...

Currently, any broker node can be elected as the controller.  As part of this KIP, the active controller will instead be selected among a small potentially smaller pool of nodes specifically configured to act as controllers.  Typically three or five nodes in the cluster will be selected to be controllers.

...

Note that as long as at least one of the provided controller addresses is valid, the broker will be able to learn about the current metadata quorum and start up.  Once the broker is in contact with the metadata quorum, the quorum bootstrap addresses will not be needed.  This makes it possible to reconfigure the metadata quorum over time.  For example, if we start with a metadata quorum of host1, host2, host3, we could replace host3 with host4 without disrupting any of the brokers.  Then we could roll the brokers to apply the new metadata quorum bootstrap configuration of host1, host2, host4 on each one.

Node IDs

Just like brokers, controller nodes will have non-negative integer node IDs.  There will be a single ID space.  In other words, no controller should share the same ID as a broker.  Even when a broker and a controller are co-located in the same JVM, they must have different node IDs.

We define a node here as a tuple consisting of a node ID and a process role. Roles are defined by the `process.roles` configuration. As explained above, in a co-located configuration, a single process may take both the "controller" and "broker" roles. The node ID for both of these roles will be defined by the `node.id` configuration. However, this is mainly for configuration convenience. Semantically, we view the co-located process as representing two distinct nodes. Each node has its own listeners and its own set of APIs which it exposes. The APIs exposed by a controller node will not be the same as those exposed by a broker node.

Automatic node ID assignment via ZooKeeper will no longer be supported in KIP-500 mode.  Node IDs must be set in the configuration fileAutomatic node ID assignment via ZooKeeper will no longer be supported in KIP-500 mode.  Node IDs must be set in the configuration file for brokers and controllers.

Networking

Controller processes will listen on a separate endpoint from brokers.  This will be true even when the broker and controller are co-located in the same JVM.

...

The only time when clients should contact a controller node directly is when they are debugging system issues.  This is similar to ZooKeeper, where we have things like zk-shell, but only for debugging.

Note that controllers do not appear in the MetadataResponses given to clients.

Metadata

The Metadata Topic

As described in KIP-500, the controller will store its data in the internal @metadata topic__cluster_metadata topic.  This topic will contain a single partition which is managed by Raft, as described in KIP-595: A Raft Protocol for the Metadata Quorum.

...

Metadata changes need to be persisted to the @metadata __cluster_metadata log before we apply them on the other nodes in the cluster.  This means waiting for the metadata log's last stable offset to advance to the offset of the change.  After that point, we are guaranteed not to lose the change as long as we uphold the Raft invariants.

...

However, this "future state" may never be committed.  For example, the active controller might fail, truncating some of its future state.  Therefore, the active controller must not make this future state "visible" to the rest of the cluster until it has been made persistent – that is, until it becomes current state.  In the case of the @metadata topic__cluster_metadata topic, the replication protocol itself neatly takes care of this for us.  In the case of controller RPCs like AlterIsr, the controller handles this by not sending back a response until the designated change has been persisted.

...

The active controller makes changes to the metadata by appending records to the log. Each record has a null key, and this format for its value:

1. an unsigned varint specifying the frame type (currently 0)
2. an unsigned varint specifying the record type.
3. an unsigned varint specifying the record version
4. the payload in Kafka RPC format

For example, if we wanted to encode a TopicRecord, we might have 1 0 encoded as a varint, 1 encoded as a varint, followed by 0 as the record version, followed by the serialized topic data.

The frame type, record type, and version will typically only take one byte each, for a total header size overhead of two three bytes.

Record Format Versions

...

In the post-KIP-500 world, controller shutdown is handled by the broker heartbeat system instead.  In its periodic heartbeats, the broker asks the controller if it can transition into the controlled shutdown state.  It does this by setting the ShouldShutdown WantShutDown boolean.  This motivates the controller to move all of the leaders off of that broker.  Once they are all moved, the controller responds to the heartbeat with ControlledShutdownOk ShouldShutDown = true.  At that point, the broker knows it's safe to begin the shutdown process proper.

...

The main change in the broker state machine is that the RECOVERING_FROM_UNCLEAN_SHUTDOWN state has been renamed to RECOVERY.  Also, unlike previously, the broker will always pass through RECOVERY (although it may only stay in this state for a very short amount of time).

Public Interfaces

meta.properties

When a storage directory is in use by a cluster running in kip-500 mode, it will have a new version of the meta.properties file.  Since the current version is 0, the new version will be 1.   Just as in version 0, meta.properties will continue to be a Java Properties file.  This essentially means that it is a plain text file where each line has the format key=value.

In version 0 of meta.properties, the cluster.id field is optional.  In contrast, in version 1 it is mandatory.

In version 0 of meta.properties, the cluster.id field is serialized in hexadecimal.  In contrast, in version 1 it is serialized in base64.

Version 1 of meta.properties adds the controller.id field.  Nodes which are acting as controllers will have this field.  If the node is also a broker, it will have the broker.id field as well.

During broker startup, if each directory does not have a broker.id, one will be added with the current broker id.  Similarly, during controller startup, if each directory does not have a controller id, one will be added with the current controller id.

If the broker id and/or controller id is present but does not match the configured id, we will throw an exception and abort the startup process.

Here is an example of a version 1 meta.properties file:

$ cat /tmp/kafka-logs/meta.properties
#
#Tue Dec 01 10:08:08 PST 2020
cluster.id=3Db5QLSqSZieL3rJBUUegA
version=1
broker.id=0

kafka-storage.sh

There will be a new command-line tool, kafka-storage.sh.

Partition Reassignment

Just like the ZooKeeper-based controller, the quorum controller will implement the KIP-455 partition reassignment API. This API specifies both alter and list operations.

The alter operation specifies a partition, plus a target replica list. If the target replica list is a subset of the partition's current replica list, and the new replica list contains at least one member of the current ISR, then the reassignment can be completed immediately. In this case, the controller issues a PartitionChangeRecord changing the replica set and ISR (if appropriate).

On the other hand, if the new replica list has additional replicas, or specifies a subset of replicas that doesn't intersect with the current ISR, then we add the additional replicas to the replica list, and wait for the ISR to catch up.

So, for example, if the old replica list was [1, 2, 3], and the new list was [3, 4, 5], we would proceed as follows:

• change the replica list to [1, 2, 3, 4, 5], and set addingReplicas to [4, 5]
• Wait for an alterIsr which adds both broker 4 and 5
• Change the replica list to [3, 4, 5 ] and prune the ISR accordingly. Remove addingReplicas.

Just like with the ZooKeeper-based controller, the new controller stores reassignments as "hard state." Reassignments will continue even after a controller failover or broker shutdown and restart. Reassignment state appears in controller snapshots, as addingReplicas and removingReplicas.

Unlike the old controller, the quorum controller does not prevent topics that are undergoing reassignment from being deleted. If a topic is deleted, then any partition reassignments that it had are terminated.

Typically, reassignments which require ISR changes are completed by a leader adding some new replicas, via the alterIsr RPC. (Note that another possible way for a reassignment to be completed is via a broker being added to a partition ISR during unfencing). 

When a broker makes an alterIsr change that completes a reassignment, the resulting ISR will be different than requested. For example, if the leader is 1, and the current ISR is 1, 2, 3, and the target replica set is 1, 2, 4, when the leader tries to change the ISR to 1, 2, 3, 4, the controller will change it to 1, 2, 4 instead, completing the reassignment. Since the alterIsr response returns the actual ISR which was actually applied, the leader will apply this new ISR instead.

