Metrics based custom scheduler plugin

[jp-sivaprasad]

Description

Metrics based custom scheduler plugin

Motivation and Context

Metrics based custom scheduler plugin. Schedules cluster with lowest number of queries (running + queued queries).
Support for implementation of third party custom schedulers to be used by Presto router.

Impact

Test Plan

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• Please make sure your submission complies with our contributing guide, in particular code style and commit standards.
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Release Notes

Please follow release notes guidelines and fill in the release notes below.

== RELEASE NOTES ==

Metrics based custom scheduler plugin Changes
* Add support for Metrics based custom scheduler plugin

[tdcmeehan]

You need to add the parent tag to make this module consistent with the others:

    <parent>
        <groupId>com.facebook.presto</groupId>
        <artifactId>presto-root</artifactId>
        <version>0.291-SNAPSHOT</version>
    </parent>

[steveburnett]

LGTM! (docs)

[tdcmeehan]

@auden-woolfson can you PTAL?

[auden-woolfson]

Would it be better to set this as a default value in the code, and leave this file empty?

[jp-sivaprasad]

The property name (router-scheduler.name) is added in the property file so that clients implementing a new scheduler plugin can just update the value to the name of the new scheduler. Also since the Metric based scheduler is just a sample implementation, it may not be good to set it as the default value in code.

[auden-woolfson]

Ok... maybe it would make more sense to add a new optional property to the existing router-config.json file ("custom-scheduler-name" or something similar) instead of creating an entirely new file for this one property. This way would also allow for customers to use multiple different custom schedulers in the future.

[jp-sivaprasad]

There can be more properties in future for new schedulers, like cpu/memory thresholds related to specific schedulers, which can use this property file

[steveburnett]

Thanks for the release note!

New release note guidelines. Please remove the manual PR link in the following format from the release note entries for this PR.

:pr:`12345`

I have updated the Release Notes Guidelines to remove the examples of manually adding the PR link.