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

Motivation

Apache Kafka is in the process of moving from storing metadata in Apache Zookeeper, to storing metadata in an internal Raft topic.  KIP-500 described the overall architecture and plan.  The purpose of this KIP is to go into detail about how the Kafka Controller will change during this transition.

Proposed Changes

Deployment

KIP-500 Mode

Once this KIP is implemented, system administrators will have the option of running in KIP-500 mode.  In this mode, we will not use ZooKeeper  The alternative mode where KIP-500 support is not enabled will be referred to as legacy mode.

KIP-500 mode must be enabled for the entire cluster, not just for specific nodes.  Initially, this mode will be considered experimental and not ready for production.  As we do more testing and gain more confidence, we will remove the experimental label.  Eventually, in a future release, KIP-500 mode will be the only supported mode.  Since dropping support for legacy mode is an incompatible change, it will need to happen in a major release, of course.

Initially, we will not support upgrading a cluster from legacy mode to KIP-500 mode.  This is in keeping with the experimental nature of KIP-500 mode.  A future KIP will describe and implement an upgrade process from legacy mode to KIP-500 mode.

Nodes

Currently, a ZooKeeper cluster must be deployed when running Kafka.  This KIP will eliminate that requirement, as well as the requirement to configure the addresses of the zookeeper nodes on each broker.

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 pool of nodes specifically configured to act as controllers.  Typically three or five nodes in the cluster will be selected to be controllers.

System administrators will be able to choose whether to run separate controller nodes, or whether to run controller nodes which are co-located with broker nodes.  Kafka will provide support for running a controller in the same JVM as a broker, in order to save memory and enable single-process test deployments.

The addresses and ports of the controller nodes must be configured on each broker, so that the broker can contact the controller quorum when starting up.  This is similar to how we configure the ZooKeeper quorum on each node today.

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.

Automatic 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.

In a well-run Kafka deployment, controller ports, like ZooKeeper ports, should be firewalled off from clients.  This will prevent clients from disrupting the cluster by flooding the controller ports with requests.  In the realm of ACLs, this translates to controllers requiring CLUSTERACTION on CLUSTER for all operations.

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.

Metadata

The Metadata Topic

As described in KIP-500, the controller will store its data in the internal __kafka_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.

The leader of the controller quorum will be the active controller.  The followers will function as hot standbys, ready to take over when the active leader fails or resigns.  The metadata will be stored in memory on all of the controllers.

Persistence and Visibility

Metadata changes need to be persisted to the __kafka_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.

Changes that we haven't yet persisted are referred to as "uncommitted."  The active controller may have several of these uncommitted changes in flight at any given time.  In essence, the controller's in-memory state is always a little bit in the future compared to the current state.  This allows the active controller to continue doing things while it waits for the previous changes to be committed to the Raft log.

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 __kafka_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.

                      earlier offsets
                          +---+
     visible change ----> |   |
                          +---+       last stable offset
     visible change ----> |   | <---- standby controllers'
                          +---+       in-memory state
     pending change ----> |   |
                          +---+
     pending change ----> |   |
                          +---+
     pending change ----> |   |
                          +---+
     pending change ----> |   | <---- active controller's
                          +---+       in-memory state
                      latest offset

Record Formats

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 record type.
2. an unsigned varint specifying the record version
3. the payload in Kafka RPC format

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

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

Record Format Versions

There are two ways to evolve the format of a KIP-500 record.  One is to add KIP-482 optional tagged fields.  These will be ignored by older software, but can contain additional data for new software to handle.  The other choice is to bump the version of the record.

In the pre-KIP-500 world, we had the inter-broker protocol (IBP) setting to control what RPC versions the controller used to communicate with the brokers.  This allowed us to evolve the inter-broker RPC format over time.  We also used it to gate many other features, such as metadata format changes.  In the post-KIP-500 world, the analogous setting is the metadata.format KIP-584 feature flag. This setting controls the snapshot and delta formats which the controller will use.

Snapshot Implementation

As time goes on, the number of records will grow and grow, even if the total size of the metadata stays constant.  Therefore, periodically, we need to consolidate all the metadata deltas into a snapshot.

