✨ Allow clusters without explicit availability zones

[mkjpryor]

What this PR does / why we need it:

This PR adds the ability to create clusters without explicitly setting availability zones. The use case is discussed in detail in #1252.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #1252

Special notes for your reviewer:

Adds an additional, backwards-compatible flag to the OpenStack cluster spec.

TODOs:

• [ ] squashed commits
• if necessary:
  • [ ] includes documentation
  • [ ] adds unit tests

/hold

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[k8s-ci-robot]

Hi @mkjpryor. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[mkjpryor]

@mdbooth

Turns out it was basically as easy as I thought. This works like a dream for me. Can I get an /ok-to-test please?

[apricote]

/ok-to-test

[mkjpryor]

/retest

[mkjpryor]

@jichenjc

I added some docs for the new option - can you review and suggest changes if required?

[mkjpryor]

@mdbooth

It might be an idea to merge the workers part of this fix separately. It's self-contained and very simple.

Happy to do this.

1. We're working round behaviour which is defined by CAPI. We should discuss this with CAPI before making an API change in case they have any better ideas/imminent plans.

I'm not actually sure that we are. The InfraCluster.status.failureDomains field is explicitly optional in the spec (see https://cluster-api.sigs.k8s.io/developer/providers/cluster-infrastructure.html#infracluster-resources) and all this flag does is explicitly say that we don't care about AZs.

However I don't disagree with your comment that there might be a better approach.

On that second point, I have in mind something like:

  failureDomainModel: (AvailabilityZone|ServerGroup|None)

instead of IgnoreFailureDomain. This is barely a half-baked thought so read nothing into the detail of it, but the critical difference is that it defines what it is rather than what it is not.

This could actually work quite well - the only other thing I can think of is host aggregates.

I guess for my specific case I would use failureDomain: ServerGroup which would put the control plane nodes in a server group with either soft-anti-affinity or anti-affinity policies (could be configurable). The way this could work in code is:

1. OpenStackCluster reconciliation in CAPO creates a server group
2. The ID of the server group is reported using OpenStackCluster.status.failureDomains with the flag that identifies it as suitable for control plane nodes
3. This will cause CAPI to create control plane nodes with the server group ID as the failureDomain
4. CAPO knows to use the failureDomain as the server group when creating the server

What do you think?

[mkjpryor]

And I guess failureDomainModel: None would be basically what I have implemented when ignoreAvailabilityZones: true.

[mkjpryor]

@mdbooth

What if I change this PR to have failureDomainModel: AvailabilityZone | None instead of the flag, leaving us open for additional modes in the future?

Then submit another PR for #1256 that implements failureDomainModel: ServerGroup.

How does that sound as a plan?

[k8s-ci-robot]

@mkjpryor: PR needs rebase.

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/remove-lifecycle stale

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