "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"}
]}
]}
]}
]
}
Handling
When the leader receives a RemoveVoter request it will do the following:
- Wait for until there are no uncommitted add or remove voter records.
- Append the RemoveVoterRecord to the log.
- The KRaft internal listener will read this record from the log and remove the voter from the voter set.
- Wait for the RemoveVoterRecord to commit using the majority of new configuration.
- Send the RemoveVoter response to the client.
- 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:
- NOT_LEADER_FOR_PARTITION - when the request is sent to a replica that is not the leader.
- VOTER_ALREADY_REMOVED - when the request contains a replic ID and UUID that is already not in the committed voter set.
- INVALID_REQUEST - when the request contains a replica ID and UUID that is not a voter.
- INVALID_REQUEST - when the controller.quorum.voters is used and the request contains a replica ID that would cause the replica ID to get removed from the voter set. In other words RemoveVoter can be used to remove replica UUIDs not replica IDs when the controller.quorum.voters configuration is used.
The broker will handle this request by forwarding the request to the active controller.
Fetch
The Fetch response version is bump and the fields remain unchanged.
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.
{
"apiKey": 1,
"type": "request",
"listeners": ["zkBroker", "broker", "controller"],
"name": "FetchRequest",
"validVersions": "0-14",
"flexibleVersions": "12+",
"fields": [
{ "name": "ClusterId", "type": "string", "versions": "12+", "nullableVersions": "12+", "default": "null", "taggedVersions": "12+", "tag": 0, "ignorable": true,
"about": "The clusterId if known. This is used to validate metadata fetches prior to broker registration." },
{ "name": "ReplicaId", "type": "int32", "versions": "0+", "entityType": "brokerId",
"about": "The replica ID of the follower, of -1 if this request is from a consumer." },
...
{ "name": "Topics", "type": "[]FetchTopic", "versions": "0+",
"about": "The topics to fetch.", "fields": [
{ "name": "Topic", "type": "string", "versions": "0-12", "entityType": "topicName", "ignorable": true,
"about": "The name of the topic to fetch." },
{ "name": "TopicId", "type": "uuid", "versions": "13+", "ignorable": true,
"about": "The unique topic ID"},
{ "name": "Partitions", "type": "[]FetchPartition", "versions": "0+",
"about": "The partitions to fetch.", "fields": [
{ "name": "Partition", "type": "int32", "versions": "0+",
"about": "The partition index." },
{ "name": "ReplicaUuid", "type": "uuid", "versions": "14+", "nullableVersions": "14+", "default": "null",
"about": "The replica generated UUID. null otherwise." },
...
]}
]}
]
}
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
The FetchSnapshot response version is bump and the fields remain unchanged.
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,
"type": "request",
"listeners": ["controller"],
"name": "FetchSnapshotRequest",
"validVersions": "0-1",
"flexibleVersions": "0+",
"fields": [
{ "name": "ClusterId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null", "taggedVersions": "0+", "tag": 0,
"about": "The clusterId if known, this is used to validate metadata fetches prior to broker
registration" },
{ "name": "ReplicaId", "type": "int32", "versions": "0+", "default": "-1", "entityType": "brokerId",
"about": "The replica ID of the follower" },
{ "name": "MaxBytes", "type": "int32", "versions": "0+", "default": "0x7fffffff",
"about": "The maximum bytes to fetch from all of the snapshots" },
{ "name": "Topics", "type": "[]TopicSnapshot", "versions": "0+",
"about": "The topics to fetch", "fields": [
{ "name": "Name", "type": "string", "versions": "0+", "entityType": "topicName",
"about": "The name of the topic to fetch" },
{ "name": "Partitions", "type": "[]PartitionSnapshot", "versions": "0+",
"about": "The partitions to fetch", "fields": [
{ "name": "Partition", "type": "int32", "versions": "0+",
"about": "The partition index" },
{ "name": "ReplicaUuid", "type": "uuid", "versions": "1+", "default": "null",
"about": "The replica UUID of the follower" },
{ "name": "CurrentLeaderEpoch", "type": "int32", "versions": "0+",
"about": "The current leader epoch of the partition, -1 for unknown leader epoch" },
{ "name": "SnapshotId", "type": "SnapshotId", "versions": "0+",
"about": "The snapshot endOffset and epoch to fetch", "fields": [
{ "name": "EndOffset", "type": "int64", "versions": "0+" },
{ "name": "Epoch", "type": "int32", "versions": "0+" }
]},
{ "name": "Position", "type": "int64", "versions": "0+",
"about": "The byte position within the snapshot to start fetching from" }
]}
]}
]
}
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
The Vote response version is bump and the fields remain unchanged.
