[GitHub] [solr] HoustonPutman commented on pull request #1577: SOLR-16719: Let AffinityPlacementFactory have an anti-affinity label

["HoustonPutman (via GitHub)" <gi...@apache.org>]

HoustonPutman commented on PR #1577:
URL: https://github.com/apache/solr/pull/1577#issuecomment-1516879031

   > Let users configure the "spread", which is exactly what the PR is doing for now (new features such as the maxSkew could be added in the future)
   
   Yep 👍 
   
   > Make anti-affinity always "required" when configured: Attempt to spread the replicas, but fail if more than one replica is placed on the same affinity group.
   
   Yes, but have the requirement be configurable. So anti-affinity could have a parameter "maxAffinity" (awful name, we would need a better one), which is a rule that "the maximum replicas in this affinityGroup is `<maxAffinity>`". 
   
   > It probably makes sense to keep both features using the same system property for defining the affinity groups (or "domains"?), right? I see no reason to have both features enabled but using different labels.
   
   If we have both of these options available (anti-affinity and spread), I can imagine that they might be used with the different affinityGroup "keys". Like you would have spread setup for "availabilityZone" and anti-affinity setup for "rack".  In that case, you want to spread the nodes across availabilityZones as best as possible, and you want to avoid putting two replicas on the same rack.
   
   Thinking about it more given my answer above, I guess this does make more sense as a single option, with separate toggles like "maxSkew" and "maxAffinity" (still and awful name). And maybe we eventually allow for two of these to be configured.
   
   So in my example you would have the `spread` configured for both `availabilityZone` and `rack`. `availabilityZone` could have a `maxAffinity` of `-1` or `0`, meaning there is no issue with there being tons of replicas in that `availabilityZone`. But `rack` would have a `maxAffinity` of `2`, saying you don't want more than 2 replicas in a single `rack`. 
   
   I know that having multiple `affinityKeys` specified is not yet supported, just kind of thinking out loud. So in the end, I do think it makes sense to keep both features using the same system property, just in the future allow for defining multiple of them.
   
   


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