klsince commented on code in PR #14477:
URL: https://github.com/apache/pinot/pull/14477#discussion_r1881014964


##########
pinot-plugins/pinot-minion-tasks/pinot-minion-builtin-tasks/src/main/java/org/apache/pinot/plugin/minion/tasks/upsertcompactmerge/UpsertCompactMergeTaskGenerator.java:
##########
@@ -0,0 +1,424 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.pinot.plugin.minion.tasks.upsertcompactmerge;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
+import com.google.common.collect.BiMap;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.commons.lang3.StringUtils;
+import org.apache.helix.task.TaskState;
+import org.apache.pinot.common.exception.InvalidConfigException;
+import org.apache.pinot.common.metadata.segment.SegmentZKMetadata;
+import org.apache.pinot.common.restlet.resources.ValidDocIdsMetadataInfo;
+import org.apache.pinot.common.restlet.resources.ValidDocIdsType;
+import org.apache.pinot.common.utils.SegmentUtils;
+import org.apache.pinot.controller.helix.core.PinotHelixResourceManager;
+import 
org.apache.pinot.controller.helix.core.minion.generator.BaseTaskGenerator;
+import 
org.apache.pinot.controller.helix.core.minion.generator.TaskGeneratorUtils;
+import org.apache.pinot.controller.util.ServerSegmentMetadataReader;
+import org.apache.pinot.core.common.MinionConstants;
+import org.apache.pinot.core.minion.PinotTaskConfig;
+import org.apache.pinot.spi.annotations.minion.TaskGenerator;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.TableType;
+import org.apache.pinot.spi.config.table.UpsertConfig;
+import org.apache.pinot.spi.utils.TimeUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+
+@TaskGenerator
+public class UpsertCompactMergeTaskGenerator extends BaseTaskGenerator {
+
+  private static final Logger LOGGER = 
LoggerFactory.getLogger(UpsertCompactMergeTaskGenerator.class);
+  private static final String DEFAULT_BUFFER_PERIOD = "2d";
+  private static final int DEFAULT_NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST = 500;
+
+  public static class SegmentMergerMetadata {
+    private final SegmentZKMetadata _segmentZKMetadata;
+    private final long _validDocIds;
+    private final long _invalidDocIds;
+
+    SegmentMergerMetadata(SegmentZKMetadata segmentZKMetadata, long 
validDocIds, long invalidDocIds) {
+      _segmentZKMetadata = segmentZKMetadata;
+      _validDocIds = validDocIds;
+      _invalidDocIds = invalidDocIds;
+    }
+
+    public SegmentZKMetadata getSegmentZKMetadata() {
+      return _segmentZKMetadata;
+    }
+
+    public long getValidDocIds() {
+      return _validDocIds;
+    }
+
+    public long getInvalidDocIds() {
+      return _invalidDocIds;
+    }
+  }
+
+  public static class SegmentSelectionResult {
+
+    private final Map<Integer, List<List<SegmentMergerMetadata>>> 
_segmentsForCompactMergeByPartition;
+
+    private final List<String> _segmentsForDeletion;
+
+    SegmentSelectionResult(Map<Integer, List<List<SegmentMergerMetadata>>> 
segmentsForCompactMergeByPartition,
+        List<String> segmentsForDeletion) {
+      _segmentsForCompactMergeByPartition = segmentsForCompactMergeByPartition;
+      _segmentsForDeletion = segmentsForDeletion;
+    }
+
+    public Map<Integer, List<List<SegmentMergerMetadata>>> 
getSegmentsForCompactMergeByPartition() {
+      return _segmentsForCompactMergeByPartition;
+    }
+
+    public List<String> getSegmentsForDeletion() {
+      return _segmentsForDeletion;
+    }
+  }
+
+  @Override
+  public String getTaskType() {
+    return MinionConstants.UpsertCompactMergeTask.TASK_TYPE;
+  }
+
+  @Override
+  public List<PinotTaskConfig> generateTasks(List<TableConfig> tableConfigs) {
+    String taskType = MinionConstants.UpsertCompactMergeTask.TASK_TYPE;
+    List<PinotTaskConfig> pinotTaskConfigs = new ArrayList<>();
+    for (TableConfig tableConfig : tableConfigs) {
+
+      String tableNameWithType = tableConfig.getTableName();
+      LOGGER.info("Start generating task configs for table: {}", 
tableNameWithType);
+
+      if (tableConfig.getTaskConfig() == null) {
+        LOGGER.warn("Task config is null for table: {}", tableNameWithType);
+        continue;
+      }
+
+      // Only schedule 1 task of this type, per table
+      Map<String, TaskState> incompleteTasks =
+          TaskGeneratorUtils.getIncompleteTasks(taskType, tableNameWithType, 
_clusterInfoAccessor);
+      if (!incompleteTasks.isEmpty()) {
+        LOGGER.warn("Found incomplete tasks: {} for same table: {} and task 
type: {}. Skipping task generation.",
+            incompleteTasks.keySet(), tableNameWithType, taskType);
+        continue;
+      }
+
+      Map<String, String> taskConfigs = 
tableConfig.getTaskConfig().getConfigsForTaskType(taskType);
+      List<SegmentZKMetadata> allSegments = 
_clusterInfoAccessor.getSegmentsZKMetadata(tableNameWithType);
+
+      // Get completed segments and filter out the segments based on the 
buffer time configuration
+      List<SegmentZKMetadata> candidateSegments =
+          getCandidateSegments(taskConfigs, allSegments, 
System.currentTimeMillis());
+
+      if (candidateSegments.isEmpty()) {
+        LOGGER.info("No segments were eligible for compactMerge task for 
table: {}", tableNameWithType);
+        continue;
+      }
+
+      // get server to segment mappings
+      PinotHelixResourceManager pinotHelixResourceManager = 
_clusterInfoAccessor.getPinotHelixResourceManager();
+      Map<String, List<String>> serverToSegments = 
pinotHelixResourceManager.getServerToSegmentsMap(tableNameWithType);
+      BiMap<String, String> serverToEndpoints;
+      try {
+        serverToEndpoints = 
pinotHelixResourceManager.getDataInstanceAdminEndpoints(serverToSegments.keySet());
+      } catch (InvalidConfigException e) {
+        throw new RuntimeException(e);
+      }
+
+      ServerSegmentMetadataReader serverSegmentMetadataReader =
+          new ServerSegmentMetadataReader(_clusterInfoAccessor.getExecutor(),
+              _clusterInfoAccessor.getConnectionManager());
+
+      // Number of segments to query per server request. If a table has a lot 
of segments, then we might send a
+      // huge payload to pinot-server in request. Batching the requests will 
help in reducing the payload size.