Request
Changes:
- Candidate Id was moved out of the topic partition maps
- Candidate Uuid was added to the PartitionData
- VoterId was added to the top level
- VoterUuId was added to PartitionData
{
"apiKey": 52,
"type": "request",
"listeners": ["controller"],
"name": "VoteRequest",
"validVersions": "0-1",
"flexibleVersions": "0+",
"fields": [
{ "name": "ClusterId", "type": "string", "versions": "0+", "nullableVersions": "0+", "default": "null" },
{ "name": "CandidateId", "type": "int32", "versions": "1+", "entityType": "brokerId",
"about": "The ID of the voter sending the request" },
{ "name": "VoterId", "type": "int32", "versions": "1+", "entityType": "brokerId",
"about": "The ID of the replica receiving the request to vote." },
{ "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+", "fields": [
{ "name": "PartitionIndex", "type": "int32", "versions": "0+",
"about": "The partition index." },
{ "name": "CandidateEpoch", "type": "int32", "versions": "0+",
"about": "The bumped epoch of the candidate sending the request"},
{ "name": "CandidateId", "type": "int32", "versions": "0", "entityType": "brokerId",
"about": "The ID of the voter sending the request"},
{ "name": "CandidateUuid", "type": "uuid", "versions": "1+",
"about": "The candidate generated UUID, null otherwise" },
{ "name": "VoterUuid", "type": "uuid", "versions": "1+", "nullableVersions": "1+", "default": "null" }
"about": "The replica generated UUID of the replica receiving the request to vote, null otherwise" },
{ "name": "LastOffsetEpoch", "type": "int32", "versions": "0+",
"about": "The epoch of the last record written to the metadata log"},
{ "name": "LastOffset", "type": "int64", "versions": "0+",
"about": "The offset of the last record written to the metadata log"}
]}
]}
]
}
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:
- 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.
Request
- LeaderId was moved out of the topic partition maps
- VoterId was added to the top level
- VoterUuId was added to PartitionData
- Allow tagged fields for version greater than or equal to 1.
{
"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": "LeaderId", "type": "int32", "versions": "1+", "entityType": "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+", "fields": [
{ "name": "PartitionIndex", "type": "int32", "versions": "0+",
"about": "The partition index." },
{ "name": "VoterUuid", "type": "uuid", "versions": "1+",
"about": "The replica UUID of the receiving replica." },
{ "name": "LeaderId", "type": "int32", "versions": "0", "entityType": "brokerId",
"about": "The ID of the newly elected leader"},
{ "name": "LeaderEpoch", "type": "int32", "versions": "0+",
"about": "The epoch of the newly elected leader"}
]}
]}
]
}
Handling
This request will be handle as described in KIP-595 with the following additional errors:
- INVALID_REQUEST - when the voter ID and UUID doesn't match.
- 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
The version of the response is increase and the fields remain unchanged.
Request
- LeaderId was moved out of the topic partition maps
- VoterId was added to the request
- VoterUuid was added to the Partitions
- ReplicaUuid was added to PreferredSuccessors
- Allow tagged fields for versions greater than or equal to 1.
