Status

Current state: Under Discussion

Discussion thread: here

JIRA: TBD

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

Motivation

Kafka exposes many pluggable API for users to bring their custom plugins. For complex and critical plugins it's important to have metrics to monitor their behavior. Plugins wanting to emit metrics can use the Metrics class from the Kafka API but when creating a new Metrics instance it does not inherits the tags from the component it depends on (for example from a producer for a custom partitioner), or the registered metrics reporters. As most plugins are configurable, a workaround is to reimplement the metric reporters logic and in some case for tags too but that is cumbersome. Also by creating a separate Metrics instance, these metrics are separate from the client's and in case multiple clients are running in the same JVM, for example multiple producers, it can be hard to identify the specific client that is associated with some plugin metrics.

This issue also applies to connectors and tasks in Kafka Connect. For example MirrorMaker2 creates its own Metrics object and has logic to add the metric reporters from the configuration.

In this proposal, a "plugin" is an interface users can implement and that is instantiated by Kafka. For example, a class implementing org.apache.kafka.server.policy.CreateTopicPolicy is considered a plugin as it's instantiated by brokers. On the other hand a class implementing org.apache.kafka.clients.producer.Callback is not considered a plugin as it's instantiated in user logic.

Public Interfaces

I propose introducing a new interface: Monitorable. If a plugin implements this interface, the withPluginMetrics() method will be called when the plugin is instantiated (after configure() if the plugin also implements Configurable). This will allow plugins to adds their own metrics to the component (producer, consumer, etc) that instantiated them.

package org.apache.kafka.common.metrics;

public interface Monitorable {

    /**
     * Get the PluginMetrics instance from the client that instantiates the plugin.
     */
    void withPluginMetrics(PluginMetrics metrics);

}

The PluginMetrics interface has methods to add and remove metrics and sensors. Plugins will only be able to remove metrics and sensors they created. Metrics created via this class will have their group set to "plugins" and include tags that uniquely identify the plugin.

public interface PluginMetrics extends Closeable {

    /**
     * Create a {@link MetricName} with the given name, description and tags. The group will be set to "plugins"
     * Tags to uniquely identify the plugins are automatically added to the provided tags
     *
     * @param name        The name of the metric
     * @param description A human-readable description to include in the metric
     * @param tags        additional key/value attributes of the metric
     */
    MetricName metricName(String name, String description, Map<String, String> tags);

    /**
     * Add a metric to monitor an object that implements {@link MetricValueProvider}. This metric won't be associated with any
     * sensor. This is a way to expose existing values as metrics.
     *
     * @param metricName The name of the metric
     * @param metricValueProvider The metric value provider associated with this metric
     * @throws IllegalArgumentException if a metric with same name already exists.
     */
    void addMetric(MetricName metricName, MetricValueProvider<?> metricValueProvider);

    /**
     * Remove a metric if it exists.
     *
     * @param metricName The name of the metric
     */
    void removeMetric(MetricName metricName);

    /**
     * Create a sensor with the given unique name. The name must only be unique for the plugin, so different plugins can use the same names.
     *
     * @param name The sensor name
      * @return The sensor
     * @throws IllegalArgumentException if a sensor with same name already exists for this plugin
     */
    Sensor sensor(String name);

    /**
     * Remove a sensor (if it exists) and its associated metrics.
     *
     * @param name The name of the sensor to be removed
     */
    void removeSensor(String name);
}

The PluginMetrics interface implements Closeable. Calling the close() method removed all metrics and sensors created by this plugin.

New methods will be added to AbstractConfig so new plugin instances implementing Monitorable can get a PluginMetrics instance.

/**
 * Get a configured instance of the give class specified by the given configuration key. If the object implements
 * Configurable configure it using the configuration. If the object implements Monitorable, set the appropriate PluginMetrics for that instance.
 *
 * @param key The configuration key for the class.
 * @param t The interface the class should implement.
 * @param metrics The metrics registry to use to build the PluginMetrics instance if the class implements Monitorable.
 * @return A configured instance of the class.
 */
public <T> T getConfiguredInstance(String key, Class<T> t, Metrics metrics);

/**
 * Get a configured instance of the give class specified by the given configuration key. If the object implements
 * Configurable configure it using the configuration. If the object implements Monitorable, set the appropriate PluginMetrics for that instance.
 *
 * @param key The configuration key for the class.
 * @param t The interface the class should implement.
 * @param configOverrides override origin configs.
 * @param metrics The metrics registry to use to build the PluginMetrics instance if the class implements Monitorable.
 * @return A configured instance of the class.
 */
public <T> T getConfiguredInstance(String key, Class<T> t, Map<String, Object> configOverrides, Metrics metrics);

/**
 * Get a list of configured instances of the given class specified by the given configuration key. The configuration
 * may specify either null or an empty string to indicate no configured instances. If an instance implements Monitorable, 
 * set the appropriate PluginMetrics for that instance. In both cases, this method returns an empty list to indicate no configured instances.
 * @param key The configuration key for the class.
 * @param t The interface the class should implement.
 * @param configOverrides Configuration overrides to use.
 * @param metrics The metrics registry to use to build the PluginMetrics instances for each class that implements Monitorable.
 * @return The list of configured instances.
 */
public <T> List<T> getConfiguredInstances(String key, Class<T> t, Map<String, Object> configOverrides, Metrics metrics);