+      int numSegmentsBatchPerServerRequest = Integer.parseInt(
+          
taskConfigs.getOrDefault(MinionConstants.UpsertCompactMergeTask.NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST,
+              String.valueOf(DEFAULT_NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST)));
+
+      Map<String, List<ValidDocIdsMetadataInfo>> validDocIdsMetadataList =
+          
serverSegmentMetadataReader.getSegmentToValidDocIdsMetadataFromServer(tableNameWithType,
 serverToSegments,
+              serverToEndpoints, null, 60_000, 
ValidDocIdsType.SNAPSHOT.toString(),
+              numSegmentsBatchPerServerRequest);
+
+      Map<String, SegmentZKMetadata> candidateSegmentsMap =
+          
candidateSegments.stream().collect(Collectors.toMap(SegmentZKMetadata::getSegmentName,
 Function.identity()));
+
+      Set<String> alreadyMergedSegments = 
getAlreadyMergedSegments(allSegments);
+
+      SegmentSelectionResult segmentSelectionResult =
+          processValidDocIdsMetadata(taskConfigs, candidateSegmentsMap, 
validDocIdsMetadataList, alreadyMergedSegments);
+
+      if (!segmentSelectionResult.getSegmentsForDeletion().isEmpty()) {
+        pinotHelixResourceManager.deleteSegments(tableNameWithType, 
segmentSelectionResult.getSegmentsForDeletion(),
+            "0d");
+        LOGGER.info(
+            "Deleted segments containing only invalid records for table: {}, 
number of segments to be deleted: {}",
+            tableNameWithType, 
segmentSelectionResult.getSegmentsForDeletion());
+      }
+
+      int numTasks = 0;
+      int maxTasks = 
Integer.parseInt(taskConfigs.getOrDefault(MinionConstants.TABLE_MAX_NUM_TASKS_KEY,
+          String.valueOf(MinionConstants.DEFAULT_TABLE_MAX_NUM_TASKS)));
+      for (Map.Entry<Integer, List<List<SegmentMergerMetadata>>> entry
+          : 
segmentSelectionResult.getSegmentsForCompactMergeByPartition().entrySet()) {
+        if (numTasks == maxTasks) {
+          break;
+        }
+        List<List<SegmentMergerMetadata>> groups = entry.getValue();
+        // no valid groups found in the partition to merge
+        if (groups.isEmpty()) {
+          continue;
+        }
+        // there are no groups with more than 1 segment to merge
+        // TODO this can be later removed if we want to just do single-segment 
compaction from this task
+        if (groups.get(0).size() <= 1) {
+          continue;
+        }
+        // TODO see if multiple groups of same partition can be added
+        Map<String, String> configs = new 
HashMap<>(getBaseTaskConfigs(tableConfig,
+            groups.get(0).stream()
+                .map(x -> 
x.getSegmentZKMetadata().getSegmentName()).collect(Collectors.toList())));
+        configs.put(MinionConstants.DOWNLOAD_URL_KEY, 
getDownloadUrl(groups.get(0)));
+        configs.put(MinionConstants.UPLOAD_URL_KEY, 
_clusterInfoAccessor.getVipUrl() + "/segments");
+        configs.put(MinionConstants.ORIGINAL_SEGMENT_CRC_KEY, 
getSegmentCrcList(groups.get(0)));
+        pinotTaskConfigs.add(new 
PinotTaskConfig(MinionConstants.UpsertCompactMergeTask.TASK_TYPE, configs));
+        numTasks++;
+      }
+      LOGGER.info("Finished generating {} tasks configs for table: {}", 
numTasks, tableNameWithType);
+    }
+    return pinotTaskConfigs;
+  }
+
+  @VisibleForTesting
+  public static SegmentSelectionResult processValidDocIdsMetadata(
+      Map<String, String> taskConfigs,
+      Map<String, SegmentZKMetadata> candidateSegmentsMap,
+      Map<String, List<ValidDocIdsMetadataInfo>> validDocIdsMetadataInfoMap,
+      Set<String> alreadyMergedSegments) {
+    Map<Integer, List<SegmentMergerMetadata>> segmentsEligibleForCompactMerge 
= new HashMap<>();
+    List<String> segmentsForDeletion = new ArrayList<>();
+    for (String segmentName : validDocIdsMetadataInfoMap.keySet()) {
+      // check if segment is part of completed segments
+      if (!candidateSegmentsMap.containsKey(segmentName)) {
+        LOGGER.warn("Segment {} is not found in the completed segments list, 
skipping it for {}", segmentName,
+            MinionConstants.UpsertCompactMergeTask.TASK_TYPE);
+        continue;
+      }
+      SegmentZKMetadata segment = candidateSegmentsMap.get(segmentName);
+      for (ValidDocIdsMetadataInfo validDocIdsMetadata : 
validDocIdsMetadataInfoMap.get(segmentName)) {
+        long totalInvalidDocs = validDocIdsMetadata.getTotalInvalidDocs();
+        long totalValidDocs = validDocIdsMetadata.getTotalValidDocs();
+
+        // Skip segments if the crc from zk metadata and server does not 
match. They may be getting reloaded.
+        if (segment.getCrc() != 
Long.parseLong(validDocIdsMetadata.getSegmentCrc())) {
+          LOGGER.warn("CRC mismatch for segment: {}, (segmentZKMetadata={}, 
validDocIdsMetadata={})", segmentName,
+              segment.getCrc(), validDocIdsMetadata.getSegmentCrc());
+          continue;
+        }
+
+        // segments eligible for deletion with no valid records
+        long totalDocs = validDocIdsMetadata.getTotalDocs();
+        if (totalInvalidDocs == totalDocs) {
+          segmentsForDeletion.add(segmentName);

Review Comment:
   `break;` early here? 