If the new ISR would does not contain the leader which made the alterIsr call, the controller returns FENCED_LEADER_EPOCH. This will notify the broker that it should wait until it can replay the latest partition change from the log. That partition change record will make this broker a follower rather than a leader.

Public Interfaces

meta.properties

When a storage directory is in use by a cluster running in kip-500 mode, it will have a new version of the meta.properties file.  Since the current version is 0, the new version will be 1.   Just as in version 0, meta.properties will continue to be a Java Properties file.  This essentially means that it is a plain text file where each line has the format key=value.

In version 0 of meta.properties, the cluster.id field is optional.  In contrast, in version 1 it is mandatory.

In version 0 of meta.properties, the cluster.id field is serialized in hexadecimal.  In contrast, in version 1 it is serialized in base64.

Version 1 of meta.properties replaces the broker.id field with node.id.

For servers running in kip-500 mode, the `meta.properties` file must be present in every log directory. The process will raise an error during startup if if either the meta.properties file does not exist or if the node.id found does not match what the value from the configuration file.

Here is an example of a version 1 meta.properties file:

$ cat /tmp/kafka-logs/meta.properties
#
#Tue Dec 01 10:08:08 PST 2020
cluster.id=3Db5QLSqSZieL3rJBUUegA
version=1
node.id=0

kafka-storage.sh

There will be a new command-line tool, kafka-storage.sh.

$ ./bin/kafka-storage.sh -h
usage: kafka-storage [-h] {info,format,random-uuid} ...

The Kafka storage tool.

positional arguments:
  {info,format,random-uuid}
    info                 Get information about the Kafka log directories on this node.
    format               Format the Kafka log directories on this node.
    random-uuid          Print a random UUID.

optional arguments:
  -h, --help             show this help message and exit

kafka-storage.sh will have three subcommands: info, format, and random-uuid.

info

$ ./bin/kafka-storage.sh info -h
usage: kafka-storage info [-h] --config CONFIG

optional arguments:
  -h, --help             show this help message and exit
  --config CONFIG, -c CONFIG
                         The Kafka configuration file to use.

The info command will give information about the configured storage directories.  Example output:

Found log directory:
  /tmp/kafka-logs

Found metadata: MetaProperties(clusterId=51380268-1036-410d-a8fc-fb3b55f48033)

format

$ ./bin/kafka-storage.sh format -h
usage: kafka-storage format [-h] --config CONFIG --cluster-id CLUSTER_ID [--ignore-formatted]

optional arguments:
  -h, --help             show this help message and exit
  --config CONFIG, -c CONFIG
                         The Kafka configuration file to use.
  --cluster-id CLUSTER_ID, -t CLUSTER_ID
                         The cluster ID to use.
  --ignore-formatted, -g

When running kip-500 mode, the storage directories must be formatted using this command prior to starting up the brokers and controllers.

If any of the storage directories are formatted, the command will normally fail.  This behavior can be changed by passing the --ignore-formatted option.  When this option is passed, the format command will skip over already formatted directories rather than failing.

random-uuid

$ ./bin/kafka-storage.sh random-uuid -h
usage: kafka-storage random-uuid [-h]

optional arguments:
  -h, --help             show this help message and exit

The random-uuid command prints out a random UUID to stdout.

$ ./bin/kafka-storage.sh random-uuid
8XUwXa9qSyi9tSOquGtauQ

kafka-cluster.sh

There will also be new command-line tool named kafka-cluster.sh.

$ ./bin/kafka-cluster.sh -h
usage: kafka-cluster [-h] {id,unregister} ...

The Kafka cluster tool.

positional arguments:
  {cluster-id,unregister}
    cluster-id           Get information about the ID of a cluster.
    unregister           Unregister a broker ID.

optional arguments:
  -h, --help             show this help message and exit

kafka-storage.sh will have two subcommands: id and unregister.

cluster-id

$ ./bin/kafka-cluster.sh cluster-id -h
usage: kafka-cluster cluster-id [-b,-c,-h]

optional arguments:
  -b, --bootstrap-server a list of host/port pairs to use for establishing the connection to the kafka cluster.
  -c, --config           a property file containing configs to be passed to Admin Client.
  -h, --help             show this help message and exit

The ID command prints out the cluster id

unregister

$ ./bin/kafka-cluster.sh unregister -h
usage: kafka-cluster unregister [-h] [--bootstrap-server BOOTSTRAP_SERVER] [--config CONFIG] [--id ID]

optional arguments:
  -h, --help             show this help message and exit
  --bootstrap-server BOOTSTRAP_SERVER, -b BOOTSTRAP_SERVER
                         A list of host/port pairs to use for establishing the connection to the kafka cluster.
  --config CONFIG, -c CONFIG
                         A property file containing configs to passed to AdminClient.
  --id ID, -i ID         The ID of the broker to unregister.

The decommission command removes the registration of a specific broker ID.  It will use make an UnregisterBrokerRequest in order to do this.

Changes to kafka-dump-log-segments.sh

kafka-dump-log-seguments.sh will have two new flags: --cluster-metadata-decoder, and --skip-record-metadata.

The --cluster-metadata-decoder flag will tell the DumpLogSegments tool to decode the records as KIP-500 metadata.  Each record will be output in the following JSON format:

payload: {"type":<record type enum name>, "version":<record version number>, "data":<record JSON>}

Example output:

 payload: {"type":"TOPIC_RECORD","version":0,"data":{"name":"bar","topicId":"GU_rXds2FGppL1JqXYpx2g"}}
 payload: {"type":"PARTITION_RECORD","version":0,"data":{"partitionId":0,"topicId":"GU_rXds2FGppL1JqXYpx2g","replicas":[1],"isr":[1],"removingReplicas":null,"addingReplicas":null,"leader":1,"leaderEpoch":0,"partitionEpoch":0}}
 payload: {"type":"PARTITION_CHANGE_RECORD","version":0,"data":{"partitionId":0,"topicId":"GU_rXds2FGppL1JqXYpx2g","leader":-1}}
 payload: {"type":"PARTITION_CHANGE_RECORD","version":0,"data":{"partitionId":0,"topicId":"WCnrza5uWKeerYa7HCNpOg","leader":-1}}
 payload: {"type":"PARTITION_CHANGE_RECORD","version":0,"data":{"partitionId":0,"topicId":"GU_rXds2FGppL1JqXYpx2g","leader":-1}}
 payload: {"type":"PARTITION_CHANGE_RECORD","version":0,"data":{"partitionId":0,"topicId":"WCnrza5uWKeerYa7HCNpOg","leader":-1}}
 payload: {"type":"FENCE_BROKER_RECORD","version":0,"data":{"id":1,"epoch":0}}

The --skip-record-metadata flag will skip printing metadata for each record.  However, metadata for each record batch will still be printed when this flag is present.

kafka-metadata-shell.sh

The Kafka Metadata shell is a new command which allows users to interactively examine the metadata stored in a KIP-500 cluster.

It can read the metadata from the controllers directly, by connecting to them, or from a metadata snapshot on disk.  In the former case, the quorum voters must be specified by passing the --controllers flag; in the latter case, the snapshot file should be specified via --snapshot.

$ ./bin/kafka-metadata-shell.sh -h
usage: metadata-tool [-h] [--controllers CONTROLLERS] [--config CONFIG] [--snapshot SNAPSHOT] [command [command ...]]