Like the metadata log, the snapshot is made up of records.  However, unlike the log, in which there may be multiple records describing a single entity, the snapshot will only contain the minimum number of records needed to describe all the entities.

Snapshots are local to each replica.  For example, replica A may have a snapshot at offset 100, and deltas up to offset 150, whereas replica B may have a snapshot at 125 and deltas up to offset 150.  Any snapshot must be usable as a starting point for loading the entire state of metadata.  In other words, a new controller node must be able to load the a snapshot, and then apply all the edits which follow it, and come up-to-date.

The currently active controller will monitor the offset of the latest snapshot made by all replicas, including itself.  The snapshotting state of each node is considered soft state: it is not persisted anywhere in the log, but purely communicated by heartbeats and stored in memory by the active controller.

When the active controller decides that a standby controller should start a snapshot, it will communicate that information in its response to the periodic heartbeat sent by that node.  When the active controller decides that it itself should create a snapshot, it will first try to give up the leadership of the Raft quorum in order to avoid a unnecessary delays while writing the snapshot.

Because the snapshots are centrally coordinated by the active controller, we can avoid initiating more than one snapshot at once.  The controller will also snapshot less frequently when too many members of the quorum have fallen behind.  Specifically, if losing a node would probably impact availability, we will use a separate set of configurations for determining when to snapshot.

Broker Registration and State Management

Broker Heartbeats

Every distributed system needs a way of managing cluster membership.  Prior to KIP-500, Kafka brokers registered ephemeral znodes in order to register themselves as part of the cluster.  The Kafka controller passively consumed the registration information from Zookeeper.

In the post-KIP-500 world there is no ZooKeeper and no ephemeral znodes.  Instead, each broker sends a ControllerHeartbeat request to the active controller every few seconds.

This heartbeat acts as a registration.  However, the controller has a choice about whether to accept it.  It will reject brokers whose metadata is too stale, or whose IDs have been claimed by another broker.  It will also reject brokers that do not support the minimum feature level of all KIP-584 features that are enabled.

When the broker accepts the registration, it grants or renews a broker ID lease associating the broker process with its ID.  Leases are time-bounded. The length of the lease is 10 times the length of the configured broker heartbeat interval, which puts it at 30 seconds by default.

A broker cannot continue using a lease indefinitely after sending a single heartbeat.  When brokers are rejected by the controller, or otherwise unable to renew their lease before it expires, they enter the "fenced" state.

Fencing

Brokers that don't have a broker ID lease are said to be "fenced."  When a broker is fenced, it cannot process any client requests.  This prevents brokers which are not receiving metadata updates or that are not receiving and processing them fast enough from causing issues to clients.

Brokers start up in the fenced state, and can leave this state only by sending a heartbeat to the active controller and getting back a response that tells them they can become active.

Controlled Shutdown

In the pre-KIP-500 world, brokers triggered a controller shutdown by making an RPC to the controller.  When the controller returned a successful result from this RPC, the broker knew that it could shut down.

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 SHUTDOWN state.  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 a nextState of SHUTDOWN.

Broker ID Conflicts

Clearly, in a correctly managed cluster, there should be no broker ID conflicts.  Each broker should be configured with a unique ID.  However, we want the system to be robust against misconfigurations.  Therefore, if there are two brokers that claim the same ID, the controller will choose only one and tell the other to fence itself.

When a broker first starts up, before it has received any responses from the controller, it will always "win" broker ID conflicts.  However, once it has communicated with the controller, it may lose subsequent conflicts if its broker epoch is stale.  (See KIP-380 for some background on broker epoch.)  The reason for favoring new processes is to accommodate the common case where a process is killed with kill -9 and then restarted.  We want it to be able to reclaim its old ID quickly in this case.

The Broker State Machine

This state machine ties together a few of the previous sections, such as broker registration and shutdown.