{
"apiKey": 54,
"type": "request",
"listeners": ["controller"],
"name": "EndQuorumEpochRequest",
"validVersions": "0-1",
"flexibleVersions": "1+",
"fields": [
{ "name": "ClusterId", "type": "string", "versions": "0+",
"nullableVersions": "0+", "default": "null"},
{ "name": "LeaderId", "type": "int32", "versions": "1+", "entityType": "brokerId",
"about": "The current leader ID that is resigning." },
{ "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+", "fields": [
{ "name": "PartitionIndex", "type": "int32", "versions": "0+",
"about": "The partition index." },
{ "name": "VoterUuid", "type": "uuid", "versions": "1+",
"about": "The replica UUID of the receiving replica." },
{ "name": "LeaderId", "type": "int32", "versions": "0", "entityType": "brokerId",
"about": "The current leader ID that is resigning"},
{ "name": "LeaderEpoch", "type": "int32", "versions": "0+",
"about": "The current epoch"},
{ "name": "PreferredSuccessors", "type": "[]ReplicaInfo", "versions": "0+",
"about": "A sorted list of preferred successors to start the election", "fields": [
{ "name": "ReplicaId", "type": "int32", "versions": "0+", "entityType": "brokerId" },
{ "name": "ReplicaUuid", "type": "uuid", "versions": "1+" }
]}
]}
]}
]
}
Handling
This request will be handle as described in KIP-595 with the following additional errors:
- INVALID_REQUEST - when the voter ID and UUID doesn't match.
- 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
- Add ReplicaUuid to ReplicaState
{
"apiKey": 55,
"type": "response",
"name": "DescribeQuorumResponse",
"validVersions": "0-2",
"flexibleVersions": "0+",
"fields": [
{ "name": "ErrorCode", "type": "int16", "versions": "0+",
"about": "The top level error code."},
{ "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+", "fields": [
{ "name": "PartitionIndex", "type": "int32", "versions": "0+",
"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": "HighWatermark", "type": "int64", "versions": "0+"},
{ "name": "CurrentVoters", "type": "[]ReplicaState", "versions": "0+" },
{ "name": "Observers", "type": "[]ReplicaState", "versions": "0+" }
]}
]}
],
"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,
"about": "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
| NAME | TAGS | TYPE | NOTE |
|---|---|---|---|
| number-of-voters | type=raft-metrics | gauge | number of voters for the cluster metadata topic partition. |
| number-of-possible-voters | type=raft-metrics | guage | number of observer that could be promoted to voters. |
| pending-add-voter | type=raft-metrics | guage | 1 if there is a pending add voter operation, 0 otherwise. |
| pending-remove-voter | type=raft-metrics | guage | 1 if there is a pending remove voter operation, 0 otherwise. |
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:
--describe
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.
> bin/kafka-metadata-quorum.sh --describe
ClusterId: SomeClusterId
LeaderId: 0
LeaderEpoch: 15
HighWatermark: 234130
MaxFollowerLag: 34
MaxFollowerLagTimeMs: 15
CurrentVoters: [{"id": 0, "uuid": "UUID1"}, {"id": 1, "uuid": "UUID2"}, {"id": 2, "uuid": "UUID2"}]
CouldBeVoters: [{"id": 3, "uuid": "UUID3"}]
--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-voter
This command is used to add new voters to the topic partition. The flags --replicaId and --replicaUuid must be specified. A future KIP will describe how the user can specify endpoint information for the replica.
--remove-voter
This command is used to remove voters from the topic partition. The flags --replicaId and --replicaUuid 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.
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 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.
References
- Ongaro, Diego, and John Ousterhout. "In search of an understandable consensus algorithm." 2014 {USENIX} Annual Technical Conference ({USENIX}{ATC} 14). 2014.
Ongaro, Diego. Consensus: Bridging theory and practice. Diss. Stanford University, 2014.
- KIP-595: A Raft Protocol for the Metadata Quorum
- KIP-630: Kafka Raft Snapshot
- KIP-631: The Quorum-based Kafka Controller