The Apache Kafka metadata tool

positional arguments:
  command                The command to run.

optional arguments:
  -h, --help             show this help message and exit
  --controllers CONTROLLERS, -C CONTROLLERS
                         The quorum voter connection string to use.
  --config CONFIG, -c CONFIG
           

$ ./bin/kafka-storage.sh -h
usage: kafka-storage [-h] {info,format,random-uuid} ...

The Kafka storage tool.

positional arguments:
  {info,format,random-uuid}
    info                 Get information about the Kafka log directories on this node.
    format               Format the Kafka log directories on this node.
The configuration file to use.
  --snapshot SNAPSHOT, -s SNAPSHOT
             random-uuid          Print a random UUID.

optional arguments:
  -h, --help             show this help message and exit

kafka-storage.sh will have three subcommands: info, format, and random-uuid.

info

The snapshot file to read.

The metadata tool works by replaying the log and storing the state into in-memory nodes.  These nodes are presented in a fashion similar to filesystem directories.  For browsing the nodes, several commands are supported:

>> help
Welcome to the Apache Kafka metadata shell.

usage:  {cat,cd,exit,find,help,history,ls,man,pwd} ...

positional arguments:
  {cat,cd,exit,find,help,history,ls,man,pwd}
    cat                  Show the contents of metadata nodes.
    cd           

$ ./bin/kafka-storage.sh info -h
usage: kafka-storage info [-h] --config CONFIG

optional arguments:
  -h, --help             show this help message and exit
  --config CONFIG, -c CONFIG
        Set the current working directory.
    exit         The Kafka configuration file to use.

The info command will give information about the configured storage directories.  Example output:

Found log directory:
  /tmp/kafka-logs

Found metadata: MetaProperties(clusterId=51380268-1036-410d-a8fc-fb3b55f48033)

format

$ ./bin/kafka-storage.sh format -h
usage: kafka-storage format [-h] --config CONFIG --cluster-id CLUSTER_ID [--ignore-formatted]

optional arguments:
  -h, --helpExit the metadata shell.
    find                 Search for nodes in the directory hierarchy.
  show this help message and exit
  --config CONFIG, -c CONFIG
                     Display this help message.
    history              Print command history.
 The Kafka configuration filels to use.
  --cluster-id CLUSTER_ID, -t CLUSTER_ID
            List metadata nodes.
    man       The cluster ID to use.
  --ignore-formatted, -g

When running kip-500 mode, the storage directories must be formatted using this command prior to starting up the brokers and controllers.

If any of the storage directories are formatted, the command will normally fail.  This behavior can be changed by passing the --ignore-formatted option.  When this option is passed, the format command will skip over already formatted directories rather than failing.

random-uuid

$ ./bin/kafka-storage.sh random-uuid -h
usage: kafka-storage random-uuid [-h]

optional arguments:
  -h, --help       Show the help text for a specific command.
    pwd               show this help messagePrint and exit

The random-uuid command prints out a random UUID to stdout.

$ ./bin/kafka-storage.sh random-uuid
8XUwXa9qSyi9tSOquGtauQ

kafka-cluster.sh

There will also be new command-line tool named kafka-cluster.sh.

$ ./bin/kafka-cluster.sh -h
usage: kafka-cluster [-h] {id,decommission} ...

The Kafka cluster tool.

positional arguments:
  {cluster-id,decommission}
    cluster-id           Get information about the ID of a cluster.
    decommission         Decommission a broker ID.

optional arguments:
  -h, --help             show this help message and exit

kafka-storage.sh will have two subcommands: id and decommission.

cluster-id

$ ./bin/kafka-cluster.sh cluster-id -h
usage: kafka-cluster cluster-id [-b,-c,-h]

optional arguments:
  -b, --bootstrap-server a list of host/port pairs to use for establishing the connection to the kafka cluster.
  -c, --config           a property file containing configs to be passed to Admin Client.
  -h, --help             show this help message and exit

The ID command prints out the cluster id

decommission

$ ./bin/kafka-cluster.sh decommission -h
usage: kafka-cluster decommission [-h] [--bootstrap-server BOOTSTRAP_SERVER] [--config CONFIG] [--id ID]

optional arguments:
  -h, --help             show this help message and exit
  --bootstrap-server BOOTSTRAP_SERVER, -b BOOTSTRAP_SERVER
                         A list of host/port pairs to use for establishing the connection to the kafka cluster.
  --config CONFIG, -c CONFIG
                         A property file containing configs to passed to AdminClient.
  --id ID, -i ID         The ID of the broker to decommission.

The decommission command removes the registration of a specific broker ID.  It will use make a DecommissionBrokerRequest in order to do this.

Configurations

...

null

broker

controller

broker,controller

...

If this is null (absent) then we are in legacy mode.

Otherwise, we are in KIP-500 mode and this configuration determines what roles this process should play: broker, controller, or both.

...

If non-null, this must be a comma-separated list of listener names.

When communicating with the controller quorum, the broker will always use the first listener in this list.

...

A comma-separated list of the names of the listeners used by the KIP-500 controller. This configuration is required if this process is a KIP-500 controller. The legacy controller will not use this configuration

Despite the similar name, note that this is different from the "control plane listener" introduced by KIP-291.

...

A comma-separated list of the configured listeners.  For example,

INTERNAL://192.1.1.8:9092, EXTERNAL://10.1.1.5:9093, CONTROLLER://192.1.1.8:9094

...

If non-null, this must be a comma-separated list of all the controller voters, in the format:

{controller-id}@{controller-host):{controller-port}

...

When in KIP-500 mode, each node must have this configuration, in order to find out how to communicate with the controller quorum.

Note that this replaces the "quorum.voters" config described in KIP-595.

This configuration is required for both brokers and controllers.

...

a 32-bit ID

...

The controller id for this server.  Only required if this server is a controller.

...

controller.quorum.election.timeout.ms

...

the current working directory.

The interface of the metadata tool is currently considered unstable and may change when KIP-500 becomes production-ready.

Configurations

Deprecated Configurations

New Error Codes

DUPLICATE_BROKER_REGISTRATION

There will be a new error code, DUPLICATE_BROKER_REGISTRATION, that the active controller will return when a broker tries to register with an ID that is currently in use.

INVALID_CLUSTER_ID

There will be a new error code, INVALID_CLUSTER_ID, that the controller will return if the broker tries to register with the wrong cluster ID.

AdminClient

There will be a new AdminClient RPC, unregisterBroker.

UnregisterBrokerResult unregisterBroker(int brokerId, UnregisterBrokerOptions options)

public class UnregisterBrokerResult {
    public KafkaFuture<Void> all();
}

public class UnregisterBrokerOptions extends AbstractOptions<UnregisterBrokerOptions> {
}

RPCs

Obsoleting the Metadata Propagation RPCs

As discussed earlier, the new controller will use FetchRequest to fetch metadata from the active controller.  The details of how Raft fetching will work are spelled out in KIP-595: A Raft Protocol for the Metadata Quorum.

Since we propagate the metadata via Raft, we will no longer need to send out LeaderAndIsrRequest, UpdateMetadataRequest, and StopReplicaRequest.  These requests will be sent out only when we're in legacy mode, not when we're in KIP-500 mode.  Eventually we will add some support for these requests to the new controller, in order to support rolling upgrade from a pre-KIP-500 release. However, for the purpose of this KIP, the new controller will not use these requests.

