xyuanlu commented on code in PR #3043:
URL: https://github.com/apache/helix/pull/3043#discussion_r2165637115


##########
helix-core/src/main/java/org/apache/helix/controller/stages/MessageGenerationPhase.java:
##########
@@ -290,7 +325,67 @@ private void generateMessage(final Resource resource, 
final BaseControllerDataPr
     } // end of for-each-partition
   }
 
-  private boolean shouldCreateSTCancellation(Message pendingMessage, String 
desiredState,
+  /**
+   * Calculate the current active replica count based on state model type.
+   * The count includes replicas in top states, secondary top states 
(excluding OFFLINE),
+   * and ERROR states since helix considers them active.Count excludes OFFLINE 
and DROPPED states.
+   * @param currentStateMap Map of instance name to current state for this 
partition, representing
+   *          the actual state of each replica before any pending transitions
+   * @param stateModelDef State model definition containing the state 
hierarchy and transition rules
+   *          used to determine which states are considered active
+   * @return The number of replicas currently in active states, used to 
determine the
+   *         currentActiveReplicaNumber for the partition.
+   */
+  private int calculateCurrentActiveReplicaCount(Map<String, String> 
currentStateMap,
+      StateModelDefinition stateModelDef) {
+    List<String> activeSecondaryTopStates = 
getActiveSecondaryTopStates(stateModelDef);
+    return (int) currentStateMap.values().stream()
+        .filter(state -> stateModelDef.getTopState().contains(state) // Top 
states (MASTER, ONLINE,
+                                                                     // LEADER)
+            || activeSecondaryTopStates.contains(state) // Active secondary 
states (SLAVE, STANDBY,
+                                                      // BOOTSTRAP)
+            || HelixDefinedState.ERROR.name().equals(state) // ERROR states 
(still considered
+                                                            // active)
+        // DROPPED and OFFLINE are automatically excluded by 
getActiveSecondaryTopStates()
+        ).count();
+  }
+
+  /**
+   * Get active secondary top states - states that are not non-serving states 
like OFFLINE and DROPPED.
+   * Reasons for elimination:
+   * - getSecondTopStates() can include OFFLINE as a secondary top state in 
some state models.
+   * Example - OnlineOffline:
+   * getSecondTopStates() = ["OFFLINE"] as it transitions to ONLINE.
+   * After filtering: activeSecondaryTopStates=[] (removes "OFFLINE" as it's 
not a serving state).
+   * @param stateModelDef State model definition containing state hierarchy 
information
+   */
+  private List<String> getActiveSecondaryTopStates(StateModelDefinition 
stateModelDef) {
+    return stateModelDef.getSecondTopStates().stream()
+        // Remove non-serving states
+        .filter(state -> !OFFLINE.equals(state) && 
!HelixDefinedState.DROPPED.name().equals(state))
+        .collect(Collectors.toList());
+  }
+
+  /**
+   * Determines if the given state is considered active based on the state 
model type.
+   * Active states include: top states, active secondary top states (excluding 
OFFLINE),
+   * and ERROR states. Active states exclude OFFLINE and DROPPED states.
+   * ERROR state replicas are always considered active in HELIX as they do not
+   * affect availability.
+   * @param state The state to check (can be current state or target state)
+   * @param stateModelDef State model definition containing state hierarchy 
information
+   * @return true if the state is considered active, false otherwise
+   */
+  private boolean isStateActive(String state, StateModelDefinition 
stateModelDef) {

Review Comment:
   nit:
   alignment



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