kafka_2.13-3.1.2.tgz

KRaft (aka KIP-500) mode Preview Release ========================================================= # Introduction It is now possible to run Apache Kafka without Apache ZooKeeper! We call this the [Kafka Raft metadata mode](https://cwiki.apache.org/confluence/display/KAFKA/KIP-500%3A+Replace+ZooKeeper+with+a+Self-Managed+Metadata+Quorum), typically shortened to `KRaft mode`. `KRaft` is intended to be pronounced like `craft` (as in `craftsmanship`). It is currently *PREVIEW AND SHOULD NOT BE USED IN PRODUCTION*, but it is available for testing in the Kafka 3.1 release. When the Kafka cluster is in KRaft mode, it does not store its metadata in ZooKeeper. In fact, you do not have to run ZooKeeper at all, because it stores its metadata in a KRaft quorum of controller nodes. KRaft mode has many benefits -- some obvious, and some not so obvious. Clearly, it is nice to manage and configure one service rather than two services. In addition, you can now run a single process Kafka cluster. Most important of all, KRaft mode is more scalable. We expect to be able to [support many more topics and partitions](https://www.confluent.io/kafka-summit-san-francisco-2019/kafka-needs-no-keeper/) in this mode. # Quickstart ## Warning KRaft mode in Kafka 3.1 is provided for testing only, *NOT* for production. We do not yet support upgrading existing ZooKeeper-based Kafka clusters into this mode. There may be bugs, including serious ones. You should *assume that your data could be lost at any time* if you try the preview release of KRaft mode. ## Generate a cluster ID The first step is to generate an ID for your new cluster, using the kafka-storage tool: ~~~~ $ ./bin/kafka-storage.sh random-uuid xtzWWN4bTjitpL3kfd9s5g ~~~~ ## Format Storage Directories The next step is to format your storage directories. If you are running in single-node mode, you can do this with one command: ~~~~ $ ./bin/kafka-storage.sh format -t <uuid> -c ./config/kraft/server.properties Formatting /tmp/kraft-combined-logs ~~~~ If you are using multiple nodes, then you should run the format command on each node. Be sure to use the same cluster ID for each one. ## Start the Kafka Server Finally, you are ready to start the Kafka server on each node. ~~~~ $ ./bin/kafka-server-start.sh ./config/kraft/server.properties [2021-02-26 15:37:11,071] INFO Registered kafka:type=kafka.Log4jController MBean (kafka.utils.Log4jControllerRegistration$) [2021-02-26 15:37:11,294] INFO Setting -D jdk.tls.rejectClientInitiatedRenegotiation=true to disable client-initiated TLS renegotiation (org.apache.zookeeper.common.X509Util) [2021-02-26 15:37:11,466] INFO [Log partition=__cluster_metadata-0, dir=/tmp/kraft-combined-logs] Loading producer state till offset 0 with message format version 2 (kafka.log.Log) [2021-02-26 15:37:11,509] INFO [raft-expiration-reaper]: Starting (kafka.raft.TimingWheelExpirationService$ExpiredOperationReaper) [2021-02-26 15:37:11,640] INFO [RaftManager nodeId=1] Completed transition to Unattached(epoch=0, voters=[1], electionTimeoutMs=9037) (org.apache.kafka.raft.QuorumState) ... ~~~~ Just like with a ZooKeeper based broker, you can connect to port 9092 (or whatever port you configured) to perform administrative operations or produce or consume data. ~~~~ $ ./bin/kafka-topics.sh --create --topic foo --partitions 1 --replication-factor 1 --bootstrap-server localhost:9092 Created topic foo. ~~~~ # Deployment ## Controller Servers In KRaft mode, only a small group of specially selected servers can act as controllers (unlike the ZooKeeper-based mode, where any server can become the Controller). The specially selected controller servers will participate in the metadata quorum. Each controller server is either active, or a hot standby for the current active controller server. You will typically select 3 or 5 servers for this role, depending on factors like cost and the number of concurrent failures your system should withstand without availability impact. Just like with ZooKeeper, you must keep a majority of the controllers alive in order to maintain availability. So if you have 3 controllers, you can tolerate 1 failure; with 5 controllers, you can tolerate 2 failures. ## Process Roles Each Kafka server now has a new configuration key called `process.roles` which can have the following values: * If `process.roles` is set to `broker`, the server acts as a broker in KRaft mode. * If `process.roles` is set to `controller`, the server acts as a controller in KRaft mode. * If `process.roles` is set to `broker,controller`, the server acts as both a broker and a controller in KRaft mode. * If `process.roles` is not set at all then we are assumed to be in ZooKeeper mode. As mentioned earlier, you can't currently transition back and forth between ZooKeeper mode and KRaft mode without reformatting. Nodes that act as both brokers and controllers are referred to as "combined" nodes. Combined nodes are simpler to operate for simple use cases and allow you to avoid some fixed memory overheads associated with JVMs. The key disadvantage is that the controller will be less isolated from the rest of the system. For example, if activity on the broker causes an out of memory condition, the controller part of the server is not isolated from that OOM condition. ## Quorum Voters All nodes in the system must set the `controller.quorum.voters` configuration. This identifies the quorum controller servers that should be used. All the controllers must be enumerated. This is similar to how, when using ZooKeeper, the `zookeeper.connect` configuration must contain all the ZooKeeper servers. Unlike with the ZooKeeper config, however, `controller.quorum.voters` also has IDs for each node. The format is id1@host1:port1,id2@host2:port2, etc. So if you have 10 brokers and 3 controllers named controller1, controller2, controller3, you might have the following configuration on controller1: ``` process.roles=controller node.id=1 listeners=CONTROLLER://controller1.example.com:9093 controller.quorum.voters=1@controller1.example.com:9093,2@controller2.example.com:9093,3@controller3.example.com:9093 ``` Each broker and each controller must set `controller.quorum.voters`. Note that the node ID supplied in the `controller.quorum.voters` configuration must match that supplied to the server. So on controller1, node.id must be set to 1, and so forth. Note that there is no requirement for controller IDs to start at 0 or 1. However, the easiest and least confusing way to allocate node IDs is probably just to give each server a numeric ID, starting from 0. Note that clients never need to configure `controller.quorum.voters`; only servers do. ## Kafka Storage Tool As described above in the QuickStart section, you must use the `kafka-storage.sh` tool to generate a cluster ID for your new cluster, and then run the format command on each node before starting the node. This is different from how Kafka has operated in the past. Previously, Kafka would format blank storage directories automatically, and also generate a new cluster UUID automatically. One reason for the change is that auto-formatting can sometimes obscure an error condition. For example, under UNIX, if a data directory can't be mounted, it may show up as blank. In this case, auto-formatting would be the wrong thing to do. This is particularly important for the metadata log maintained by the controller servers. If two controllers out of three controllers were able to start with blank logs, a leader might be able to be elected with nothing in the log, which would cause all metadata to be lost. # Missing Features We don't support any kind of upgrade right now, either to or from KRaft mode. This is an important gap that we are working on. Finally, the following Kafka features have not yet been fully implemented: * Support for certain security features: configuring a KRaft-based Authorizer, setting up SCRAM, delegation tokens, and so forth