Obsoleting the Controlled Shutdown RPC

The broker heartbeat mechanism replaces the controlled shutdown RPC.  Therefore, we will not need to support the this RPC any more in the controller-- except for compatibility during upgrades, which will be described further in a follow-on KIP.

Topic Identifiers

This KIP builds on top of the work done in KIP-516 to improve how partitions are tracked.

Because each topic is identified by a unique topic UUID, we can implement topic deletion with a single record, RemoveTopicRecord.  Upon replaying this record, each broker will delete the associated topic if it is present.

Of course, some brokers may be down when the topic is deleted.  In fact, some brokers may never see the RemoveTopicRecord.  This record may get collapsed into one of the periodic metadata snapshots.  If this happens, the record will be reflected in the snapshot through the absence of a broker record, not its presence.  Therefore, during the startup process, brokers must compare the log directories that they have with the ones contained in the latest metadata.  The appropriate time to do this is at the start of the RECOVERY phase.  At this point, the broker has the latest metadata.

BrokerRegistration

Required ACLs: CLUSTERACTION on CLUSTER

{
  "apiKey": 57,
  "type": "request",
  "name": "BrokerRegistrationRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "BrokerId", "type": "int32", "versions": "0+",
      "about": "The broker ID." },
	{ "name": "ClusterId", "type": "string", "versions": "0+",
	  "about": "The cluster id of the broker process." },
	{ "name": "IncarnationId", "type": "uuid", "versions": "0+",
	  "about": "The incarnation id of the broker process." },
    { "name": "CurrentMetadataOffset", "type": "int64", "versions": "0+",
      "about": "The highest metadata offset which the broker has reached." },
    { "name": "Listeners", "type": "[]Listener",
      "about": "The listeners of this broker", "versions": "0+", "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." }
      ]
    { "name": "Features", "type": "[]Feature",
      "about": "The features on this broker

Deprecated Configurations

...

New Error Codes

DUPLICATE_BROKER_REGISTRATION

There will be a new error code, DUPLICATE_BROKER_REGISTRATION, that the active controller will return when a broker tries to register with an ID that is currently in use.

INVALID_CLUSTER_ID

There will be a new error code, INVALID_CLUSTER_ID, that the controller will return if the broker tries to register with the wrong cluster ID.

AdminClient

There will be a new AdminClient RPC, decommissionBroker.

DecommissionBrokerResult decommissionBroker(int brokerId, DecommissionBrokerOptions options)

public class DecommissionBrokerResult {
    public KafkaFuture<Void> all();
}

public class DecommissionBrokerOptions extends AbstractOptions<DecommissionBrokerOptions> {
}

RPCs

Obsoleting the Metadata Propagation RPCs

As discussed earlier, the new controller will use FetchRequest to fetch metadata from the active controller.  The details of how Raft fetching will work are spelled out in KIP-595: A Raft Protocol for the Metadata Quorum.

Since we propagate the metadata via Raft, we will no longer need to send out LeaderAndIsrRequest, UpdateMetadataRequest, and StopReplicaRequest.  These requests will be sent out only when we're in legacy mode, not when we're in KIP-500 mode.  Eventually we will add some support for these requests to the new controller, in order to support rolling upgrade from a pre-KIP-500 release. However, for the purpose of this KIP, the new controller will not use these requests.

Obsoleting the Controlled Shutdown RPC

The broker heartbeat mechanism replaces the controlled shutdown RPC.  Therefore, we will not need to support the this RPC any more in the controller-- except for compatibility during upgrades, which will be described further in a follow-on KIP.

Topic Identifiers

This KIP builds on top of the work done in KIP-516 to improve how partitions are tracked.

Because each topic is identified by a unique topic UUID, we can implement topic deletion with a single record, RemoveTopicRecord.  Upon replaying this record, each broker will delete the associated topic if it is present.

Of course, some brokers may be down when the topic is deleted.  In fact, some brokers may never see the RemoveTopicRecord.  This record may get collapsed into one of the periodic metadata snapshots.  If this happens, the record will be reflected in the snapshot through the absence of a broker record, not its presence.  Therefore, during the startup process, brokers must compare the log directories that they have with the ones contained in the latest metadata.  The appropriate time to do this is at the start of the RECOVERY phase.  At this point, the broker has the latest metadata.

BrokerRegistration

Required ACLs: CLUSTERACTION on CLUSTER

{
  "apiKey": 57,
  "type": "request",
  "name": "BrokerRegistrationRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "BrokerId", "type": "int32", "versions": "0+",
 "fields": [
    "about": "The broker ID." },
	{ "name": "ClusterIdName", "type": "uuidstring", "versions": "0+", "mapKey": true,
	          "about": "The cluster id of the broker process." },
	 feature name." }
        { "name": "IncarnationIdMinSupportedVersion", "type": "uuidint16", "versions": "0+",
	          "about": "The incarnationminimum idsupported of the broker processfeature level." },
        { "name": "CurrentMetadataOffsetMaxSupportedVersion", "type": "int64int16", "versions": "0+",
,
          "about": "The highest metadata offset which the broker has reached." maximum supported feature level." }
      ]
    },
    { "name": "ListenersRack", "type": "[]Listenerstring",
      "aboutversions": "The listeners of this broker0+", "versionsnullableVersions": "0+",
      "fieldsabout": "The [
rack which this broker is in." }
  ]
}

{
  "nameapiKey": "Name"57,
  "type": "stringresponse",
  "versionsname": "0+BrokerRegistrationResponse",
  "mapKeyvalidVersions": true"0",
          "aboutflexibleVersions": "The name of the endpoint." }0+",
    "fields": [
    { "name": "HostThrottleTimeMs", "type": "stringint32", "versions": "0+",
          "about": "TheDuration hostname." },
        { "name": "Port", "type": "int16", "versions": "0+",
          "about": "The portin milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
        { "name": "SecurityProtocolErrorCode", "type": "int16", "versions": "0+",
          "about": "The security protocol error code, or 0 if there was no error." },
    {  ]
    { "name"name": "BrokerEpoch", "type": "int64", "versions": "Features0+", "typedefault": "[]Feature-1",
      "about": "The features on this broker", "versions": "0+", "fields": [
        { "name": "Name", 's assigned epoch, or -1 if none was assigned." }
  ]
}

BrokerHeartbeat

Required ACLs: CLUSTERACTION on CLUSTER

As described earlier, the broker periodically sends out a heartbeat request to the active controller.

