Add concept doc on how exclusive placement works

[danielvegamyhre]

What would you like to be added:

Add concept doc on how exclusive placement works

Why is this needed:

Users often have questions about how it works and it is helpful info for them to be able to debug issues on their own

[k8s-triage-robot]

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/lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all issues.

This bot triages un-triaged issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 30d of inactivity since lifecycle/rotten was applied, the issue is closed

You can:

• Mark this issue as fresh with /remove-lifecycle rotten
• Close this issue with /close
• Offer to help out with Issue Triage

Please send feedback to sig-contributor-experience at kubernetes/community.

/lifecycle rotten

[danielvegamyhre]

/remove-lifecycle stale

[danielvegamyhre]

/remove-lifecycle rotten

[googs1025]

/assign
I will try this!

[googs1025]

Is this issue meant to demonstrate how to use and operate, or is it to be explained through code?

[danielvegamyhre]

__

Is this issue meant to demonstrate how to use and operate, or is it to be explained through code?

It's meant to explain how exclusive placement is implemented. I.e., a leader pod (index 0) from each Job is created, with pod affinity/anti-affinity rules ensuring only 1 leader pod lands in each topology domain (e.g., rack, node pool, etc). All follower pods (non-0 indexes) are blocked from creation until their corresponding leader pod has been scheduled. Once a leader pod from a job is scheduled, the follower pods in that Job will be created and have nodeSelectors injected which ensure they land in the same topology domain as the leader.

[googs1025]

__

Is this issue meant to demonstrate how to use and operate, or is it to be explained through code?

It's meant to explain how exclusive placement is implemented. I.e., a leader pod (index 0) from each Job is created, with pod affinity/anti-affinity rules ensuring only 1 leader pod lands in each topology domain (e.g., rack, node pool, etc). All follower pods (non-0 indexes) are blocked from creation until their corresponding leader pod has been scheduled. Once a leader pod from a job is scheduled, the follower pods in that Job will be created and have nodeSelectors injected which ensure they land in the same topology domain as the leader.

OK, I understand, I will also demonstrate with examples