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Status

Current state: Draft

Discussion thread: here

JIRA: here [

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

Motivation

Kafka exposes many pluggable points for users to bring their custom plugins. For complex and critical plugins it's important to have metrics to monitor their behavior. Plugins can use the Metrics class from the Kafka API but when creating a new Metrics instance, it does not inherits the tags from the components 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 tags and metric reporters logic but that cumbersome.

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

Public Interfaces

I propose introducing a new interface Monitorable that plugins can implement.

package org.apache.kafka.common.metrics;

public interface Monitorable {

    /**
     * Get the Metrics instance from the component that instantiates the plugin.
     */
    void withMetrics(Metrics Metrics);

}

For connectors and tasks, I propose adding a metrics(Metrics metrics) method to the SinkConnectorContext, SourceConnectorContext, SinkTaskContext and SourceTaskContext interfaces. To

/**
 * 
 */
default void metrics(Metrics metrics) {
    return null;
}

Proposed Changes

When instantiating a class via the Utils.newInstance() helper methods, if it implements Monitorable and a Metrics object is available, useMetrics() will be called with the current Metrics instance. It will be always called after configure().

Compatibility, Deprecation, and Migration Plan

This is a new feature so it has no impact on deprecation and does not need a migration plan. Regarding compatibility, plugins and connectors that start using this feature will have to handle Metrics not being available to support older broker/Connect versions. For regular plugins they should be able to function without a call to useMetrics(), for connectors and tasks they should work even if the metrics() method returns null.

Test Plan

Describe in few sentences how the KIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.