{
  "apiKey": 58,
  "type": "stringrequest",
  "versionsname": "0+BrokerHeartbeatRequest",
  "mapKeyvalidVersions": true"0",
          "about": "The feature name." }
    "flexibleVersions": "0+",
  "fields": [
    { "name": "MinSupportedVersionBrokerId", "type": "int16int32", "versions": "0+",
          "about": "The minimum supported feature levelbroker ID." },
        { "name": "MaxSupportedVersionBrokerEpoch", "type": "int16int64", "versions": "0+",
 "default": "-1",
        "about": "The maximum supported feature level." }
      ]
    broker epoch." },
    { "name": "RackCurrentMetadataOffset", "type": "stringint64", "versions": "0+", "nullableVersions": "0+",
      "about": "The rackOne more than the highest metadata offset which thisthe broker ishas inreached." },
  ]
}

{
  "apiKey": 57,
 { "typename": "responseWantFence",
  "nametype": "BrokerRegistrationResponsebool",
  "validVersionsversions": "0+",
      "flexibleVersionsabout": "0+",
  "fields": [True if the broker wants to be fenced, false otherwise." }
    { "name": "ThrottleTimeMsWantShutDown", "type": "int32bool", "versions": "0+",
      "about": "DurationTrue in milliseconds for which if the request was throttled duebroker wants to ainitiate quota violation, or zero if the request did not violate any quotacontrolled shutdown." },
  ]
}   

{
  "nameapiKey": "ErrorCode"58,
  "type": "int16response",
  "versionsname": "0+BrokerHeartbeatResponse",
      "aboutvalidVersions": "0"The error code, or 0 if there was no error." },,
  "flexibleVersions": "0+",
  "fields": [
    { "name": "BrokerEpochThrottleTimeMs", "type": "int64int32", "versions": "0+",
      "defaultabout": "-1",
      "about": "The broker's assigned epochDuration in milliseconds for which the request was throttled due to a quota violation, or -1zero if none was assignedthe request did not violate any quota." },
  ]
}

BrokerHeartbeat

Required ACLs: CLUSTERACTION on CLUSTER

As described earlier, the broker periodically sends out a heartbeat request to the active controller.

{
  "apiKey": 58,
  "type": "request",
   { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The error code, or 0 if there was no error." },
    { "name": "BrokerHeartbeatRequestIsCaughtUp",
  "validVersionstype": "0bool",
  "flexibleVersionsversions": "0+",
      "fieldsabout": [ "True if the broker has approximately caught up with the latest metadata." },
    { "name": "BrokerIdIsFenced", "type": "int32bool", "versions": "0+",
      "about": "TheTrue if the broker IDis fenced." },
    { "name": "BrokerEpochShouldShutDown", "type": "int64bool", "versions": "0+",
      "defaultabout": "-1",
      "about": "The broker epoch." },
  True if the broker should proceed with its shutdown." }
  ]
}

The controller will wait to unfence a broker until it sends a heartbeat where ShouldFence is false and CurrentMetadataOffset is caught up.

If the heartbeat request has ShouldShutDown set, the controller will try to move all the leaders off of the broker.

The controller will set ControlledShutdownOk if the broker is cleared to execute a controlled shutdown.  In other words, if it has no leaderships.

The controller will return NOT_CONTROLLER if it is not active.  Brokers will always return NOT_CONTROLLER for these RPCs.

UnregisterBroker

Required ACLs: ALTER on CLUSTER

The UnregisterBrokerRequest asks the controller to unregister a broker from the cluster.

{
  "apiKey": 59,
    { "name": "CurrentMetadataOffset", "type": "int64request", "versions": "0+",
      "aboutname": "One more than the highest metadata offset which the broker has reached." },
    { "nameUnregisterBrokerRequest",
  "validVersions": "ShouldFence0",
 "type": "bool", "versions"flexibleVersions": "0+",
      "aboutfields": "True if the broker wants to be fenced, false otherwise." }[
    { "name": "ShouldShutDownBrokerId", "type": "boolint32", "versions": "0+",
      "about": "True ifThe the broker wantsID to initiate controlled shutdownunregister." }
  ]
}   

{
  "apiKey": 5859,
  "type": "response",
  "name": "BrokerHeartbeatResponseUnregisterBrokerResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "0+",
      "about": "Duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The error code, or 0 if there was no error." },
  ]
}

The valid response codes are:

• NONE if the unregistration succeeded or if the broker was already unregistered.
• NOT_CONTROLLER if the node that the request was sent to is not the controller
• UNSUPPORTED_VERSION if KIP-500 mode is not enabled

Record Formats

RegisterBrokerRecord

{
  "apiKey": 0,
   { "name": "IsCaughtUp", "type": "boolmetadata", "versions": "0+",
      "about": "True if the broker has approximately caught up with the latest metadata." }"name": "RegisterBrokerRecord"
,
  "validVersions": "0",
    "fields": [
	{ "name": "IsFencedBrokerId", "type": "boolint32", "versions": "0+",
	      "about": "TrueThe if the broker is fencedid." },
    	{ "name": "ControlledShutdownOkIncarnationId", "type": "booluuid", "versions": "0+",
    	  "about": "True if the broker can execute a controlled shutdown now." }
  ]
}

The controller will wait to unfence a broker until it sends a heartbeat where ShouldFence is false and CurrentMetadataOffset is caught up.

If the heartbeat request has ShouldShutDown set, the controller will try to move all the leaders off of the broker.

The controller will set ControlledShutdownOk if the broker is cleared to execute a controlled shutdown.  In other words, if it has no leaderships.

The controller will return NOT_CONTROLLER if it is not active.  Brokers will always return NOT_CONTROLLER for these RPCs.

DecommissionBroker

Required ACLs: ALTER on CLUSTER

The DecomissionBrokerRequest asks the controller to unregister a broker from the cluster.

{
  "apiKey": 59,
  "type": "request",
  "name": "DecommissionBrokerRequest",
  "validVersions": "0",
  "flexibleVersions"The incarnation id of the broker process." },
	{ "name": "BrokerEpoch", "type": "int64", "versions": "0+",
	  "about": "The broker epoch assigned by the controller." },
	{ "name": "EndPoints", "type": "[]BrokerEndpoint", "versions": "0+", "nullableVersions": "0+",
	  "about": "The endpoints that can be used to communicate with this broker.", "fields": [
		{ "name": "Name", "type": "string", "versions": "0+", "mapKey": true,
		  "fieldsabout": [
    "The name of the endpoint." },
		{ "name": "BrokerIdHost", "type": "int32string", "versions": "0+",
		      "about": "The broker ID to decommissionhostname." }
  ]
}   

{
  "apiKey,
		{ "name": 59,
  "type": "response",
  "name"Port", "type": "DecommissionBrokerResponseuint16",
  "validVersionsversions": "0+",
		  "flexibleVersionsabout": "0+",
  "fields": [
    The port." },
		{ "name": "ThrottleTimeMsSecurityProtocol", "type": "int32int16", "versions": "0+",
      "about": "Duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." }
    		  "about": "The security protocol." }
	]},
	{ "name": "ErrorCodeFeatures", "type": "int16[]BrokerFeature", "versions": "0+",
    "nullableVersions": "0+",
	  "about": "The errorfeatures code,that orthis 0 if there was no errorbroker supports." },
 "fields": ]
}

The valid response codes are:

• NONE if the decommissioning succeeded or if the broker was already decommissioned
• NOT_CONTROLLER if the node that the request was sent to is not the controller
• UNSUPPORTED_VERSION if KIP-500 mode is not enabled

Record Formats

RegisterBrokerRecord

{
  "apiKey": 0,
  [
		{ "name": "Name", "type": "metadatastring",
  "nameversions": "RegisterBrokerRecord0+"
,
  "validVersionsmapKey": "0"true,
		  "fieldsabout": [
"The name of the feature." },
		{ "name": "IdMinVersion", "type": "int32int16", "versions": "0+",
		  "about": "The minimum feature level that this broker idsupports." },
		{ "name": "IncarnationIdMaxVersion", "type": "uuidint16", "versions": "0+",
		  "about": "The incarnationmaximum feature idlevel ofthat thethis broker processsupports." }
	]},
	{ "name": "BrokerEpochRack", "type": "int64string", "versions": "0+", "nullableVersions": "0+",
	  "about": "The broker epoch assigned by the controllerrack." },
	{ "name": "EndPoints", "type": ]
}