/**
 * Get a list of configured instances of the given class specified by the given configuration key. The configuration
 * may specify either null or an empty string to indicate no configured instances. If an instance implements Monitorable, 
 * set the appropriate PluginMetrics for that instance. In both cases, this method returns an empty list to indicate no configured instances.
 * @param classNames The list of class names of the instances to create.
 * @param t The interface the class should implement.
 * @param configOverrides Configuration overrides to use.
 * @param metrics The metrics registry to use to build the PluginMetrics instances for each class that implements Monitorable.
 * @param configKey The value used to set the config tag for metrics from this plugin.
 * @return The list of configured instances.
 */
public <T> List<T> getConfiguredInstances(List<String> classNames, Class<T> t, Map<String, Object> configOverrides, Metrics metrics, String configKey)

Proposed Changes

When instantiating a class, if it implements Monitorable, the withPluginMetrics() method will be called. If the class is also Configurable, withPluginMetrics() will be always called after configure(). Metrics registered by plugins will inherit the prefix/namespace from the current Metrics instance, these are: kafka.producer, kafka.consumer, kafka.connect, kafka.streams and kafka.server. Tags will be added to metrics and sensors tags created via the PluginMetrics interface to unique identify each instance.

For all plugins apart from Connectors, Tasks, Converters, Transformations and Predicates, a tag containing the configuration name (config) will be added. For example, metrics registered by a custom Serializer named MySerializer configured via key.serializer will have the following name: kafka.producer:type=plugins,client-id=producer-1,class=MySerializer,config=key.serializer

For Connectors and Converters, the name of the connector (connector) will be added as a tag. Tasks will add the connector name and the task id (task) added as tags. Transformations and Predicates will have the connector name, the task id and their alias (alias) added as tags. For example for a task: kafka.connect:type=plugins,class=MyTask,connector=my-sink,task=0

For configurations that accept a list of classes, for example interceptor.classes, if the same class is provided multiple times, their metrics may collide. This is deemed highly unlikely to occur as there are no use cases where providing multiple times the same class is useful.

Supported plugins

The goal is for all plugins, apart from MetricsReporter and KafkaMetricsReporter (as these plugins are created before the Metrics instance), to support this feature:

Broker
- KafkaPrincipalBuilder: principal.builder.class
- ReplicaSelector: replica.selector.class
- AlterConfigPolicy: alter.config.policy.class.name
- Authorizer: authorizer.class.name
- ClientQuotaCallback: client.quota.callback.class
- CreateTopicPolicy: create.topic.policy.class.name

Producer
- Serializer: key.serializer, value.serializer
- Partitioner: partitioner.class
- ProducerInterceptor: interceptor.classes

Consumer
- Deserializer: key.deserializer, value.deserializer
- ConsumerPartitionAssignor: partition.assignment.strategy
- ConsumerInterceptor: interceptor.classes

Connect
- Converter: key.converter, value.converter
- ConnectorClientConfigOverridePolicy: connector.client.config.override.policy
- HeaderConverter: header.converter
- ConnectRestExtension: rest.extension.classes
- Connector
- Task
- Transformation
- Predicate

Streams
- DeserializationExceptionHandler: default.deserialization.exception.handler
- Serde: default.key.serde, default.list.key.serde.inner, default.list.key.serde.type, default.list.value.serde.inner, default.list.value.serde.type, windowed.inner.class.serde
- ProductionExceptionHandler: default.production.exception.handler
- TimestampExtractor: default.timestamp.extractor
- RocksDBConfigSetter: rocksdb.config.setter

Common
- ConfigProvider: config.providers
- AuthenticateCallbackHandler: sasl.client.callback.handler.class, sasl.login.callback.handler.class, sasl.server.callback.handler.class
- Login: sasl.login.class
- SslEngineFactory: ssl.engine.factory.class
- SecurityProviderCreator: security.providers

MirrorMaker

- ReplicationPolicy: replication.policy.class
- ConfigPropertyFilter: config.property.filter.class
- GroupFilter: group.filter.class
- TopicFilter: topic.filter.class
- ForwardingAdmin: forwarding.admin.class

Example usage

For example if we create a custom ProducerInterceptor

public class MyInterceptor<K, V> implements ProducerInterceptor<K, V>, Monitorable {

    private Sensor sensor;

    public void withPluginMetrics(PluginMetrics metrics) {
        sensor = metrics.sensor("onSend");
        MetricName rate = metrics.metricName("rate", "Average number of calls per second.", Collections.emptyMap());
        MetricName total = metrics.metricName("total", "Total number of calls.", Collections.emptyMap());
        sensor.add(rate, new Rate());
        sensor.add(total, new CumulativeCount());
    }

    @Override
    public ProducerRecord<K, V> onSend(ProducerRecord<K, V> record) {
        sensor.record();
        return record;
    }
    
    ...
}

If the producer using this plugin has its client-id set to producer-1, the metrics created by this plugin will have the following name: kafka.producer:type=plugins,client-id=producer-1,class=MySerializer,config=interceptor.classes and these attributes: rate and total.

Compatibility, Deprecation, and Migration Plan

This is a new feature so it has no impact on deprecation and does not need a migration plan. Plugins should be able to function even if their withPluginMetrics() method is not called.

Test Plan

This feature will be tested using unit and integration tests.

Rejected Alternatives

• Create a dedicated Metrics instance for plugins: A dedicated instance could have its own prefix/namespace (for example kafka.consumer.plugins). This would allow grouping metrics from all plugins but it requires instantiating another Metrics instance and new metrics reporters.
  
• Let plugins create their own Metrics instance: Instead of passing the Metrics instance to plugins we could pass all the values necessary (metrics reporters, configs, etc ...) to create and configure a Metrics instance. This is impractical as it requires passing a lot of values around and plugins still have to have logic to use them.
• Provide the Metrics instance to Kafka Connect Connectors and Tasks via their context: We would have 2 different mechanisms, one for Connectors/Tasks and one of all other plugins, for the same feature. Also using the Connector and Task contexts has an impact on compatibility.