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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

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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 add their own metrics to the component (broker, producer, consumer, etc) that instantiated them.

package org.apache.kafka.common.metrics;

public interface Monitorable 

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 {

    /**
     * Set 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 {

    /**
     * CreateSet athe {@linkPluginMetrics MetricName}instance withfrom the givencomponent name,that descriptioninstantiates andthe tagsplugin.
 The group will be set to "plugins"* Plugins can register and unregister metrics using the given PluginMetrics
     * Tagsat toany uniquelypoint identifyin thetheir pluginslifecycle areprior automaticallyto addedtheir toclose themethod providedbeing tagscalled.
     *
     * @paramPlugin namemust call the close() method on this PluginMetrics Theinstance namein of the metrictheir close()
     * @param description A human-readable description method to includeclear inall themetrics metric
     * @param tags        additional key/value attributes of the metricthey registered.
     */
    MetricNamevoid metricNamewithPluginMetrics(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.

/**
 * Implementations are thread safe so plugins may use the PluginMetrics instance from multiple threads
 */
public interface PluginMetrics extends Closeable {

    /*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.
     *
     * @paramCreate metricNamea The{@link nameMetricName} ofwith the metric
given name, description and tags. *The @paramgroup metricValueProviderwill Thebe metricset value provider associated with this metric
     * @throws IllegalArgumentException if a metric with same name already exists.to "plugins"
     * Tags to uniquely identify the plugins are automatically added to the provided tags
     */
    void addMetric(MetricName metricName, MetricValueProvider<?> metricValueProvider);

* @param name     /**
   The name *of Removethe a metric if it exists.
     *
 @param description A human-readable *description @paramto metricNameinclude Thein name of the metric
     * @param @throwstags IllegalArgumentException if the plugin has not already createdadditional akey/value metricattributes withof thisthe namemetric
     */
    voidMetricName removeMetric(MetricName metricNamemetricName(String name, String description, Map<String, String> tags);

    /**
     * CreateAdd a sensormetric with the given unique name. The name must only be unique for the plugin, so different plugins can use the same namesto 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 namemetricName The sensor name
 of the metric
     * @return@param metricValueProvider The sensor metric value provider associated with this metric
     * @throws IllegalArgumentException if a sensormetric 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
     * @throws IllegalArgumentException if the plugin has not already created a sensor with this name
     */
    void removeSensor(String name);
}

The PluginMetrics interface implements Closeable. Calling the close() method removed all metrics and sensors created by this plugin. It will be the responsibility of the plugin that create metrics to call close() of their PluginMetrics instance to remove their metrics.

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

.
     */
    void addMetric(MetricName metricName, MetricValueProvider<?> metricValueProvider);

    /**
     * Remove a metric if it exists.
     *
     * @param metricName The name of the metric
     * @throws IllegalArgumentException if the plugin has not already created a metric with this name
     */
    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
     * @throws IllegalArgumentException if the plugin has not already created a sensor with this name
     */
    void removeSensor(String name);
}

The PluginMetrics interface implements Closeable. Calling the close() method removed all metrics and sensors created by this plugin. It will be the responsibility of the plugin that creates metrics to call close() of the PluginMetrics instance they were given to remove their metrics.

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 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.
 * 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 overrideConfiguration overrides originto configsuse.
 * @param metrics The metrics registry to use to build the PluginMetrics instanceinstances iffor theeach class that implements Monitorable.
 * @return AThe configured instancelist of theconfigured classinstances.
 */
public <T> TList<T> getConfiguredInstancegetConfiguredInstances(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. classes.   
 * @param metricsclassNames The metrics registry to uselist of class names of the instances to build the PluginMetrics instances for eachcreate.
 * @param t The interface the class thatshould implements Monitorableimplement.
 * @return@param TheconfigOverrides listConfiguration ofoverrides configuredto instancesuse.
 */
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 key The configuration key for the classes.   
 * @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.
 * @return The list of configured instances.
 */
public <T> List<T> getConfiguredInstances(String key, List<String> classNames, Class<T> t, Map<String, Object> configOverrides, Metrics metrics)

The Converter interface will extend Closeable (like HeaderConverter already does), the logic of the Converter class stays untouched. The Connect runtime will be updated to call close() when the converter instances can be released.

public interface Converter extends Closeable {

    @Override
    default void close() throws IOException {
        // no-op
    }
}

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,config=key.serializer,class=MySerializer,

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

...

 @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, List<String> classNames, Class<T> t, Map<String, Object> configOverrides, Metrics metrics)

The Converter interface will extend Closeable (like HeaderConverter already does), the logic of the Converter class stays untouched. The Connect runtime will be updated to call close() when the converter instances can be released.

public interface Converter extends Closeable {

    @Override
    default void close() throws IOException {
        // no-op
    }
}

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,config=key.serializer,class=MySerializer,

For Connectors and Converters, the name of the connector will be added as a tag (connector). Tasks will add the name of the connector (connector) and the task id (task) as tags. Transformations and Predicates will have the connector name, the task id and their alias (transformation/predicate) added as tags. For example:

• for a task: kafka.connect:type=plugins,connector=my-sink,task=0
• for a predicate: kafka.connect:type=plugins,connector=my-sink,task=0,predicate=my-predicate

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.

This proposal supersedes KIP-608 which only aimed at providing this functionality to Authorizer plugins. KIP-608 was adopted in 2020 but never implemented. This KIP proposes to alter the names of the metrics from KIP-608 to the following so all plugins use the same naming format:

Supported plugins

The goal is allow this feature to be used by all plugins that are Closeable, AutoCloseable or have a close() methods apart from MetricsReporter since instances are created before the Metrics instance. Also all Connect connector plugins will be supported. The javadoc of all supported plugins will be updated to mention they are able to implement the Monitorable interface to define their own metrics.

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The following plugins will not support this feature:

If we decide that some of these plugins would benefit from being able to emit metrics, we could make the necessary API changes in the future to support them.

...

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

    private Sensor sensor;
    private PluginMetrics metrics;

    public void withPluginMetrics(PluginMetrics metrics) {
        this.metrics = 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;
    }

    @Override
    public void close() {
        try {
            if (metrics != null) metrics.close();
        } catch (IOException e) { // Even though ProducerInterceptor extends AutoCloseable which has "void close() throws Exception;", ProducerInterceptor has its own close() method without "throws"!
            throw new RuntimeException(e);
        }
    }
}

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