somandal commented on code in PR #15617:
URL: https://github.com/apache/pinot/pull/15617#discussion_r2086941117


##########
pinot-controller/src/main/java/org/apache/pinot/controller/helix/core/rebalance/TableRebalancer.java:
##########
@@ -1524,67 +1535,276 @@ private static void handleErrorInstance(String 
tableNameWithType, String segment
     }
   }
 
+  /**
+   * Uses the default LOGGER
+   */
+  @VisibleForTesting
+  static Map<String, Map<String, String>> getNextAssignment(Map<String, 
Map<String, String>> currentAssignment,
+      Map<String, Map<String, String>> targetAssignment, int 
minAvailableReplicas, boolean enableStrictReplicaGroup,
+      boolean lowDiskMode, int batchSizePerServer, 
Object2IntOpenHashMap<String> segmentPartitionIdMap,
+      PartitionIdFetcher partitionIdFetcher) {
+    return getNextAssignment(currentAssignment, targetAssignment, 
minAvailableReplicas, enableStrictReplicaGroup,
+        lowDiskMode, batchSizePerServer, segmentPartitionIdMap, 
partitionIdFetcher, LOGGER);
+  }
+
   /**
    * Returns the next assignment for the table based on the current assignment 
and the target assignment with regard to
    * the minimum available replicas requirement. For strict replica-group 
mode, track the available instances for all
    * the segments with the same instances in the next assignment, and ensure 
the minimum available replicas requirement
    * is met. If adding the assignment for a segment breaks the requirement, 
use the current assignment for the segment.
+   * <p>
+   * For strict replica group routing only (where the segment assignment is 
not StrictRealtimeSegmentAssignment)
+   * if batching is enabled, the instances assigned for the same partitionId 
can be different for different segments.
+   * For strict replica group routing with StrictRealtimeSegmentAssignment on 
the other hand, the assignment for a given
+   * partitionId will be the same across all segments. We can treat both cases 
similarly by creating a mapping from
+   * partitionId -> unique set of instance assignments -> currentAssignment. 
With StrictRealtimeSegmentAssignment,
+   * this map will have a single entry for 'unique set of instance 
assignments'.
+   * <p>
+   * TODO: Ideally if strict replica group routing is enabled then 
StrictRealtimeSegmentAssignment should be used, but
+   *       this is not enforced in the code today. Once enforcement is added, 
then the routing side and assignment side
+   *       will be equivalent and all segments belonging to a given 
partitionId will be assigned to the same set of
+   *       instances. Special handling to check each group of assigned 
instances can be removed in that case.
    */
-  @VisibleForTesting
-  static Map<String, Map<String, String>> getNextAssignment(Map<String, 
Map<String, String>> currentAssignment,
+  private static Map<String, Map<String, String>> 
getNextAssignment(Map<String, Map<String, String>> currentAssignment,
       Map<String, Map<String, String>> targetAssignment, int 
minAvailableReplicas, boolean enableStrictReplicaGroup,
-      boolean lowDiskMode) {
-    return enableStrictReplicaGroup ? 
getNextStrictReplicaGroupAssignment(currentAssignment, targetAssignment,
-        minAvailableReplicas, lowDiskMode)
+      boolean lowDiskMode, int batchSizePerServer, 
Object2IntOpenHashMap<String> segmentPartitionIdMap,
+      PartitionIdFetcher partitionIdFetcher, Logger tableRebalanceLogger) {
+    return enableStrictReplicaGroup
+        ? getNextStrictReplicaGroupAssignment(currentAssignment, 
targetAssignment, minAvailableReplicas, lowDiskMode,
+        batchSizePerServer, segmentPartitionIdMap, partitionIdFetcher, 
tableRebalanceLogger)
         : getNextNonStrictReplicaGroupAssignment(currentAssignment, 
targetAssignment, minAvailableReplicas,
-            lowDiskMode);
+            lowDiskMode, batchSizePerServer);
   }
 