UnregisterBrokerRecord

{
  "apiKey": 1,
  "type "[]BrokerEndpoint", "versions": "0+metadata",
  "nullableVersionsname": "0+UnregisterBrokerRecord",
	  "aboutvalidVersions": "The endpoints that can be used to communicate with this broker.0",
  "fields": [
		{ "name": "NameBrokerId", "type": "stringint32", "versions": "0+", "mapKey": true,
		  "about": "The name of the endpointbroker id." },
		{ "name": "HostBrokerEpoch", "type": "stringint64", "versions": "0+",
		  "about": "The broker hostnameepoch." }
  ]
}

TopicRecord

{
  "apiKey": 2,
  "type,
		{ "name": "Portmetadata",
  "typename": "int16TopicRecord",
  "versionsvalidVersions": "0+",
		  "about": "The port." },
		
  "fields": [
        { "name": "SecurityProtocolTopicName", "type": "int16string", "versions": "0+",
		          "about": "The securitytopic protocolname." }
	]},
	        { "name": "FeaturesTopicId", "type": "[]BrokerFeatureuuid", "versions": "0+", "nullableVersions": "0+",
	
          "about": "The unique featuresID thatof this broker supportstopic." }
  ]
}

PartitionRecord

{
  "apiKey": 3,
  "type": "metadata",
 , "fields": [
		{ "name": "Name", "type": "string", "versions": "0+", "mapKey": true,
		  "about": "The name of the feature." },
		{ "name": "MinVersionPartitionRecord",
 "type": "int16validVersions", "versions": "0+",
		  "aboutfields": "The[
 minimum feature level that this broker supports." },
		{ "name": "MaxVersionPartitionId", "type": "int16int32", "versions": "0+",
		  "aboutdefault": "The maximum feature level that this broker supports"-1",
      "about": "The partition id." }
	]},
	    { "name": "RackTopicId", "type": "stringuuid", "versions": "0+", "nullableVersions": "0+",
	
      "about": "The broker rackThe unique ID of this topic." },
    ]
}

UnregisterBrokerRecord

{
  "apiKeyname": 1"Replicas",
  "type":  "metadata[]int32",
  "nameversions":  "UnregisterBrokerRecord0+",
      "validVersionsabout": "0"The replicas of this partition, sorted by preferred order." },
  "fields": [
	  { "name": "IdIsr", "type":  "[]int32", "versions":  "0+",
	      "about": "The broker id.in-sync replicas of this partition" },
	    { "name": "EpochRemovingReplicas", "type":  "int64[]int32", "versions":  "0+",
	      "about": "The broker epoch": "The replicas that we are in the process of removing." },
    ]
}

TopicRecord

{
  "apiKeyname": 2"AddingReplicas",
  "type":  "metadata[]int32",
  "nameversions":  "TopicRecord0+",
      "validVersionsabout": "0",
  "fields": [
    The replicas that we are in the process of adding." },
    { "name": "NameLeader", "type": "stringint32", "versions": "0+",
    "default": "-1",
      "about": "The topic name lead replica, or -1 if there is no leader." },
        { "name": "TopicIdLeaderEpoch", "type": "int32", "versions": "uuid0+", "versionsdefault": "0+-1",
          "about": "The unique ID of this topicAn epoch that gets incremented each time we change the leader." }
  ]
}

...

ConfigRecord

{         
  "apiKey": 34,
  "type": "metadata",
  "name": "PartitionRecordConfigRecord",
  "validVersions": "0",
  "fields": [
    { "name": "PartitionIdResourceType", "type": "int32int8", "versions": "0+", "default": "-1",
      "about": "The partition id." },
    { "name": "TopicId", "type": "uuid", "versions": "0+",
      "about": "The unique ID of this topic of resource this configuration applies to." },
    { "name": "ReplicasResourceName", "type":  "[]int32string", "versions":  "0+",
      "about": "The replicasname of thisthe partition,resource sortedthis byconfiguration preferredapplies orderto." },         
    { "name": "IsrName", "type":  "[]int32string", "versions":  "0+",
      "about": "The in-syncname replicasof ofthe thisconfiguration partitionkey." },                  
    { "name": "RemovingReplicasValue", "type":  "[]int32string", "versions":  "0+",     
      "about": "The replicas that we are in the process of removing." },
    { "name": "AddingReplicas", "type":  "[]int32", "versions":  "0+",
      "about": "The replicas that we are in the process of adding." }, value of the configuration." }
  ]           
} 

PartitionChangeRecord

{
  "apiKey": 5,
  "type": "metadata",
  "name": "PartitionChangeRecord",
  "validVersions": "0",
  "fields": [
    { "name": "LeaderPartitionId", "type": "int32", "versions": "0+", "default": "-1",
      "about": "The lead replica, or -1 if there is no leaderpartition id." },
    { "name": "LeaderEpochTopicId", "type": "int32uuid", "versions": "0+", "default": "-1",
      "about": "AnThe epochunique thatID getsof incremented each time we change the leaderthis topic." },
  ]
}

ConfigRecord

{         
  "apiKey": 4,
  "type  { "name": "metadataIsr",
  "nametype":  "ConfigRecord[]int32",
  "validVersionsdefault": "0null",
  "fieldsentityType": ["brokerId",
     { "nameversions": "ResourceType0+", "typenullableVersions": "int80+", "versionstaggedVersions": "0+", "tag": 0,
      "about": "The type of resource this configuration applies tonull if the ISR didn't change; the new in-sync replicas otherwise." },
    { "name": "ResourceNameLeader", "type": "int32", "default": "string-2", "entityType": "brokerId",
      "versions": "0+", "taggedVersions": "0+", "tag": 1,
      "about": "The name of the resource this configuration applies to." },         -1 if there is now no leader; -2 if the leader didn't change; the new leader otherwise." },
    { "name": "NameReplicas", "type": "string[]int32", "versionsdefault": "0+null",
      "aboutentityType": "The name of the configuration key." },                  
brokerId",
      { "nameversions": "Value0+", "typenullableVersions": "string0+", "versionstaggedVersions": "0+",     "tag": 2,
      "about": "Thenull valueif ofthe the configuration." }
  ]   replicas didn't change; the new replicas otherwise." },
    { "name": "RemovingReplicas",  
} 

IsrChangeRecord

{
  "apiKey": 5,
  "type"type": "[]int32", "default": "null", "entityType": "brokerId",
      "versions": "metadata0+",
  "namenullableVersions": "IsrChangeRecord0+",
  "validVersionstaggedVersions": "0+",
  "fieldstag": [3,
     { "nameabout": "PartitionId", "typenull if the removing replicas didn't change; the new removing replicas otherwise." },
    { "name": "int32AddingReplicas", "versionstype": "0+[]int32", "default": "-1null",
      "aboutentityType": "The partition id." }brokerId",
     { "nameversions": "TopicId0+", "typenullableVersions": "uuid0+", "versionstaggedVersions": "0+", "tag": 4,
      "about": "The unique ID of this topicnull if the adding replicas didn't change; the new adding replicas otherwise." },
  ]
} 

