xiangfu0 commented on code in PR #18501:
URL: https://github.com/apache/pinot/pull/18501#discussion_r3281051841


##########
pinot-controller/src/main/java/org/apache/pinot/controller/validation/RealtimeOffsetAutoResetManager.java:
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@@ -105,36 +112,81 @@ protected void processTable(String tableNameWithType, 
RealtimeOffsetAutoResetMan
       return;
     }
 
+    // Skip triggering if the controller is already handling the maximum 
number of concurrent backfills
+    int maxConcurrent = 
_controllerConf.getMaxConcurrentBackfillsPerController();
+    if (context._shouldTriggerBackfillJobs && maxConcurrent > 0
+        && _tableBackfillTopics.size() >= maxConcurrent) {
+      LOGGER.warn("Skipping backfill trigger for table {} — max concurrent 
backfills ({}) reached",
+          tableNameWithType, maxConcurrent);
+      _controllerMetrics.addMeteredTableValue(tableNameWithType,
+          ControllerMeter.OFFSET_AUTO_RESET_BACKFILL_SKIPPED_MAX_CONCURRENT, 
1L);
+      context._shouldTriggerBackfillJobs = false;
+    }
+
     if (context._shouldTriggerBackfillJobs) {
       _tableTopicsUnderBackfill.putIfAbsent(tableNameWithType, 
ConcurrentHashMap.newKeySet());
       String topicName = 
context._backfillJobProperties.get(Constants.RESET_OFFSET_TOPIC_NAME);
+      String partitionStr = 
context._backfillJobProperties.get(Constants.RESET_OFFSET_TOPIC_PARTITION);
       _tableTopicsUnderBackfill.get(tableNameWithType).add(topicName);
 
+      // Per-partition in-flight guard: _tableBackfillTopics contains the 
backfill Kafka topic names
+      // (not the main topic names), so any non-empty set indicates a backfill 
is already in flight.
+      // Track collisions per (table, topic, partition); auto-pause if 
collisions exceed the threshold.
+      String partitionKey = tableNameWithType + ":" + topicName + ":" + 
partitionStr;
+      Set<String> activeBackfillTopics = 
_tableBackfillTopics.get(tableNameWithType);
+      boolean anyBackfillInFlight = activeBackfillTopics != null && 
!activeBackfillTopics.isEmpty();

Review Comment:
   This breaker is described as per-partition, but the guard is actually 
table-wide: `anyBackfillInFlight` becomes true whenever *any* backfill topic 
exists for the table. After that, every unrelated `(topic, partition)` trigger 
increments its own collision counter and can auto-pause its source topic even 
though there was no collision on that partition. The check needs to prove that 
the active backfill matches the same source topic/partition, not just that the 
table has some backfill running.



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