   
    some of the `else keyword` can be saved with those `continued;`



##########
pinot-plugins/pinot-minion-tasks/pinot-minion-builtin-tasks/src/main/java/org/apache/pinot/plugin/minion/tasks/upsertcompactmerge/UpsertCompactMergeTaskGenerator.java:
##########
@@ -0,0 +1,424 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.pinot.plugin.minion.tasks.upsertcompactmerge;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
+import com.google.common.collect.BiMap;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.commons.lang3.StringUtils;
+import org.apache.helix.task.TaskState;
+import org.apache.pinot.common.exception.InvalidConfigException;
+import org.apache.pinot.common.metadata.segment.SegmentZKMetadata;
+import org.apache.pinot.common.restlet.resources.ValidDocIdsMetadataInfo;
+import org.apache.pinot.common.restlet.resources.ValidDocIdsType;
+import org.apache.pinot.common.utils.SegmentUtils;
+import org.apache.pinot.controller.helix.core.PinotHelixResourceManager;
+import 
org.apache.pinot.controller.helix.core.minion.generator.BaseTaskGenerator;
+import 
org.apache.pinot.controller.helix.core.minion.generator.TaskGeneratorUtils;
+import org.apache.pinot.controller.util.ServerSegmentMetadataReader;
+import org.apache.pinot.core.common.MinionConstants;
+import org.apache.pinot.core.minion.PinotTaskConfig;
+import org.apache.pinot.spi.annotations.minion.TaskGenerator;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.TableType;
+import org.apache.pinot.spi.config.table.UpsertConfig;
+import org.apache.pinot.spi.utils.TimeUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+
+@TaskGenerator
+public class UpsertCompactMergeTaskGenerator extends BaseTaskGenerator {
+
+  private static final Logger LOGGER = 
LoggerFactory.getLogger(UpsertCompactMergeTaskGenerator.class);
+  private static final String DEFAULT_BUFFER_PERIOD = "2d";
+  private static final int DEFAULT_NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST = 500;
+
+  public static class SegmentMergerMetadata {
+    private final SegmentZKMetadata _segmentZKMetadata;
+    private final long _validDocIds;
+    private final long _invalidDocIds;
+
+    SegmentMergerMetadata(SegmentZKMetadata segmentZKMetadata, long 
validDocIds, long invalidDocIds) {
+      _segmentZKMetadata = segmentZKMetadata;
+      _validDocIds = validDocIds;
+      _invalidDocIds = invalidDocIds;
+    }
+
+    public SegmentZKMetadata getSegmentZKMetadata() {
+      return _segmentZKMetadata;
+    }
+
+    public long getValidDocIds() {
+      return _validDocIds;
+    }
+
+    public long getInvalidDocIds() {
+      return _invalidDocIds;
+    }
+  }
+
+  public static class SegmentSelectionResult {
+
+    private final Map<Integer, List<List<SegmentMergerMetadata>>> 
_segmentsForCompactMergeByPartition;
+
+    private final List<String> _segmentsForDeletion;
+
+    SegmentSelectionResult(Map<Integer, List<List<SegmentMergerMetadata>>> 
segmentsForCompactMergeByPartition,
+        List<String> segmentsForDeletion) {
+      _segmentsForCompactMergeByPartition = segmentsForCompactMergeByPartition;
+      _segmentsForDeletion = segmentsForDeletion;
+    }
+
+    public Map<Integer, List<List<SegmentMergerMetadata>>> 
getSegmentsForCompactMergeByPartition() {
+      return _segmentsForCompactMergeByPartition;
+    }
+
+    public List<String> getSegmentsForDeletion() {
+      return _segmentsForDeletion;
+    }
+  }
+
+  @Override
+  public String getTaskType() {
+    return MinionConstants.UpsertCompactMergeTask.TASK_TYPE;
+  }
+
+  @Override
+  public List<PinotTaskConfig> generateTasks(List<TableConfig> tableConfigs) {
+    String taskType = MinionConstants.UpsertCompactMergeTask.TASK_TYPE;
+    List<PinotTaskConfig> pinotTaskConfigs = new ArrayList<>();
+    for (TableConfig tableConfig : tableConfigs) {
+
+      String tableNameWithType = tableConfig.getTableName();
+      LOGGER.info("Start generating task configs for table: {}", 
tableNameWithType);
+
+      if (tableConfig.getTaskConfig() == null) {
+        LOGGER.warn("Task config is null for table: {}", tableNameWithType);
+        continue;
+      }
+
+      // Only schedule 1 task of this type, per table
+      Map<String, TaskState> incompleteTasks =
+          TaskGeneratorUtils.getIncompleteTasks(taskType, tableNameWithType, 
_clusterInfoAccessor);
+      if (!incompleteTasks.isEmpty()) {
+        LOGGER.warn("Found incomplete tasks: {} for same table: {} and task 
type: {}. Skipping task generation.",
+            incompleteTasks.keySet(), tableNameWithType, taskType);
+        continue;
+      }
+
+      Map<String, String> taskConfigs = 
tableConfig.getTaskConfig().getConfigsForTaskType(taskType);
+      List<SegmentZKMetadata> allSegments = 
_clusterInfoAccessor.getSegmentsZKMetadata(tableNameWithType);
+
+      // Get completed segments and filter out the segments based on the 
buffer time configuration
+      List<SegmentZKMetadata> candidateSegments =
+          getCandidateSegments(taskConfigs, allSegments, 
System.currentTimeMillis());
+
+      if (candidateSegments.isEmpty()) {
+        LOGGER.info("No segments were eligible for compactMerge task for 
table: {}", tableNameWithType);
+        continue;
+      }
+
+      // get server to segment mappings
+      PinotHelixResourceManager pinotHelixResourceManager = 
_clusterInfoAccessor.getPinotHelixResourceManager();
+      Map<String, List<String>> serverToSegments = 
pinotHelixResourceManager.getServerToSegmentsMap(tableNameWithType);
+      BiMap<String, String> serverToEndpoints;
+      try {
+        serverToEndpoints = 
pinotHelixResourceManager.getDataInstanceAdminEndpoints(serverToSegments.keySet());
+      } catch (InvalidConfigException e) {
+        throw new RuntimeException(e);
+      }
+
+      ServerSegmentMetadataReader serverSegmentMetadataReader =
+          new ServerSegmentMetadataReader(_clusterInfoAccessor.getExecutor(),
+              _clusterInfoAccessor.getConnectionManager());
+
+      // Number of segments to query per server request. If a table has a lot 
of segments, then we might send a
+      // huge payload to pinot-server in request. Batching the requests will 
help in reducing the payload size.
