Support multiple clusters in a single namespace

[vincepri]

/kind feature

On Wed 9 January 2019 meeting, the group discussed to potentially revert the assumption that clusters and machines living in the same namespace are tied together.

Some meetings notes:

This is the PR that implemented the cluster-machines namespace #481
Related: #177
Do we want a tighter relationship between clusters / machines so that we can have multiple clusters in a single namespace
[ashish] When I started the above PR, the consensus was that we wanted one cluster per namespace #252 (comment)
[ashish] more context on the issue #252
[vince] That is the current assumption. Do we want to make that official?
[robert] Hardik has mentioned that Gardener found it useful to put multiple clusters into a namespace so that things like references to secrets can be shared
[justinsb] We may want to get an api reviewer to take a look at our API and help us answer some of these types of questions. I'm not sure if there are other examples of singletons.
[detiber] If we create a link between the two we should be aware of the use case of using machines without clusters and we'd need to make the link optional
[robertbailey] We could use labels to match them up
[robertbailey] Even if there are examples of singletons, they certainly aren't commonplace
[justinsb] kops wouldn't object to creating a dummy cluster object if necessary to adopt the machines api
[robertbailey] the other use case is to put cluster and machine resources in different kubernetes universes; so then the link would also need to be optional
[vince] If we use the label, then providers can adopt them over time
[justinsb] Label selectors are great for many-many which doesn't work well here. Maybe have a field that just points to the owning cluster (even across universes). If it doesn't exist fall back to existing behavior of same-namespace

The goal of this issue is to get an agreement if we want to support multiple clusters in a single namespace. A followup issue, after this one is closed, will be opened to discuss implementations.

[vincepri]

/close

[k8s-ci-robot]

@vincepri: Closing this issue.

In response to this:

/close

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[zexi]

Do we have a plan to implement this feature?