liuchang0520 commented on a change in pull request #6778:
URL: https://github.com/apache/pinot/pull/6778#discussion_r701506788



##########
File path: 
pinot-controller/src/main/java/org/apache/pinot/controller/helix/core/realtime/PinotLLCRealtimeSegmentManager.java
##########
@@ -1214,4 +1287,147 @@ private int 
getMaxNumPartitionsPerInstance(InstancePartitions instancePartitions
       return (numPartitions + numInstancesPerReplicaGroup - 1) / 
numInstancesPerReplicaGroup;
     }
   }
+
+  // Pre-fetch the LLC segments without deep store copy.
+  public void prefetchLLCSegmentsWithoutDeepStoreCopy(String 
tableNameWithType) {
+      TableType tableType = 
TableNameBuilder.getTableTypeFromTableName(tableNameWithType);
+      if (tableType != TableType.REALTIME) {
+        return;
+      }
+
+      TableConfig tableConfig = 
_helixResourceManager.getTableConfig(tableNameWithType);
+      if (tableConfig == null) {
+        LOGGER.warn("Failed to find table config for table: {}", 
tableNameWithType);
+        return;
+      }
+
+      PartitionLevelStreamConfig streamConfig = new 
PartitionLevelStreamConfig(tableConfig.getTableName(),
+          IngestionConfigUtils.getStreamConfigMap(tableConfig));
+      if (!streamConfig.hasLowLevelConsumerType()) {
+        return;
+      }
+
+      long currentTimeMs = getCurrentTimeMs();
+      List<String> segmentNames = 
ZKMetadataProvider.getLLCRealtimeSegments(_propertyStore, tableNameWithType);
+      for (String segmentName : segmentNames) {
+        try {
+          if (!isLLCSegmentWithinValidationRange(segmentName, currentTimeMs)) {
+            continue;
+          }
+
+          LLCRealtimeSegmentZKMetadata segmentZKMetadata = 
getSegmentZKMetadata(tableNameWithType, segmentName, new Stat());
+          // Cache the committed LLC segments without segment store download 
url
+          if (segmentZKMetadata.getStatus() == Status.DONE &&
+              
CommonConstants.Segment.METADATA_URI_FOR_PEER_DOWNLOAD.equals(segmentZKMetadata.getDownloadUrl()))
 {
+            cacheLLCSegmentNameForUpload(tableNameWithType, segmentName);
+          }
+        } catch (Exception e) {
+          _controllerMetrics.addValueToTableGauge(tableNameWithType,
+              
ControllerGauge.NUMBER_OF_ERRORS_FOR_LLC_SEGMENTS_ZK_METADATA_PREFETCH, 1L);
+          LOGGER.error("Failed to fetch the LLC segment {} ZK metadata", 
segmentName);
+        }
+      }
+  }
+
+  /**
+   * Only validate recently created LLC segment for missing deep store 
download url.
+   * The time range check is based on segment name. This step helps to 
alleviate ZK access.
+   */
+  private boolean isLLCSegmentWithinValidationRange(String segmentName, long 
currentTimeMs) {
+    LLCSegmentName llcSegmentName = new LLCSegmentName(segmentName);
+    long creationTimeMs = llcSegmentName.getCreationTimeMs();
+    return currentTimeMs - creationTimeMs < 
_validationRangeForLLCSegmentsDeepStoreCopyMs;
+  }
+
+  /**
+   * Fix the missing LLC segment in deep store by asking servers to upload, 
and add segment store download uri in ZK.
+   * Since uploading to segment store involves expensive compression step 
(first tar up the segment and then upload), we don't want to retry the 
uploading.
+   * Segment without segment store copy can still be downloaded from peer 
servers.
+   * @see <a 
href="https://cwiki.apache.org/confluence/display/PINOT/By-passing+deep-store+requirement+for+Realtime+segment+completion#BypassingdeepstorerequirementforRealtimesegmentcompletion-Failurecasesandhandling";>By-passing
 deep-store requirement for Realtime segment completion:Failure cases and 
handling</a>
+   */
+  public void uploadToSegmentStoreIfMissing(TableConfig tableConfig) {
+    String realtimeTableName = tableConfig.getTableName();
+    if (_isStopping) {
+      LOGGER.info("Skipped fixing segment store copy of LLC segments for table 
{}, because segment manager is stopping.", realtimeTableName);
+      return;
+    }
+
+    Queue<String> segmentQueue = 
_llcSegmentMapForUpload.get(realtimeTableName);
+    if (segmentQueue == null || segmentQueue.isEmpty()) {
+      return;
+    }
+
+    // Store the segments to be fixed again in the case of fix failure, or 
skip in this round
+    Queue<String> segmentsNotFixed = new LinkedList<>();
+    RetentionStrategy retentionStrategy = new TimeRetentionStrategy(
+        
TimeUnit.valueOf(tableConfig.getValidationConfig().getRetentionTimeUnit().toUpperCase()),
+        
Long.parseLong(tableConfig.getValidationConfig().getRetentionTimeValue()));
+
+    // Iterate through LLC segments and upload missing segment store copy by 
following steps:
+    //  1. Ask servers which have online segment replica to upload to segment 
store. Servers return segment store download url after successful uploading.
+    //  2. Update the LLC segment ZK metadata by adding segment store download 
url.
+    while (!segmentQueue.isEmpty()) {
+      String segmentName = segmentQueue.poll();
+      // Check if it's null in case of the while condition doesn't stand true 
anymore in the step of dequeue. Dequeue returns null if queue is empty.
+      if (segmentName == null) {

Review comment:
       You are right. I removed the redundant null check here. 




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