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[placement][ptg] Aggregate on root spans whole tree policy:


On Apr 9, 2019, at 8:00 AM, Chris Dent <cdent+os at anticdent.org> wrote:
> 
> * Last PTG for Stein, we decided the following policies and have done so in Stein
> 
>    A) Aggregate on root spans whole tree for ``members_of=``
>       requests in 'GET /allocation_candidates'
>    B) This spanning policy doesn't apply to granular requests
>       ``members_of<N>=`` or to requests in 'GET /resource_providers'
>    C) This change is a bug fix without microversion
> 
>  However, I now feel the policy B is weird. Consider a case where
>  only granular requests are used in the request. If operator puts
>  aggA on root, aggA applies the child or not depends on cases how
>  you created the request. That's very difficult for operators to
>  debug...

It seems that a lot of the other efforts around trees have the goal of keeping trees and subtrees more integral, rather than separate pieces. Given that, it would make sense that if the root is in an aggregate, the entire tree is in the agg.

I'm trying to remember why we decided on policy B, but my brain is failing me. 


-- Ed Leafe