+      int numSegmentsBatchPerServerRequest = Integer.parseInt(
+          
taskConfigs.getOrDefault(MinionConstants.UpsertCompactMergeTask.NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST,
+              String.valueOf(DEFAULT_NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST)));
+
+      Map<String, List<ValidDocIdsMetadataInfo>> validDocIdsMetadataList =
+          
serverSegmentMetadataReader.getSegmentToValidDocIdsMetadataFromServer(tableNameWithType,
 serverToSegments,
+              serverToEndpoints, null, 60_000, 
ValidDocIdsType.SNAPSHOT.toString(),
+              numSegmentsBatchPerServerRequest);
+
+      Map<String, SegmentZKMetadata> candidateSegmentsMap =
+          
candidateSegments.stream().collect(Collectors.toMap(SegmentZKMetadata::getSegmentName,
 Function.identity()));
+
+      Set<String> alreadyMergedSegments = 
getAlreadyMergedSegments(allSegments);
+
+      SegmentSelectionResult segmentSelectionResult =
+          processValidDocIdsMetadata(taskConfigs, candidateSegmentsMap, 
validDocIdsMetadataList, alreadyMergedSegments);
+
+      if (!segmentSelectionResult.getSegmentsForDeletion().isEmpty()) {
+        pinotHelixResourceManager.deleteSegments(tableNameWithType, 
segmentSelectionResult.getSegmentsForDeletion(),
+            "0d");
+        LOGGER.info(
+            "Deleted segments containing only invalid records for table: {}, 
number of segments to be deleted: {}",
+            tableNameWithType, 
segmentSelectionResult.getSegmentsForDeletion());
+      }
+
+      int numTasks = 0;
+      int maxTasks = 
Integer.parseInt(taskConfigs.getOrDefault(MinionConstants.TABLE_MAX_NUM_TASKS_KEY,
+          String.valueOf(MinionConstants.DEFAULT_TABLE_MAX_NUM_TASKS)));
+      for (Map.Entry<Integer, List<List<SegmentMergerMetadata>>> entry
+          : 
segmentSelectionResult.getSegmentsForCompactMergeByPartition().entrySet()) {
+        if (numTasks == maxTasks) {
+          break;
+        }
+        List<List<SegmentMergerMetadata>> groups = entry.getValue();
+        // no valid groups found in the partition to merge
+        if (groups.isEmpty()) {
+          continue;
+        }
+        // there are no groups with more than 1 segment to merge
+        // TODO this can be later removed if we want to just do single-segment 
compaction from this task
+        if (groups.get(0).size() <= 1) {
+          continue;
+        }
+        // TODO see if multiple groups of same partition can be added
+        Map<String, String> configs = new 
HashMap<>(getBaseTaskConfigs(tableConfig,
+            groups.get(0).stream()
+                .map(x -> 
x.getSegmentZKMetadata().getSegmentName()).collect(Collectors.toList())));
+        configs.put(MinionConstants.DOWNLOAD_URL_KEY, 
getDownloadUrl(groups.get(0)));
+        configs.put(MinionConstants.UPLOAD_URL_KEY, 
_clusterInfoAccessor.getVipUrl() + "/segments");
+        configs.put(MinionConstants.ORIGINAL_SEGMENT_CRC_KEY, 
getSegmentCrcList(groups.get(0)));
+        pinotTaskConfigs.add(new 
PinotTaskConfig(MinionConstants.UpsertCompactMergeTask.TASK_TYPE, configs));
+        numTasks++;
+      }
+      LOGGER.info("Finished generating {} tasks configs for table: {}", 
numTasks, tableNameWithType);
+    }
+    return pinotTaskConfigs;
+  }
+
+  @VisibleForTesting
+  public static SegmentSelectionResult processValidDocIdsMetadata(
+      Map<String, String> taskConfigs,

Review Comment:
   format?



##########
pinot-plugins/pinot-minion-tasks/pinot-minion-builtin-tasks/src/main/java/org/apache/pinot/plugin/minion/tasks/upsertcompactmerge/UpsertCompactMergeTaskGenerator.java:
##########
@@ -0,0 +1,424 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.pinot.plugin.minion.tasks.upsertcompactmerge;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
+import com.google.common.collect.BiMap;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.commons.lang3.StringUtils;
+import org.apache.helix.task.TaskState;
+import org.apache.pinot.common.exception.InvalidConfigException;
+import org.apache.pinot.common.metadata.segment.SegmentZKMetadata;
+import org.apache.pinot.common.restlet.resources.ValidDocIdsMetadataInfo;
+import org.apache.pinot.common.restlet.resources.ValidDocIdsType;
+import org.apache.pinot.common.utils.SegmentUtils;
+import org.apache.pinot.controller.helix.core.PinotHelixResourceManager;
+import 
org.apache.pinot.controller.helix.core.minion.generator.BaseTaskGenerator;
+import 
org.apache.pinot.controller.helix.core.minion.generator.TaskGeneratorUtils;
+import org.apache.pinot.controller.util.ServerSegmentMetadataReader;
+import org.apache.pinot.core.common.MinionConstants;
+import org.apache.pinot.core.minion.PinotTaskConfig;
+import org.apache.pinot.spi.annotations.minion.TaskGenerator;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.TableType;
+import org.apache.pinot.spi.config.table.UpsertConfig;
+import org.apache.pinot.spi.utils.TimeUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+
+@TaskGenerator
+public class UpsertCompactMergeTaskGenerator extends BaseTaskGenerator {
+
+  private static final Logger LOGGER = 
LoggerFactory.getLogger(UpsertCompactMergeTaskGenerator.class);
+  private static final String DEFAULT_BUFFER_PERIOD = "2d";
+  private static final int DEFAULT_NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST = 500;
+
+  public static class SegmentMergerMetadata {
+    private final SegmentZKMetadata _segmentZKMetadata;
+    private final long _validDocIds;
+    private final long _invalidDocIds;
+
+    SegmentMergerMetadata(SegmentZKMetadata segmentZKMetadata, long 
validDocIds, long invalidDocIds) {
+      _segmentZKMetadata = segmentZKMetadata;
+      _validDocIds = validDocIds;
+      _invalidDocIds = invalidDocIds;
+    }
+
+    public SegmentZKMetadata getSegmentZKMetadata() {
+      return _segmentZKMetadata;
+    }
+
+    public long getValidDocIds() {
+      return _validDocIds;
+    }
+
+    public long getInvalidDocIds() {
+      return _invalidDocIds;
+    }
+  }
+
+  public static class SegmentSelectionResult {
+
+    private final Map<Integer, List<List<SegmentMergerMetadata>>> 
_segmentsForCompactMergeByPartition;
+
+    private final List<String> _segmentsForDeletion;
+
+    SegmentSelectionResult(Map<Integer, List<List<SegmentMergerMetadata>>> 
segmentsForCompactMergeByPartition,
+        List<String> segmentsForDeletion) {
+      _segmentsForCompactMergeByPartition = segmentsForCompactMergeByPartition;
+      _segmentsForDeletion = segmentsForDeletion;
+    }
+
+    public Map<Integer, List<List<SegmentMergerMetadata>>> 
getSegmentsForCompactMergeByPartition() {
+      return _segmentsForCompactMergeByPartition;
+    }
+
+    public List<String> getSegmentsForDeletion() {
+      return _segmentsForDeletion;
+    }
+  }
+
+  @Override
+  public String getTaskType() {
+    return MinionConstants.UpsertCompactMergeTask.TASK_TYPE;
+  }
+
+  @Override
+  public List<PinotTaskConfig> generateTasks(List<TableConfig> tableConfigs) {
+    String taskType = MinionConstants.UpsertCompactMergeTask.TASK_TYPE;
+    List<PinotTaskConfig> pinotTaskConfigs = new ArrayList<>();
+    for (TableConfig tableConfig : tableConfigs) {
+
+      String tableNameWithType = tableConfig.getTableName();
+      LOGGER.info("Start generating task configs for table: {}", 
tableNameWithType);
+
+      if (tableConfig.getTaskConfig() == null) {
+        LOGGER.warn("Task config is null for table: {}", tableNameWithType);
+        continue;
+      }
+
+      // Only schedule 1 task of this type, per table
+      Map<String, TaskState> incompleteTasks =
+          TaskGeneratorUtils.getIncompleteTasks(taskType, tableNameWithType, 
_clusterInfoAccessor);
+      if (!incompleteTasks.isEmpty()) {
+        LOGGER.warn("Found incomplete tasks: {} for same table: {} and task 
type: {}. Skipping task generation.",
+            incompleteTasks.keySet(), tableNameWithType, taskType);
+        continue;
+      }
+
+      Map<String, String> taskConfigs = 
tableConfig.getTaskConfig().getConfigsForTaskType(taskType);
+      List<SegmentZKMetadata> allSegments = 
_clusterInfoAccessor.getSegmentsZKMetadata(tableNameWithType);
+
+      // Get completed segments and filter out the segments based on the 
buffer time configuration
+      List<SegmentZKMetadata> candidateSegments =
+          getCandidateSegments(taskConfigs, allSegments, 
System.currentTimeMillis());
+
+      if (candidateSegments.isEmpty()) {
+        LOGGER.info("No segments were eligible for compactMerge task for 
table: {}", tableNameWithType);
+        continue;
+      }
+
+      // get server to segment mappings
+      PinotHelixResourceManager pinotHelixResourceManager = 
_clusterInfoAccessor.getPinotHelixResourceManager();
+      Map<String, List<String>> serverToSegments = 
pinotHelixResourceManager.getServerToSegmentsMap(tableNameWithType);
+      BiMap<String, String> serverToEndpoints;
+      try {
+        serverToEndpoints = 
pinotHelixResourceManager.getDataInstanceAdminEndpoints(serverToSegments.keySet());
+      } catch (InvalidConfigException e) {
+        throw new RuntimeException(e);
+      }
+
+      ServerSegmentMetadataReader serverSegmentMetadataReader =
+          new ServerSegmentMetadataReader(_clusterInfoAccessor.getExecutor(),
+              _clusterInfoAccessor.getConnectionManager());
+
+      // Number of segments to query per server request. If a table has a lot 
of segments, then we might send a
+      // huge payload to pinot-server in request. Batching the requests will 
help in reducing the payload size.