AccessControlRecord

{
 { "nameapiKey": "Isr"6,
  "type":  "[]int32metadata",
  "versionsname":  "0+AccessControlRecord",
      "aboutvalidVersions": "0"The,
 in-sync replicas of this partition" }, "fields": [
    { "name": "LeaderResourceType", "type": "int32int8", "versions": "0+", "default": "-10+",
      "about": "The lead replica, or -1 if there is no leader.resource type" },
    { "name": "LeaderEpochResourceName", "type": "int32string", "versions": "0+", "defaultnullableVersions": "-10+",
      "about": "AnThe epochresource thatname, getsor incrementednull eachif timethis weis changefor the default leaderresource." },
   ]
} 

AccessControlRecord

{
  "apiKeyname": 6"PatternType",
  "type": "metadataint8",
  "nameversions": "AccessControlRecord0+",
      "validVersionsabout": "0",
  "fields": [The pattern type (literal, prefixed, etc.)" },
    { "name": "ResourceTypePrincipal", "type": "int8string", "versions": "0+",
      "about": "The resourceprincipal typename." },
    { "name": "ResourceName", "type": "stringHost", "versionstype": "0+string", "nullableVersionsversions": "0+",
      "about": "The resource name, or null if this is for the default resourcehost." },
    { "name": "PatternTypeOperation", "type": "int8", "versions": "0+",
      "about": "The patternoperation type (literal, prefixed, etc.)" },
    { "name": "PrincipalPermissionType", "type": "stringint8", "versions": "0+",
      "about": "The principal namepermission type (allow, deny)." },
  ]
} 

FenceBrokerRecord

{
 { "nameapiKey": "Host"7,
  "type": "stringmetadata",
  "versionsname": "0+FenceBrokerRecord",
      "aboutvalidVersions": "0"The host." },,
  "fields": [
    { "name": "OperationBrokerId", "type": "int8int32", "versions": "0+",
      "about": "The operation typeThe broker ID to fence. It will be removed from all ISRs." },
    { "name": "PermissionTypeBrokerEpoch", "type": "int8int64", "versions": "0+",
      "about": "The epoch of permissionthe typebroker (allow, deny)to fence." }
  ]
} 

...

UnfenceBrokerRecord

{
  "apiKey": 78,
  "type": "metadata",
  "name": "FenceBrokerRecordUnfenceBrokerRecord",
  "validVersions": "0",
  "fields": [
    { "name": "IdBrokerId", "type": "int32", "versions": "0+",
      "about": "The broker ID to fence. It will be removed from all ISRs.unfence." }
    { "name": "EpochBrokerEpoch", "type": "int64", "versions": "0+",
      "about": "The epoch of the broker to fenceunfence." }
  ]
} 

...

RemoveTopic

{
  "apiKey": 89,
  "type": "metadata",
  "name": "UnfenceBrokerRecordRemoveTopicRecord",
  "validVersions": "0",
  "fields": [
    { "name": "IdTopicId", "type": "int32uuid", "versions": "0+",
      "about": "The broker IDtopic to unfenceremove." }
All associated partitions will { "name": "Epoch", "type": "int64", "versions": "0+",
      "about": "The epoch of the broker to unfencebe removed as well." }
  ]
} 

...

DelegationTokenRecord

{
  "apiKey": 910,
  "type": "metadata",
  "name": "RemoveTopicRecordDelegationTokenRecord",
  "validVersions": "0",
  "fields": [
    { "name": "IdOwner", "type": "uuidstring", "versions": "0+",
      "about": "The topicdelegation totoken remove. All associated partitions will be removed as wellowner." },
    { "name": "Renewers", "type": "[]string", "versions": "0+",
      "about": "The principals which have renewed this token." },
  ]
} 

DelegationTokenRecord

{
   { "apiKeyname": 10"IssueTimestamp",
  "type": "metadataint64",
  "nameversions": "DelegationTokenRecord0+",
      "validVersionsabout": "0",
  "fields": [The time at which this timestamp was issued." },
    { "name": "OwnerMaxTimestamp", "type": "stringint64", "versions": "0+",
      "about": "The delegation time at which this token owner cannot be renewed any more." },
    { "name": "RenewersExpirationTimestamp", "type": "[]stringint64", "versions": "0+",
      "about": "The principalsnext whichtime haveat renewedwhich this token must be renewed." },
    { "name": "IssueTimestampTokenId", "type": "int64string", "versions": "0+",
      "about": "The token id." },
  ]
} 

UserScramCredentialRecord

{
  "apiKey": 11,
  "type": "metadata",
  "name": "UserScramCredentialRecord",
  "validVersions": "0",
  "fields": [ time at which this timestamp was issued." },
    { "name": "MaxTimestampUserName", "type": "int64string", "versions": "0+",
      "about": "The time at which this token cannot be renewed any moreuser name." },
    { "name": "ExpirationTimestampCredentialInfos", "type": "int64[]CredentialInfo", "versions": "0+",
      "about": "The mechanism nextand timerelated atinformation whichassociated thiswith tokenthe mustuser's beSCRAM renewedcredential.", "fields": },[
      { "name": "TokenIdMechanism", "type": "stringint8", "versions": "0+",
        "about": "The tokenSCRAM idmechanism." },
    ]
} 

UserScramCredentialRecord

{
  "apiKey  { "name": 11"Salt",
  "type": "metadatabytes",
  "nameversions": "UserScramCredentialRecord0+",
  "validVersions      "about": "0",
  "fields": [
A random salt generated by the client." },
      { "name": "NameSaltedPassword", "type": "stringbytes", "versions": "0+",
        "about": "The usersalted namepassword." },
      { "name": "CredentialInfosIterations", "type": "[]CredentialInfoint32", "versions": "0+",
        "about": "The mechanismnumber andof relatediterations informationused associatedin with the user's SCRAM credential.", "fields": [ }]}
  ]
} 

FeatureLevelRecord

{
  "apiKey    { "name": "Mechanism"12,
  "type": "int8metadata",
  "versionsname": "0+FeatureLevelRecord",
        "aboutvalidVersions": "The0",
 SCRAM mechanism."fields": },[
      { "name": "SaltName", "type": "bytesstring", "versions": "0+",
 "mapKey": true,
      "about": "AThe random salt generated by the clientfeature name." },
      { "name": "SaltedPasswordMinFeatureLevel", "type": "bytesint16", "versions": "0+",
        "about": "The salted password current finalized minimum feature level of this feature for the cluster." },
      { "name": "IterationsMaxFeatureLevel", "type": "int32int16", "versions": "0+",
        "about": "The numbercurrent finalized maximum feature level of iterationsthis usedfeature infor the SCRAM credentialcluster." }]}
  ]
} 

...

FailedReplicasRecord

{
  "apiKey": 1213,
  "type": "metadata",
  "name": "FeatureLevelRecordFailedReplicasRecord",
  "validVersions": "0",
  "fields": [
    { "name": "NameBrokerId", "type": "stringint32", "versions": "0+",
      "about": "The broker id." },
 "mapKey   { "name": "Topics", "type": "[]TopicWithFailures", "versions": true"0+",
      "about": "The feature name." }, topics with failed replicas.", "fields": [
      { "name": "MinFeatureLevelTopicId", "type": "int16uuid", "versions": "0+",
        "about": "The current finalized minimum feature level of this feature for the clustertopic UUID." },
      { "name": "MaxFeatureLevelPartitions", "type": "int16[]int32", "versions": "0+",
        "about": "The currentpartition finalizedids." maximum} feature level of this
 feature for the cluster." ]}
  ]
} 

...

