Overlord incremental select worker strategy

[jakubmatyszewski]

Description

I was trying to use existing fillCapacity select worker strategy in an effort to implement MiddleManager autoscaling, but since MM instances that I use are created by k8s statefulsets it wasn't consistent enough. Proposed strategy fills capacity based on worker's hostname, with assumption that host is k8s FQDN - eg. druid-middlemanager-0.druid-middlemanager.druid-test.svc.cluster.local:8091.

Similar problem was discussed in #8801.
I'm not sure if this feature is desirable and this is my first java admission, so I'll take every bit of critique here. I can add tests if it will appear that this feature might be useful and we want to add it to codebase.

Key changed/added classes in this PR

• FillIncrementallyWorkerSelectStrategy
• FillIncrementallyWithAffinityWorkerSelectStrategy
• FillIncrementallyWithCategorySpecWorkerSelectStrategy

This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[abhishekagarwal87]

thank you for your contribution. can you elaborate a bit on following

but since MM instances that I use are created by k8s statefulsets it wasn't consistent enough.

what is not consistent?

[jakubmatyszewski]

thank you for your contribution. can you elaborate a bit on following

but since MM instances that I use are created by k8s statefulsets it wasn't consistent enough.

what is not consistent?

For statefulset to scale down we need to take into consideration that:

When Pods are being deleted, they are terminated in reverse order, from {N-1..0}.

But with existing fillCapacity overlord strategy it doesn't care on what host ordinal index it accumulates tasks - it just checks which middlemanager instance has highest currCapacityUsed value. So in scenarios when there are multiple middlemanagers with same amount of currCapacityUsed it seems like this strategy picks middlemanagers at random.

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