+      int numSegmentsBatchPerServerRequest = Integer.parseInt(
+          
taskConfigs.getOrDefault(MinionConstants.UpsertCompactMergeTask.NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST,
+              String.valueOf(DEFAULT_NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST)));
+
+      Map<String, List<ValidDocIdsMetadataInfo>> validDocIdsMetadataList =
+          
serverSegmentMetadataReader.getSegmentToValidDocIdsMetadataFromServer(tableNameWithType,
 serverToSegments,
+              serverToEndpoints, null, 60_000, 
ValidDocIdsType.SNAPSHOT.toString(),
+              numSegmentsBatchPerServerRequest);
+
+      Map<String, SegmentZKMetadata> candidateSegmentsMap =
+          
candidateSegments.stream().collect(Collectors.toMap(SegmentZKMetadata::getSegmentName,
 Function.identity()));
+
+      Set<String> alreadyMergedSegments = 
getAlreadyMergedSegments(allSegments);
+
+      SegmentSelectionResult segmentSelectionResult =
+          processValidDocIdsMetadata(taskConfigs, candidateSegmentsMap, 
validDocIdsMetadataList, alreadyMergedSegments);
+
+      if (!segmentSelectionResult.getSegmentsForDeletion().isEmpty()) {
+        pinotHelixResourceManager.deleteSegments(tableNameWithType, 
segmentSelectionResult.getSegmentsForDeletion(),
+            "0d");
+        LOGGER.info(
+            "Deleted segments containing only invalid records for table: {}, 
number of segments to be deleted: {}",
+            tableNameWithType, 
segmentSelectionResult.getSegmentsForDeletion());
+      }
+
+      int numTasks = 0;
+      int maxTasks = 
Integer.parseInt(taskConfigs.getOrDefault(MinionConstants.TABLE_MAX_NUM_TASKS_KEY,
+          String.valueOf(MinionConstants.DEFAULT_TABLE_MAX_NUM_TASKS)));
+      for (Map.Entry<Integer, List<List<SegmentMergerMetadata>>> entry
+          : 
segmentSelectionResult.getSegmentsForCompactMergeByPartition().entrySet()) {
+        if (numTasks == maxTasks) {
+          break;
+        }
+        List<List<SegmentMergerMetadata>> groups = entry.getValue();
+        // no valid groups found in the partition to merge
+        if (groups.isEmpty()) {
+          continue;
+        }
+        // there are no groups with more than 1 segment to merge
+        // TODO this can be later removed if we want to just do single-segment 
compaction from this task
+        if (groups.get(0).size() <= 1) {
+          continue;
+        }
+        // TODO see if multiple groups of same partition can be added
+        Map<String, String> configs = new 
HashMap<>(getBaseTaskConfigs(tableConfig,
+            groups.get(0).stream()
+                .map(x -> 
x.getSegmentZKMetadata().getSegmentName()).collect(Collectors.toList())));
+        configs.put(MinionConstants.DOWNLOAD_URL_KEY, 
getDownloadUrl(groups.get(0)));
+        configs.put(MinionConstants.UPLOAD_URL_KEY, 
_clusterInfoAccessor.getVipUrl() + "/segments");
+        configs.put(MinionConstants.ORIGINAL_SEGMENT_CRC_KEY, 
getSegmentCrcList(groups.get(0)));
+        pinotTaskConfigs.add(new 
PinotTaskConfig(MinionConstants.UpsertCompactMergeTask.TASK_TYPE, configs));
+        numTasks++;
+      }
+      LOGGER.info("Finished generating {} tasks configs for table: {}", 
numTasks, tableNameWithType);
+    }
+    return pinotTaskConfigs;
+  }
+
+  @VisibleForTesting
+  public static SegmentSelectionResult processValidDocIdsMetadata(
+      Map<String, String> taskConfigs,
+      Map<String, SegmentZKMetadata> candidateSegmentsMap,
+      Map<String, List<ValidDocIdsMetadataInfo>> validDocIdsMetadataInfoMap,
+      Set<String> alreadyMergedSegments) {
+    Map<Integer, List<SegmentMergerMetadata>> segmentsEligibleForCompactMerge 
= new HashMap<>();
+    List<String> segmentsForDeletion = new ArrayList<>();
+    for (String segmentName : validDocIdsMetadataInfoMap.keySet()) {
+      // check if segment is part of completed segments
+      if (!candidateSegmentsMap.containsKey(segmentName)) {
+        LOGGER.warn("Segment {} is not found in the completed segments list, 
skipping it for {}", segmentName,
+            MinionConstants.UpsertCompactMergeTask.TASK_TYPE);

Review Comment:
   I assume we don't need to log task type as the LOGGER prints class name 
anyway



##########
pinot-plugins/pinot-minion-tasks/pinot-minion-builtin-tasks/src/main/java/org/apache/pinot/plugin/minion/tasks/upsertcompactmerge/UpsertCompactMergeTaskGenerator.java:
##########
@@ -0,0 +1,424 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.pinot.plugin.minion.tasks.upsertcompactmerge;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
+import com.google.common.collect.BiMap;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.commons.lang3.StringUtils;
+import org.apache.helix.task.TaskState;
+import org.apache.pinot.common.exception.InvalidConfigException;
+import org.apache.pinot.common.metadata.segment.SegmentZKMetadata;
+import org.apache.pinot.common.restlet.resources.ValidDocIdsMetadataInfo;
+import org.apache.pinot.common.restlet.resources.ValidDocIdsType;
+import org.apache.pinot.common.utils.SegmentUtils;
+import org.apache.pinot.controller.helix.core.PinotHelixResourceManager;
+import 
org.apache.pinot.controller.helix.core.minion.generator.BaseTaskGenerator;
+import 
org.apache.pinot.controller.helix.core.minion.generator.TaskGeneratorUtils;
+import org.apache.pinot.controller.util.ServerSegmentMetadataReader;
+import org.apache.pinot.core.common.MinionConstants;
+import org.apache.pinot.core.minion.PinotTaskConfig;
+import org.apache.pinot.spi.annotations.minion.TaskGenerator;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.TableType;
+import org.apache.pinot.spi.config.table.UpsertConfig;
+import org.apache.pinot.spi.utils.TimeUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+
+@TaskGenerator
+public class UpsertCompactMergeTaskGenerator extends BaseTaskGenerator {
+
+  private static final Logger LOGGER = 
LoggerFactory.getLogger(UpsertCompactMergeTaskGenerator.class);
+  private static final String DEFAULT_BUFFER_PERIOD = "2d";
+  private static final int DEFAULT_NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST = 500;
+
+  public static class SegmentMergerMetadata {
+    private final SegmentZKMetadata _segmentZKMetadata;
+    private final long _validDocIds;
+    private final long _invalidDocIds;
+