QuotaRecord

{
  "apiKey": 1314,
  "type": "metadata",
  "name": "FailedReplicasRecordQuotaRecord",
  "validVersions": "0",
  "fields": [
    { "name": "Broker", "type": "int32", "versions": "0+",
      "about": "The broker id." },",
  "fields": [
    { "name": "TopicEntity", "type": "uuid[]EntityData", "versions": "0+",
      "about": "The topic UUID." }quota entity to alter.", "fields": [
      { "name": "PartitionsEntityType", "type": "[]int32string", "versions": "0+",
        "about": "The partitionentity idstype." },
  ]
} 

New Metrics

...

kafka.controller:type=KafkaController,name=MetadataLag

...

The offset delta between the latest metadata record this controller has replayed and the last stable offset of the metadata topic.

...

New name for kafka.controller:type=KafkaController,name=SnapshotLag

The offset delta between the latest stable offset of the metadata topic and the offset of the last snapshot (or the last stable offset itself, if there are no snapshots)

    { "name": "EntityName", "type": "string", "versions": "0+", "nullableVersions": "0+",
        "about": "The name of the entity, or null if the default." }
    ]},
    { "name": "Key", "type": "string", "versions": "0+",
      "about": "The quota configuration key." },
    { "name": "Value", "type": "float64", "versions": "0+",
      "about": "The value to set, otherwise ignored if the value is to be removed." },
    { "name": "Remove", "type": "bool", "versions": "0+",
      "about": "Whether the quota configuration value should be removed, otherwise set." }
  ]
} 

New Metrics

Unused Metrics in KIP-500 Mode

We will deprecate these metrics as soon as legacy mode is deprecated.  For now, they will be unused in KIP-500 mode.

Compatibility, Deprecation, and Migration Plan

As described above, this KIP outlines a new mode that the broker can run in, KIP-500 mode.  For now, this mode will be experimental, and there will be no way to migrate existing clusters from legacy mode to KIP-500 mode.  We plan on outlining how this upgrade process will work in a follow-on KIP.  We do plan on deprecating legacy mode eventually, but we are not quite ready to do it yet in this KIP.

Since KIP-500 mode is currently in a pre-alpha state, we do not guarantee that future versions will support upgrading from the current version of it yet.  Once it is more stable, we will have a more traditional binary compatibility regime.

Rejected Alternatives

Suport Automatic Broker ID Assignment

This KIP proposes to drop support for automatic broker ID assignment.  What if we decided to continue to support it?

If we were willing to take a little bit more complexity on board, it would be relatively easy to support automatic broker ID assignment.  Brokers could simply ask the active controller to assign them a new ID when starting up, just as they previously obtained one from ZooKeeper.

However, automatic controller ID assignment is a much more difficult problem.  We never actually supported automatically assigning ZooKeeper IDs, so there is no pattern to follow here.  In general, Raft assumes that nodes know their IDs before the protocol begins. We cannot rely on random assignment because the 31 bit space is not large enough. We could perhaps create a separate protocol for assigning node IDs, but it might be complex.  

In general it's not clear how useful automatic broker ID assignment really is.  Configuration management software like Puppet, Chef, or Ansible can easily create a new ID for each node's configuration file.  Therefore, it's probably best to use this compatibility break to drop support for automatic broker ID assignment.

Combined Heartbeats and Fetch Requests

The brokers are always fetching new metadata from the controller.  Why not combine these fetch requests with the heartbeat requests, so that the brokers only have to send one request rather than two?

The main reason for making them separate requests is to have better separation of concerns.  Fetching metadata is logically a bit different than sending a heartbeat, and coupling them could result in a messy design and code.  We would have to add significant extra complexity to the FetchRequest schema.  Perhaps even worse, we would need to make the timing of fetch requests line up with the timing needed for broker heartbeats

Unused Metrics in KIP-500 Mode

We will deprecate these metrics as soon as legacy mode is deprecated.  For now, they will be unused in KIP-500 mode.

...

kafka.server:type=SessionExpireListener,name=ZooKeeperExpiresPerSec

...

No longer needed when running in KIP-500 mode because we won't have any ZK sessions

Compatibility, Deprecation, and Migration Plan

As described above, this KIP outlines a new mode that the broker can run in, KIP-500 mode.  For now, this mode will be experimental, and there will be no way to migrate existing clusters from legacy mode to KIP-500 mode.  We plan on outlining how this upgrade process will work in a follow-on KIP.  We do plan on deprecating legacy mode eventually, but we are not quite ready to do it yet in this KIP.

Since KIP-500 mode is currently in a pre-alpha state, we do not guarantee that future versions will support upgrading from the current version of it yet.  Once it is more stable, we will have a more traditional binary compatibility regime.

Rejected Alternatives

Suport Automatic Broker ID Assignment

This KIP proposes to drop support for automatic broker ID assignment.  What if we decided to continue to support it?

If we were willing to take a little bit more complexity on board, it would be relatively easy to support automatic broker ID assignment.  Brokers could simply ask the active controller to assign them a new ID when starting up, just as they previously obtained one from ZooKeeper.

However, automatic controller ID assignment is a much more difficult problem.  We never actually supported automatically assigning ZooKeeper IDs, so there is no pattern to follow here.  In general, Raft assumes that nodes know their IDs before the protocol begins. We cannot rely on random assignment because the 31 bit space is not large enough. We could perhaps create a separate protocol for assigning node IDs, but it might be complex.  

In general it's not clear how useful automatic broker ID assignment really is.  Configuration management software like Puppet, Chef, or Ansible can easily create a new ID for each node's configuration file.  Therefore, it's probably best to use this compatibility break to drop support for automatic broker ID assignment.

Combined Heartbeats and Fetch Requests

The brokers are always fetching new metadata from the controller.  Why not combine these fetch requests with the heartbeat requests, so that the brokers only have to send one request rather than two?

The main reason for making them separate requests is to have better separation of concerns.  Fetching metadata is logically a bit different than sending a heartbeat, and coupling them could result in a messy design and code.  We would have to add significant extra complexity to the FetchRequest schema.  Perhaps even worse, we would need to make the timing of fetch requests line up with the timing needed for broker heartbeats.

Shared IDs between Multiple Nodes

One possibility that we've discussed is whether we could have controller IDs and broker IDs share the same ID space.  For example, could there be both a broker 1 and a controller 1?  This is not a good idea because NetworkClient assumes a single ID space.  So if there is both a controller 1 and a broker 1, we don't have a way of picking the "right" one.  We would have to add a concept of node types.  That is a fair amount of extra work.  It also is a bit conceptually messy for the network layer to understand node types, rather than just treating them all as endpoints.

Another reason not to share node IDs is that it would make migrating from a combined broker+controller node to separate nodes more difficult.  For example, let's say you have three combined nodes but the controller load is getting too high and you now want a separate controller node.  If you also have to change the controller ID, the migration becomes more difficult.  Since controller node IDs make their way into the Raft log itself, changing the ID later on is non-trivial.

External metrics and management systems often make use of the concept of node ID.  For example, someone may want to aggregate all the socket server metrics from node 1.  But if there are actually two socket server sharing this same node ID (controller and broker) this may create problems and require changes to the external system.

A related question is whether a broker and a controller could share the same port, if they are co-located in the same JVM.  We discussed this as part of KIP-590 and agreed that we would not share ports.  A shared port would make it impossible to have separate RPC handlers, complicating the code.  It would also make it impossible to have separate, stricter authentication for controllers.  Sharing a port with the broker opens us up to denial of service attacks, including unintentional ones.