   private static Map<String, Map<String, String>> 
getNextStrictReplicaGroupAssignment(
       Map<String, Map<String, String>> currentAssignment, Map<String, 
Map<String, String>> targetAssignment,
-      int minAvailableReplicas, boolean lowDiskMode) {
+      int minAvailableReplicas, boolean lowDiskMode, int batchSizePerServer,
+      Object2IntOpenHashMap<String> segmentPartitionIdMap, PartitionIdFetcher 
partitionIdFetcher,
+      Logger tableRebalanceLogger) {
     Map<String, Map<String, String>> nextAssignment = new TreeMap<>();
     Map<String, Integer> numSegmentsToOffloadMap = 
getNumSegmentsToOffloadMap(currentAssignment, targetAssignment);
+    Map<Integer, Map<Set<String>, Map<String, Map<String, String>>>>
+        partitionIdToAssignedInstancesToCurrentAssignmentMap;
+    if (batchSizePerServer == RebalanceConfig.DISABLE_BATCH_SIZE_PER_SERVER) {
+      // Don't calculate the partition id to assigned instances to current 
assignment mapping if batching is disabled
+      // since we want to update the next assignment based on all partitions 
in this case. Use partitionId as 0
+      // and a dummy set for the assigned instances.
+      partitionIdToAssignedInstancesToCurrentAssignmentMap = new TreeMap<>();
+      partitionIdToAssignedInstancesToCurrentAssignmentMap.put(0, new 
HashMap<>());
+      
partitionIdToAssignedInstancesToCurrentAssignmentMap.get(0).put(Set.of(""), 
currentAssignment);
+    } else {
+      partitionIdToAssignedInstancesToCurrentAssignmentMap =
+          
getPartitionIdToAssignedInstancesToCurrentAssignmentMap(currentAssignment, 
segmentPartitionIdMap,
+              partitionIdFetcher);
+    }
     Map<Pair<Set<String>, Set<String>>, Set<String>> assignmentMap = new 
HashMap<>();
     Map<Set<String>, Set<String>> availableInstancesMap = new HashMap<>();
-    for (Map.Entry<String, Map<String, String>> entry : 
currentAssignment.entrySet()) {
-      String segmentName = entry.getKey();
-      Map<String, String> currentInstanceStateMap = entry.getValue();
-      Map<String, String> targetInstanceStateMap = 
targetAssignment.get(segmentName);
-      SingleSegmentAssignment assignment =
-          getNextSingleSegmentAssignment(currentInstanceStateMap, 
targetInstanceStateMap, minAvailableReplicas,
-              lowDiskMode, numSegmentsToOffloadMap, assignmentMap);
-      Set<String> assignedInstances = assignment._instanceStateMap.keySet();
-      Set<String> availableInstances = assignment._availableInstances;
-      availableInstancesMap.compute(assignedInstances, (k, 
currentAvailableInstances) -> {
-        if (currentAvailableInstances == null) {
-          // First segment assigned to these instances, use the new assignment 
and update the available instances
-          nextAssignment.put(segmentName, assignment._instanceStateMap);
-          updateNumSegmentsToOffloadMap(numSegmentsToOffloadMap, 
currentInstanceStateMap.keySet(), k);
-          return availableInstances;
-        } else {
-          // There are other segments assigned to the same instances, check 
the available instances to see if adding the
-          // new assignment can still hold the minimum available replicas 
requirement
-          availableInstances.retainAll(currentAvailableInstances);
-          if (availableInstances.size() >= minAvailableReplicas) {
-            // New assignment can be added
+
+    Map<String, Integer> serverToNumSegmentsAddedSoFar = new HashMap<>();
+    for (Map<Set<String>, Map<String, Map<String, String>>> 
assignedInstancesToCurrentAssignment
+        : partitionIdToAssignedInstancesToCurrentAssignmentMap.values()) {
+      boolean anyServerExhaustedBatchSize = false;