+    SegmentMergerMetadata(SegmentZKMetadata segmentZKMetadata, long 
validDocIds, long invalidDocIds) {
+      _segmentZKMetadata = segmentZKMetadata;
+      _validDocIds = validDocIds;
+      _invalidDocIds = invalidDocIds;
+    }
+
+    public SegmentZKMetadata getSegmentZKMetadata() {
+      return _segmentZKMetadata;
+    }
+
+    public long getValidDocIds() {
+      return _validDocIds;
+    }
+
+    public long getInvalidDocIds() {
+      return _invalidDocIds;
+    }
+  }
+
+  public static class SegmentSelectionResult {
+
+    private final Map<Integer, List<List<SegmentMergerMetadata>>> 
_segmentsForCompactMergeByPartition;
+
+    private final List<String> _segmentsForDeletion;
+
+    SegmentSelectionResult(Map<Integer, List<List<SegmentMergerMetadata>>> 
segmentsForCompactMergeByPartition,
+        List<String> segmentsForDeletion) {
+      _segmentsForCompactMergeByPartition = segmentsForCompactMergeByPartition;
+      _segmentsForDeletion = segmentsForDeletion;
+    }
+
+    public Map<Integer, List<List<SegmentMergerMetadata>>> 
getSegmentsForCompactMergeByPartition() {
+      return _segmentsForCompactMergeByPartition;
+    }
+
+    public List<String> getSegmentsForDeletion() {
+      return _segmentsForDeletion;
+    }
+  }
+
+  @Override
+  public String getTaskType() {
+    return MinionConstants.UpsertCompactMergeTask.TASK_TYPE;
+  }
+
+  @Override
+  public List<PinotTaskConfig> generateTasks(List<TableConfig> tableConfigs) {
+    String taskType = MinionConstants.UpsertCompactMergeTask.TASK_TYPE;
+    List<PinotTaskConfig> pinotTaskConfigs = new ArrayList<>();
+    for (TableConfig tableConfig : tableConfigs) {
+

Review Comment:
   nit: move the for-loop logic to a helper method to be more readable



##########
pinot-plugins/pinot-minion-tasks/pinot-minion-builtin-tasks/src/main/java/org/apache/pinot/plugin/minion/tasks/upsertcompactmerge/UpsertCompactMergeTaskGenerator.java:
##########
@@ -0,0 +1,424 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.pinot.plugin.minion.tasks.upsertcompactmerge;
+
+import com.google.common.annotations.VisibleForTesting;
+import com.google.common.base.Preconditions;
+import com.google.common.collect.BiMap;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.function.Function;
+import java.util.stream.Collectors;
+import org.apache.commons.lang3.StringUtils;
+import org.apache.helix.task.TaskState;
+import org.apache.pinot.common.exception.InvalidConfigException;
+import org.apache.pinot.common.metadata.segment.SegmentZKMetadata;
+import org.apache.pinot.common.restlet.resources.ValidDocIdsMetadataInfo;
+import org.apache.pinot.common.restlet.resources.ValidDocIdsType;
+import org.apache.pinot.common.utils.SegmentUtils;
+import org.apache.pinot.controller.helix.core.PinotHelixResourceManager;
+import 
org.apache.pinot.controller.helix.core.minion.generator.BaseTaskGenerator;
+import 
org.apache.pinot.controller.helix.core.minion.generator.TaskGeneratorUtils;
+import org.apache.pinot.controller.util.ServerSegmentMetadataReader;
+import org.apache.pinot.core.common.MinionConstants;
+import org.apache.pinot.core.minion.PinotTaskConfig;
+import org.apache.pinot.spi.annotations.minion.TaskGenerator;
+import org.apache.pinot.spi.config.table.TableConfig;
+import org.apache.pinot.spi.config.table.TableType;
+import org.apache.pinot.spi.config.table.UpsertConfig;
+import org.apache.pinot.spi.utils.TimeUtils;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+
+@TaskGenerator
+public class UpsertCompactMergeTaskGenerator extends BaseTaskGenerator {
+
+  private static final Logger LOGGER = 
LoggerFactory.getLogger(UpsertCompactMergeTaskGenerator.class);
+  private static final String DEFAULT_BUFFER_PERIOD = "2d";
+  private static final int DEFAULT_NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST = 500;
+
+  public static class SegmentMergerMetadata {
+    private final SegmentZKMetadata _segmentZKMetadata;
+    private final long _validDocIds;
+    private final long _invalidDocIds;
+
+    SegmentMergerMetadata(SegmentZKMetadata segmentZKMetadata, long 
validDocIds, long invalidDocIds) {
+      _segmentZKMetadata = segmentZKMetadata;
+      _validDocIds = validDocIds;
+      _invalidDocIds = invalidDocIds;
+    }
+
+    public SegmentZKMetadata getSegmentZKMetadata() {
+      return _segmentZKMetadata;
+    }
+
+    public long getValidDocIds() {
+      return _validDocIds;
+    }
+
+    public long getInvalidDocIds() {
+      return _invalidDocIds;
+    }
+  }
+
+  public static class SegmentSelectionResult {
+
+    private final Map<Integer, List<List<SegmentMergerMetadata>>> 
_segmentsForCompactMergeByPartition;
+
+    private final List<String> _segmentsForDeletion;
+
+    SegmentSelectionResult(Map<Integer, List<List<SegmentMergerMetadata>>> 
segmentsForCompactMergeByPartition,
+        List<String> segmentsForDeletion) {
+      _segmentsForCompactMergeByPartition = segmentsForCompactMergeByPartition;
+      _segmentsForDeletion = segmentsForDeletion;
+    }
+
+    public Map<Integer, List<List<SegmentMergerMetadata>>> 
getSegmentsForCompactMergeByPartition() {
+      return _segmentsForCompactMergeByPartition;
+    }
+
+    public List<String> getSegmentsForDeletion() {
+      return _segmentsForDeletion;
+    }
+  }
+
+  @Override
+  public String getTaskType() {
+    return MinionConstants.UpsertCompactMergeTask.TASK_TYPE;
+  }
+
+  @Override
+  public List<PinotTaskConfig> generateTasks(List<TableConfig> tableConfigs) {
+    String taskType = MinionConstants.UpsertCompactMergeTask.TASK_TYPE;
+    List<PinotTaskConfig> pinotTaskConfigs = new ArrayList<>();
+    for (TableConfig tableConfig : tableConfigs) {
+
+      String tableNameWithType = tableConfig.getTableName();
+      LOGGER.info("Start generating task configs for table: {}", 
tableNameWithType);
+
+      if (tableConfig.getTaskConfig() == null) {
+        LOGGER.warn("Task config is null for table: {}", tableNameWithType);
+        continue;
+      }
+
+      // Only schedule 1 task of this type, per table
+      Map<String, TaskState> incompleteTasks =
+          TaskGeneratorUtils.getIncompleteTasks(taskType, tableNameWithType, 
_clusterInfoAccessor);
+      if (!incompleteTasks.isEmpty()) {
+        LOGGER.warn("Found incomplete tasks: {} for same table: {} and task 
