Status

Current state: WIP

Discussion thread:

JIRA: 

Motivation

Currently, all cluster configuration is managed via property files read by the brokers on startup with the exception of topic configuration. There is no mechanism to change broker configuration without having to do a rolling restart of the entire cluster. Additionally, there is no way to model configuration per-client i.e. quotas, permissions etc.

This proposal attempts to build a unified mechanism for modeling configuration across various entities like topics, clients and brokers using Zookeeper.

Public Interfaces

• We will add a new tool called ConfigChangeCommand that can manage all config changes in ZK. New methods will be added in AdminUtils to change configuration. This will be similar to the TopicCommand tool already present.
• There will be new zookeeper paths under "config" but they are not considered to be public interfaces.

Proposed Changes

This proposal will reuse pieces of the TopicQuotaManager. Similar to topic configs, we can have quota managers for Brokers and Clients (producer and consumer). These classes will listen to Zk notifications from a certain path and apply the notifications within the brokers on a per-topic, broker or clientId basis. These approaches can share significant amounts of code amongst themselves. The only thing that is different is the entity that is being modeled and how the config changes are applied.

ZNode Structure:

There should be 4 paths within config
/config/producers/<client_id>
/config/consumers/<client_id>
/config/topics/<topic_name>
/config/brokers/<broker_id>

Internally, the znodes are comma-separated key-value pairs where key represents the configuration to change.
{"version": x, "config" : {X1=Y1, X2=Y2..}

Upon startup, all brokers will load all the configs from zookeeper. In order to receive notifications, it's undesirable to watch every path under config/. We can model change notifications the same way as they are currently handled for topic based configs. Here is the workflow for changing or adding a config of any type:

• Create a znode (or modify existing) under the required path with the configs that you want to update. Example, if you want to change quotas for producer "Adi", add a path under /config/producers/Adi as shown below.
• Create a sequential znode under "config/changes/config_change_XX". This will send a notification to all the watchers. The data within the change node should indicate what has changed i.e. topic config + topic name, client config + clientId or broker config + brokerId.
• The brokers process the changed configs.

Config Precedence

These configs in Zookeeper will override any configuration read from the properties file. We also do not plan to move away from file based configuration.

Modeling Default values

In addition to overrides, we also need a mechanism to model default configuration. For example: say we have default quotas for all clients and we need to selectively override them on a per-client basis. So far, we don't have a way to change the the default values. We can do this by having a special path to store defaults.

The properties in this znode are applicable to all clients, topics and brokers respectively. Changing something here should affect all entities unless overridden explicitly.
 
/config/producers/__default
/config/consumers/__default
/config/topics/__default
/config/brokers/__default
 
Let's extend the example of quotas. Assume that we have a default quota of 5Mbytes per second per producer and we want to change it to 10M for the producer Adi. The default znode will look like this:
/config/producers/__default
{"version": 0, "config" : {quota=5M}

Overridden config:
/config/producers/Adi
{"version": 0, "config" : {quota=10M}

Broker Configuration

For Broker configs, the key can be the KafkaConfig property that we wish to override. All configs represented here should override the configs from the property file.

/config/brokers/0
{"version": 0, "config" : {log.retention.check.interval.ms=100000}

 This KIP provides a mechanism to change any broker config dynamically but identifying which configs can be changed and how the config changes should be applied within the brokers is out of scope. 

ConfigDef Changes

We will need to add a new property to the ConfigKey to indicate if a config is updatable or not. This property can be used to generate documentation so it becomes very easy to discover which properties can be dynamically changed. If a broker property is set in zookeeper and it is not a config marked "isDynamic", that property can simply be ignored. This is similar to configuring Kafka with a property it does not understand.

private static class ConfigKey {
    public final String name;
    public final Type type;
    public final String documentation;
    public final Object defaultValue;
    public final Validator validator;
    public final Importance importance;
    public final boolean required;
	public final boolean isDynamic;

    public ConfigKey(String name, Type type, Object defaultValue, Validator validator, Importance importance, String documentation, boolean required, boolean isDynamic) {
        super();
        this.name = name;
        this.type = type;
        this.defaultValue = defaultValue;
        this.validator = validator;
        this.importance = importance;
        if (this.validator != null)
            this.validator.ensureValid(name, defaultValue);
        this.documentation = documentation;
        this.required = required;
		this.isDynamic = isDynamic;
    }
 }
 
// The isDynamic property can be included in the documentation for all configs
public String toHtmlTable() {
    StringBuilder b = new StringBuilder();
    b.append("<table>\n");
    b.append("<tr>\n");
    b.append("<th>Name</th>\n");
    b.append("<th>Type</th>\n");
    b.append("<th>Default</th>\n");
    b.append("<th>Importance</th>\n");
    b.append("<th>Description</th>\n");
	b.append("<th>IsDynamic</th>\n");
}

Applying Configs within Broker

The ZK listener can have a Properties object parsed from the properties file which was used to start the KafkaServer. Anytime a change notification is received, we should do the following:

1. Parse received notification into a properties object.
2. Within KafkaConfig, for each newly changed property, verify that it can be changed dynamically. For this, ConfigDef needs to expose a getConfigKey() method that can be used to check the isDynamic flag.

All configs should always be accessed via a reference to KafkaConfig. For this, all subsystems within the broker need to be configured with a config object and not individual config values.

// Sample code only
class BrokerConfigManager {
	// current kafka config
	val kafkaConfig : KafkaConfig = ... ;
	.....
	def processConfigChanges(notifications: Seq[String]) {
		val updatedProperties = AdminUtils.fetchConfig(zkClient, configPath)
		kafkaConfig.updateProperties(updatedProperties);		
	}	
}
 
class AdminUtils {
 def fetchConfig(zkClient: ZkClient, path: String): Properties = {
	val str: String = zkClient.readData(path, true)
	val props = new Properties()
	// Read the znode, parse the JSON and return a properties object
	props
}

Config Change Notification

Currently, the change notification znode only contains the topic name. We need to add more information to distinguish whether this config change is for a topic, client or broker config. We need to distinguish producers and consumers separately because it is possible for a producer and consumer to have the same clientId. 

The notification data can be:
{"entity":"topic/producer/consumer/broker", "value" : "topic_name/client_id/broker_id"}

Compatibility, Deprecation, and Migration Plan

• TopicConfigManager has a config_change_XX sequential znode under configs/. The format of data within the config_change node is going to change hence it is important to not make any config changes using AdminTools until the cluster is fully upgraded. 

Rejected Alternatives