                        INITIAL
sent initial heartbeart,  | 
got back an epoch         |
                          V
                        ACTIVE -------------------------> SHUTDOWN
lease expired or revoked  |  ^                          |
(stale metadata,          |  | lease restored           | controller granted
 id conflict,             |  | with new broker epoch    | controlled shutdown
 unable to communicate,   V  |                          |
 etc.)                  FENCED -------------------------+ 

Public Interfaces

Configurations

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.

BrokerHeartbeat

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

{
  "apiKey": 50,
  "type": "request",
  "name": "BrokerHeartbeatRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "TargetState", "type": "int8", "versions": "0+",
      "about": "The state that the broker wants to reach." },
    { "name": "BrokerId", "type": "int32", "versions": "0+",
      "about": "The broker ID." },
    { "name": "BrokerEpoch", "type": "int64", "versions": "0+", "default": "-1",
      "about": "The broker epoch, or -1 if one has not yet been assigned." },
    { "name": "LeaseStartTimeMs", "type": "int64", "versions": "0+",
      "about": "The time which the broker wants the lease to start at in milliseconds." },
    { "name": "CurMetadataOffset", "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": "int16", "versions": "0+",
          "about": "The port." },
        { "name": "SecurityProtocol", "type": "int16", "versions": "0+",
          "about": "The security protocol." }
      ]
    }
  ]
}

{
  "apiKey": 50,
  "type": "response",
  "name": "BrokerHeartbeatResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The error code, or 0 if there was no error." },
    { "name": "ActiveControllerId", "type": "int32", "versions": "0+",
      "about": "The ID of the active controller, or -1 if the controller doesn't know." },
    { "name": "NextState", "type": "int8", "versions": "0+",
      "about": "The state to which the broker should transition." },
    { "name": "BrokerEpoch", "type": "int64", "versions": "0+", "default": "-1",
      "about": "The broker's assigned epoch, or -1 if none was assigned." },
    { "name": "LeaseEndTimeMs", "type": "int64", "versions": "0+",
      "about": "The time in milliseconds at which the lease should end. This is based on the start time that was passed, not the controller's local clock." }
  ]
}

enum BrokerState {
    UNKNOWN(0),
    INITIAL(1),
    FENCED(2),
    ACTIVE(3),
    SHUTDOWN(4);
}

As always with enums, the UNKNOWN state is used only to translate values that our software is too old to understand.

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

ControllerHeartbeat

The controllers periodically send out a heartbeat request to the active controller.

{
  "apiKey": 51,
  "type": "request",
  "name": "ControllerHeartbeatRequest",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ControllerId", "type": "int32", "versions": "0+",
      "about": "The controller ID." },
    { "name": "LastSnapshotOffset", "type": "int64", "versions": "0+",
      "about": "The offset of the last snapshot on this controller node." }
  ]
}

{
  "apiKey": 51,
  "type": "response",
  "name": "ControllerHeartbeatResponse",
  "validVersions": "0",
  "flexibleVersions": "0+",
  "fields": [
    { "name": "ErrorCode", "type": "int16", "versions": "0+",
      "about": "The error code, or 0 if there was no error." },
    { "name": "StartSnapshot", "type": "bool", "versions": "0+",
      "about": "True if we should start creating a snapshot." }
  ]
}

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

Record Formats

BrokerRecord

{
  "apiKey": 0,
  "type": "metadataRecord",
  "name": "BrokerRecord",
  "validVersions": "0",
  "fields": [
	{ "name": "BrokerId", "type": "int32", "versions": "0+",
	  "about": "The broker id." },
	{ "name": "BrokerEpoch", "type": "int64", "versions": "0+",
	  "about": "The broker epoch." },
	{ "name": "EndPoints", "type": "[]BrokerEndpoint", "versions": "0+",
	  "about": "The endpoints that can be used to communicate with this broker.", "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": "int16", "versions": "0+",
		  "about": "The port." },
		{ "name": "SecurityProtocol", "type": "int16", "versions": "0+",
		  "about": "The security protocol." }
	]},
	{ "name": "Rack", "type": "string", "versions": "0+", "nullableVersions": "0+",
	  "about": "The broker rack." }
  ]
}