type: {}. Skipping task generation.",
+            incompleteTasks.keySet(), tableNameWithType, taskType);
+        continue;
+      }
+
+      Map<String, String> taskConfigs = 
tableConfig.getTaskConfig().getConfigsForTaskType(taskType);
+      List<SegmentZKMetadata> allSegments = 
_clusterInfoAccessor.getSegmentsZKMetadata(tableNameWithType);
+
+      // Get completed segments and filter out the segments based on the 
buffer time configuration
+      List<SegmentZKMetadata> candidateSegments =
+          getCandidateSegments(taskConfigs, allSegments, 
System.currentTimeMillis());
+
+      if (candidateSegments.isEmpty()) {
+        LOGGER.info("No segments were eligible for compactMerge task for 
table: {}", tableNameWithType);
+        continue;
+      }
+
+      // get server to segment mappings
+      PinotHelixResourceManager pinotHelixResourceManager = 
_clusterInfoAccessor.getPinotHelixResourceManager();
+      Map<String, List<String>> serverToSegments = 
pinotHelixResourceManager.getServerToSegmentsMap(tableNameWithType);
+      BiMap<String, String> serverToEndpoints;
+      try {
+        serverToEndpoints = 
pinotHelixResourceManager.getDataInstanceAdminEndpoints(serverToSegments.keySet());
+      } catch (InvalidConfigException e) {
+        throw new RuntimeException(e);
+      }
+
+      ServerSegmentMetadataReader serverSegmentMetadataReader =
+          new ServerSegmentMetadataReader(_clusterInfoAccessor.getExecutor(),
+              _clusterInfoAccessor.getConnectionManager());
+
+      // Number of segments to query per server request. If a table has a lot 
of segments, then we might send a
+      // huge payload to pinot-server in request. Batching the requests will 
help in reducing the payload size.
+      int numSegmentsBatchPerServerRequest = Integer.parseInt(
+          
taskConfigs.getOrDefault(MinionConstants.UpsertCompactMergeTask.NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST,
+              String.valueOf(DEFAULT_NUM_SEGMENTS_BATCH_PER_SERVER_REQUEST)));
+
+      Map<String, List<ValidDocIdsMetadataInfo>> validDocIdsMetadataList =
+          
serverSegmentMetadataReader.getSegmentToValidDocIdsMetadataFromServer(tableNameWithType,
 serverToSegments,
+              serverToEndpoints, null, 60_000, 
ValidDocIdsType.SNAPSHOT.toString(),
+              numSegmentsBatchPerServerRequest);
+
+      Map<String, SegmentZKMetadata> candidateSegmentsMap =
+          
candidateSegments.stream().collect(Collectors.toMap(SegmentZKMetadata::getSegmentName,
 Function.identity()));
+
+      Set<String> alreadyMergedSegments = 
getAlreadyMergedSegments(allSegments);
+
+      SegmentSelectionResult segmentSelectionResult =
+          processValidDocIdsMetadata(taskConfigs, candidateSegmentsMap, 
validDocIdsMetadataList, alreadyMergedSegments);
+
+      if (!segmentSelectionResult.getSegmentsForDeletion().isEmpty()) {
+        pinotHelixResourceManager.deleteSegments(tableNameWithType, 
segmentSelectionResult.getSegmentsForDeletion(),
+            "0d");
+        LOGGER.info(
+            "Deleted segments containing only invalid records for table: {}, 
number of segments to be deleted: {}",
+            tableNameWithType, 
segmentSelectionResult.getSegmentsForDeletion());
+      }
+
+      int numTasks = 0;
+      int maxTasks = 
Integer.parseInt(taskConfigs.getOrDefault(MinionConstants.TABLE_MAX_NUM_TASKS_KEY,
+          String.valueOf(MinionConstants.DEFAULT_TABLE_MAX_NUM_TASKS)));
+      for (Map.Entry<Integer, List<List<SegmentMergerMetadata>>> entry
+          : 
segmentSelectionResult.getSegmentsForCompactMergeByPartition().entrySet()) {
+        if (numTasks == maxTasks) {
+          break;
+        }
+        List<List<SegmentMergerMetadata>> groups = entry.getValue();
+        // no valid groups found in the partition to merge
+        if (groups.isEmpty()) {
+          continue;
+        }
+        // there are no groups with more than 1 segment to merge
+        // TODO this can be later removed if we want to just do single-segment 
compaction from this task
+        if (groups.get(0).size() <= 1) {
+          continue;
+        }
+        // TODO see if multiple groups of same partition can be added
+        Map<String, String> configs = new 
HashMap<>(getBaseTaskConfigs(tableConfig,
+            groups.get(0).stream()
+                .map(x -> 
x.getSegmentZKMetadata().getSegmentName()).collect(Collectors.toList())));
+        configs.put(MinionConstants.DOWNLOAD_URL_KEY, 
getDownloadUrl(groups.get(0)));
+        configs.put(MinionConstants.UPLOAD_URL_KEY, 
_clusterInfoAccessor.getVipUrl() + "/segments");
+        configs.put(MinionConstants.ORIGINAL_SEGMENT_CRC_KEY, 
getSegmentCrcList(groups.get(0)));
+        pinotTaskConfigs.add(new 
PinotTaskConfig(MinionConstants.UpsertCompactMergeTask.TASK_TYPE, configs));
+        numTasks++;
+      }
+      LOGGER.info("Finished generating {} tasks configs for table: {}", 
numTasks, tableNameWithType);
+    }
+    return pinotTaskConfigs;
+  }
+
+  @VisibleForTesting
+  public static SegmentSelectionResult processValidDocIdsMetadata(
+      Map<String, String> taskConfigs,
+      Map<String, SegmentZKMetadata> candidateSegmentsMap,
+      Map<String, List<ValidDocIdsMetadataInfo>> validDocIdsMetadataInfoMap,
+      Set<String> alreadyMergedSegments) {
+    Map<Integer, List<SegmentMergerMetadata>> segmentsEligibleForCompactMerge 
= new HashMap<>();
+    List<String> segmentsForDeletion = new ArrayList<>();
+    for (String segmentName : validDocIdsMetadataInfoMap.keySet()) {
+      // check if segment is part of completed segments
+      if (!candidateSegmentsMap.containsKey(segmentName)) {
+        LOGGER.warn("Segment {} is not found in the completed segments list, 
skipping it for {}", segmentName,
+            MinionConstants.UpsertCompactMergeTask.TASK_TYPE);
+        continue;
+      }
+      SegmentZKMetadata segment = candidateSegmentsMap.get(segmentName);
+      for (ValidDocIdsMetadataInfo validDocIdsMetadata : 
validDocIdsMetadataInfoMap.get(segmentName)) {
+        long totalInvalidDocs = validDocIdsMetadata.getTotalInvalidDocs();
+        long totalValidDocs = validDocIdsMetadata.getTotalValidDocs();
+
+        // Skip segments if the crc from zk metadata and server does not 
match. They may be getting reloaded.