+      if (batchSizePerServer != RebalanceConfig.DISABLE_BATCH_SIZE_PER_SERVER) 
{
+        // Check if the servers of the first assignment for each unique set of 
assigned instances has any space left
+        // to move this partition. If so, let's mark the partitions as to be 
moved, otherwise we mark the partition
+        // as a whole as not moveable.
+        for (Map<String, Map<String, String>> curAssignment : 
assignedInstancesToCurrentAssignment.values()) {
+          Map.Entry<String, Map<String, String>> firstEntry = 
curAssignment.entrySet().iterator().next();
+          // All segments should be assigned to the same set of servers so it 
is enough to check for whether any server
+          // for one segment is above the limit or not
+          Map<String, String> firstEntryInstanceStateMap = 
firstEntry.getValue();
+          SingleSegmentAssignment firstAssignment =
+              getNextSingleSegmentAssignment(firstEntryInstanceStateMap, 
targetAssignment.get(firstEntry.getKey()),
+                  minAvailableReplicas, lowDiskMode, numSegmentsToOffloadMap, 
assignmentMap);
+          Set<String> serversAdded = 
getServersAddedInSingleSegmentAssignment(firstEntryInstanceStateMap,
+              firstAssignment._instanceStateMap);
+          for (String server : serversAdded) {
+            if (serverToNumSegmentsAddedSoFar.getOrDefault(server, 0) >= 
batchSizePerServer) {
+              anyServerExhaustedBatchSize = true;
+              break;
+            }
+          }
+          if (anyServerExhaustedBatchSize) {
+            break;
+          }
+        }
+      }
+      for (Map<String, Map<String, String>> curAssignment : 
assignedInstancesToCurrentAssignment.values()) {
+        getNextAssignmentForPartitionIdStrictReplicaGroup(curAssignment, 
targetAssignment, nextAssignment,
+            anyServerExhaustedBatchSize, minAvailableReplicas, lowDiskMode, 
numSegmentsToOffloadMap, assignmentMap,
+            availableInstancesMap, serverToNumSegmentsAddedSoFar);
+      }
+    }
+
+    checkIfAnyServersAssignedMoreSegmentsThanBatchSize(batchSizePerServer, 
serverToNumSegmentsAddedSoFar,
+        tableRebalanceLogger);
+    return nextAssignment;
+  }
+
+  private static void 
getNextAssignmentForPartitionIdStrictReplicaGroup(Map<String, Map<String, 
String>> curAssignment,
+      Map<String, Map<String, String>> targetAssignment, Map<String, 
Map<String, String>> nextAssignment,
+      boolean anyServerExhaustedBatchSize, int minAvailableReplicas, boolean 
lowDiskMode,
+      Map<String, Integer> numSegmentsToOffloadMap, Map<Pair<Set<String>, 
Set<String>>, Set<String>> assignmentMap,
+      Map<Set<String>, Set<String>> availableInstancesMap, Map<String, 
Integer> serverToNumSegmentsAddedSoFar) {
+    if (anyServerExhaustedBatchSize) {
+      // Exhausted the batch size for at least 1 server, just copy over the 
remaining segments as is
+      for (Map.Entry<String, Map<String, String>> entry : 
curAssignment.entrySet()) {
+        String segmentName = entry.getKey();
+        Map<String, String> currentInstanceStateMap = entry.getValue();
+        nextAssignment.put(segmentName, currentInstanceStateMap);
+      }
+    } else {
+      // Process all the partitionIds even if segmentsAddedSoFar becomes 
larger than batchSizePerServer
+      // Can only do bestEfforts w.r.t. StrictReplicaGroup since a whole 
partition must be moved together for
+      // maintaining consistency
+      for (Map.Entry<String, Map<String, String>> entry : 
curAssignment.entrySet()) {

Review Comment:
   sure, let's keep the current for now, can revisit this later if needed



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