New Feature: Multi Cluster TargetGroupBindings

[zac-nixon]

Issue

#2173

Description

Initial approach done here, but didn't handle all use cases such as instance target groups or clusters that share subnets. This new approach will work with IP and Instance targets and makes no assumptions on the clusters' composition, subnets, etc.

This PR adds a new flag to the TGB CR that allows users to specify that a TGB TG ARN might be associated to more than one cluster. This flag can be set from ingress / svc annotations as well for managed TGBs.

The basis of the solution is to maintain a per TGB config map that tracks what targets have been registered in the ELB API. When performing deregistrations, we first check the TGB config map and filter out any targets that haven't been registered in the TGB config map. We maintain an in-memory cache of this config map in order to reduce reads to the k8s API. Every reconcile cycle that changes the endpoints will also incur a write of this config map.

I've placed some warnings in the documentation about potential pitfalls of this solution, such as moving an existing TGB to multicluster support could potentially leak target deregistrations while the config map state is generated. The other warning is around flipping between multicluster / non-multicluster for the same reason.

I've load tested this feature and on a relatively big target group of 150 targets there is roughly 25 additional ms of latency during reconciles due to the write of the CM. This 25ms latency represented about a 5% latency increase in my cluster.

Tests done

• IP targets in multiple clusters
• Instance targets in multiple clusters
• NLB w/ shared target groups
• ALB w/ shared target groups.
• Load test with moderately size TGB (150 replicas)

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[k8s-ci-robot]

Hi @zac-nixon. Thanks for your PR.

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[shraddhabang]

/ok-to-test

[zac-nixon]

/retest

[zac-nixon]

/retest

[shraddhabang]

/lgtm

[shraddhabang]

/lgtm

[shraddhabang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: shraddhabang, zac-nixon
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