jackjlli commented on pull request #8441:
URL: https://github.com/apache/pinot/pull/8441#issuecomment-1085383347


   > I don't follow why we need to store the pool to instances mapping in the 
ZK. Essentially what we need is to pass the existing instance partitions to the 
`InstanceReplicaGroupPartitionSelector.selectInstances()`, and add a flag to 
enable `minimizeMovement` and use a greedy algorithm to retain as much 
instances as possible from the candidate instances from each pool.
   
   Yes the initial plan is to reuse the existing instance partitions, but the 
thing is that for the instance which no longer exists in the current list of 
instance configs (which is gone), there is no way to know which pool it came 
from; i.e. even though we know there is an empty seat there, we don't know 
which pool we should select the new instance from.
   As long as we can control the input parameters of 
`replicaPartitionSelector.selectInstances()` (the input instancePartitions is 
always empty, thus as long as we control the instance sequence of 
`poolToInstanceConfigsMap`), we can always get the deterministic 
instancePartitions as output.


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