TopicRecord

{
  "apiKey": 1,
  "type": "metadataRecord",
  "name": "TopicRecord",
  "validVersions": "0",
  "fields": [
        { "name": "Name", "type": "string", "versions": "0+",
          "about": "The topic name." },
        { "name": "TopicId", "type": "uuid", "versions": "0+",
          "about": "The unique ID of this topic." },
        { "name": "Deleting", "type": "boolean", "versions": "0+",
          "about": "True if this topic is in the process of being deleted." }
  ]
}

PartitionRecord

{
  "apiKey": 2,
  "type": "metadataRecord",
  "name": "PartitionRecord",
  "validVersions": "0",
  "fields": [
    { "name": "PartitionId", "type": "int32", "versions": "0+", "default": "-1",
      "about": "The partition id." },
    { "name": "TopicId", "type": "uuid", "versions": "0+",
      "about": "The unique ID of this topic." },
    { "name": "Replicas", "type":  "[]int32", "versions":  "0+",
      "about": "The replicas of this partition, sorted by preferred order." },
    { "name": "Isr", "type":  "[]int32", "versions":  "0+",
      "about": "The in-sync replicas of this partition" },
    { "name": "RemovingReplicas", "type":  "[]int32", "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." },
    { "name": "Leader", "type": "int32", "versions": "0+", "default": "-1",
      "about": "The lead replica, or -1 if there is no leader." },
    { "name": "LeaderEpoch", "type": "int32", "versions": "0+", "default": "-1",
      "about": "An epoch that gets incremented each time we change the ISR." }
  ]
}

ConfigRecord

{         
  "apiKey": 3,
  "type": "metadataRecord",
  "name": "ConfigRecord",
  "validVersions": "0",
  "fields": [
    { "name": "ResourceType", "type": "int8", "versions": "0+",
      "about": "The type of resource this configuration applies to." },
    { "name": "ResourceName", "type": "string", "versions": "0+",
      "about": "The name of the resource this configuration applies to." },         
    { "name": "Name", "type": "string", "versions": "0+",
      "about": "The name of the configuration key." },                  
    { "name": "Value", "type": "string", "versions": "0+",     
      "about": "The value of the configuration." }
  ]           
} 

IsrChange

{
  "apiKey": 4,
  "type": "metadataRecord",
  "name": "IsrChangeRecord",
  "validVersions": "0",
  "fields": [
    { "name": "PartitionId", "type": "int32", "versions": "0+", "default": "-1",
      "about": "The partition id." },
    { "name": "TopicId", "type": "uuid", "versions": "0+",
      "about": "The unique ID of this topic." },
    { "name": "Isr", "type":  "[]int32", "versions":  "0+",
      "about": "The in-sync replicas of this partition" },
    { "name": "Leader", "type": "int32", "versions": "0+", "default": "-1",
      "about": "The lead replica, or -1 if there is no leader." },
    { "name": "LeaderEpoch", "type": "int32", "versions": "0+", "default": "-1",
      "about": "An epoch that gets incremented each time we change the ISR." }
  ]
} 

AccessControlRecord

{
  "apiKey": 5,
  "type": "metadataRecord",
  "name": "AccessControlRecord",
  "validVersions": "0",
  "fields": [
    { "name": "ResourceType", "type": "int8", "versions": "0+",
      "about": "The resource type" },
    { "name": "ResourceName", "type": "string", "versions": "0+", "nullableVersions": "0+",
      "about": "The resource name, or null if this is for the default resource." },
    { "name": "PatternType", "type": "int8", "versions": "0+",
      "about": "The pattern type (literal, prefixed, etc.)" },
    { "name": "Principal", "type": "string", "versions": "0+",
      "about": "The principal name." },
    { "name": "Host", "type": "string", "versions": "0+",
      "about": "The host." },
    { "name": "Operation", "type": "int8", "versions": "0+",
      "about": "The operation type." },
    { "name": "PermissionType", "type": "int8", "versions": "0+",
      "about": "The permission type (allow, deny)." }
  ]
} 

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

Support 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.