Provide explicit definition of variables scope in ClusterClass

[fabriziopandini]

User Story

As a user, I would like to understand what variables are meant to be used as overrides at CP/MD levels

Detailed Description

As of today, ClusterClass allows to define variables and all those variables can be used in a Cluster's topology.variables as well as for providing override at CP and MD level.

However, in practice, it really makes sense to use variables as override only a subset of variables, the ones that are used by patches impacting CP/MD of a given class.

This is currently hard to detect, especially for users which are not the ClusterClass authors, so it will be great if we can extend the current API by providing explicit information about the scope where a variable can be used.

Anything else you would like to add:

Might be it is possible to Inferr variables scope from patches, but it doesn't seem the right way to go because it will be complex and brittle.

/kind feature

[fabriziopandini]

/area topology

[fabriziopandini]

/kind api-change

[fabriziopandini]

/triage accepted

The biggest challenge here is to make sure that a variable that is meant to be used in scope is not used somewhere else, and this is complex given that variables can be used in go templates or in external patches.

[sbueringer]

 and this is complex given that variables can be used in go templates or in external patches.

With our current implementation of external patches it's impossible to decide if a variable is used in an external patch.

We could do the following:

• introduce scope
• validate inline patches against scope
• send variables based on scope to external patching (i.e. send variables which are not cluster-scoped only as template-specific variables according to their scope)

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/lifecycle stale

[fabriziopandini]

/lifecycle frozen
/help

still something worth to discuss and address

[k8s-ci-robot]

@fabriziopandini:
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In response to this:

/lifecycle frozen
/help

still something worth to discuss and address

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[fabriziopandini]

/priority important-longterm

[fabriziopandini]

Note about the current situation:
As of today users are to set a variable only at MD level, but not if the variable is required (in this case it is required to set the variable at top level too, and what happens is that CC authors make those variables optional).

This idea of scope could help to address this use case too.
At some point, we should also consider if the idea of overrides still holds (or to simply to have "local" variables)