jackjlli commented on code in PR #11953:
URL: https://github.com/apache/pinot/pull/11953#discussion_r1396731814


##########
pinot-controller/src/main/java/org/apache/pinot/controller/helix/core/assignment/instance/InstanceReplicaGroupPartitionSelector.java:
##########
@@ -73,16 +76,65 @@ public void selectInstances(Map<Integer, 
List<InstanceConfig>> poolToInstanceCon
       Map<Integer, List<Integer>> poolToReplicaGroupIdsMap = new TreeMap<>();
       Map<Integer, Integer> replicaGroupIdToPoolMap = new TreeMap<>();
       Map<Integer, Set<String>> poolToCandidateInstancesMap = new TreeMap<>();
-      for (int replicaId = 0; replicaId < numReplicaGroups; replicaId++) {
-        // Pick one pool for each replica-group based on the table name hash
-        int pool = pools.get((tableNameHash + replicaId) % numPools);
-        poolToReplicaGroupIdsMap.computeIfAbsent(pool, k -> new 
ArrayList<>()).add(replicaId);
-        replicaGroupIdToPoolMap.put(replicaId, pool);
+      Map<String, Integer> instanceToPoolMap = new HashMap<>();
+      for (Map.Entry<Integer, List<InstanceConfig>> entry : 
poolToInstanceConfigsMap.entrySet()) {
+        Integer pool = entry.getKey();
+        List<InstanceConfig> instanceConfigsInPool = entry.getValue();
+        Set<String> candidateInstances = 
poolToCandidateInstancesMap.computeIfAbsent(pool, k -> new LinkedHashSet<>());
+        for (InstanceConfig instanceConfig : instanceConfigsInPool) {
+          String instanceName = instanceConfig.getInstanceName();
+          candidateInstances.add(instanceName);
+          instanceToPoolMap.put(instanceName, pool);
+        }
+      }
 
-        Set<String> candidateInstances =
-            poolToCandidateInstancesMap.computeIfAbsent(pool, k -> new 
LinkedHashSet<>());
-        List<InstanceConfig> instanceConfigsInPool = 
poolToInstanceConfigsMap.get(pool);
-        instanceConfigsInPool.forEach(k -> 
candidateInstances.add(k.getInstanceName()));
+      if (_minimizeDataMovement && _existingInstancePartitions != null) {
+        // Keep the same pool for the replica group if it's already been used 
for the table.
+        int existingNumPartitions = 
_existingInstancePartitions.getNumPartitions();
+        int existingNumReplicaGroups = 
_existingInstancePartitions.getNumReplicaGroups();
+        int numCommonReplicaGroups = Math.min(numReplicaGroups, 
existingNumReplicaGroups);

Review Comment:
   The reason of checking the common replica groups is that it's not always the 
case that numReplicaGroups gets incremented. If the number of replica groups is 
reduced, we don't actually care what was used for the stale RG which is no 
longer needed. That's why the common one is used here.
   
   The scenarios you mentioned can be covered by using the min heap in Line 
112, which is to gather the pool number as well as the number of times to be 
chosen. The one with the least frequent usage would always be chosen to assign 
to a RG. Keep in mind that 1 RG can only have 1 pool (RG -> pool), while 1 pool 
may have more than 1 RG (pool -> [RG]).



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