Evaluate options for controlling HPA-based scaling

[amshuman-kr]

The current implementation of the HVPA controller only controls VPA-based scaling. This is because VPA provides the updatePolicy: Off option to switch off VPA updates and only consume recommendations.

Some of the alternatives for a similar controlled HPA-based scaling are below with some preliminary evaluation.

1. Enhance HPA spec with a updatePolicy with the functionality similar to VPA.
   • Pros
     • Better/Cleaner API
   • Cons
     • Change in API might take a long time to decide and become available
2. Introduce a Scale sub-resource in the HVPA resource to act as a proxy scaleTargetRef for the HPA.
   • Pros
     • Can be done already with existing HPA
   • Cons
     • Not so clean API. The dependency/aggregation/composition hierarchy is mixed up.
     • Selector will have to be replicated from the actual scaleTargetRef.
   • Questions
     • What happens if the HPA's recommendation is not applied and hence, not reflected in the Scale sub-resource's status?

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