+        if (segment.getCrc() != 
Long.parseLong(validDocIdsMetadata.getSegmentCrc())) {
+          LOGGER.warn("CRC mismatch for segment: {}, (segmentZKMetadata={}, 
validDocIdsMetadata={})", segmentName,
+              segment.getCrc(), validDocIdsMetadata.getSegmentCrc());
+          continue;
+        }
+
+        // segments eligible for deletion with no valid records
+        long totalDocs = validDocIdsMetadata.getTotalDocs();
+        if (totalInvalidDocs == totalDocs) {
+          segmentsForDeletion.add(segmentName);
+        } else if (alreadyMergedSegments.contains(segmentName)) {
+          LOGGER.debug("Segment {} already merged. Skipping it for {}", 
segmentName,
+              MinionConstants.UpsertCompactMergeTask.TASK_TYPE);
+          continue;
+        } else {
+          Integer partitionID = 
SegmentUtils.getPartitionIdFromRealtimeSegmentName(segmentName);
+          if (partitionID == null) {
+            LOGGER.warn("Partition ID not found for segment: {}, skipping it 
for {}", segmentName,
+                MinionConstants.UpsertCompactMergeTask.TASK_TYPE);
+            continue;
+          }
+          segmentsEligibleForCompactMerge.computeIfAbsent(partitionID, k -> 
new ArrayList<>())
+              .add(new SegmentMergerMetadata(segment, totalValidDocs, 
totalInvalidDocs));
+        }
+        break;
+      }
+    }
+
+    segmentsEligibleForCompactMerge.forEach((partitionID, segmentList) ->
+        segmentList.sort(Comparator.comparingLong(o -> 
o.getSegmentZKMetadata().getCreationTime()))
+    );
+
+    // Map to store the result: each key (partition) will have a list of groups
+    Map<Integer, List<List<SegmentMergerMetadata>>> groupedSegments = new 
HashMap<>();
+
+    // Iterate over each partition and process its segments list
+    for (Map.Entry<Integer, List<SegmentMergerMetadata>> entry : 
segmentsEligibleForCompactMerge.entrySet()) {
+      int partitionID = entry.getKey();
+      List<SegmentMergerMetadata> segments = entry.getValue();
+      // task config thresholds
+      // TODO add output segment size as one of the thresholds
+      long validDocsThreshold = 
Long.parseLong(taskConfigs.getOrDefault(MinionConstants
+              .UpsertCompactMergeTask.MAX_NUM_RECORDS_PER_SEGMENT_KEY,
+          
String.valueOf(MinionConstants.UpsertCompactMergeTask.DEFAULT_MAX_NUM_RECORDS_PER_SEGMENT)));
+      long maxRecordsPerTask = 
Long.parseLong(taskConfigs.getOrDefault(MinionConstants
+              .UpsertCompactMergeTask.MAX_NUM_RECORDS_PER_TASK_KEY,
+          
String.valueOf(MinionConstants.UpsertCompactMergeTask.DEFAULT_MAX_NUM_RECORDS_PER_TASK)));
+      long maxNumSegments = 
Long.parseLong(taskConfigs.getOrDefault(MinionConstants
+              .UpsertCompactMergeTask.MAX_NUM_SEGMENTS_PER_TASK_KEY,
+          
String.valueOf(MinionConstants.UpsertCompactMergeTask.DEFAULT_MAX_NUM_SEGMENTS_PER_TASK)));
+
+      // List to store groups for the current partition
+      List<List<SegmentMergerMetadata>> groups = new ArrayList<>();
+      List<SegmentMergerMetadata> currentGroup = new ArrayList<>();
+
+      // variables to maintain current group sum
+      long currentValidDocsSum = 0;
+      long currentTotalDocsSum = 0;
+
+      for (SegmentMergerMetadata segment : segments) {
+        long validDocs = segment.getValidDocIds();
+        long invalidDocs = segment.getInvalidDocIds();
+
+        // Check if adding this segment would keep the validDocs sum within 
the threshold
+        if (currentValidDocsSum + validDocs <= validDocsThreshold && 
currentGroup.size() < maxNumSegments
+            && currentTotalDocsSum + validDocs + invalidDocs < 
maxRecordsPerTask) {
+          // Add the segment to the current group
+          currentGroup.add(segment);
+          currentValidDocsSum += validDocs;
+          currentTotalDocsSum += validDocs + invalidDocs;
+        } else {
+          // Finalize the current group and start a new one
+          if (!currentGroup.isEmpty()) {
+            groups.add(new ArrayList<>(currentGroup));  // Add the finalized 
group
+          }
+
+          // Reset current group, sums and start with the new segment
+          currentGroup = new ArrayList<>();
+          currentGroup.add(segment);
+          currentValidDocsSum = validDocs;
+          currentTotalDocsSum = validDocs + invalidDocs;
+        }
+      }
+      // Add the last group
+      if (!currentGroup.isEmpty()) {
+        groups.add(new ArrayList<>(currentGroup)); // Add a copy of the 
current group
+      }
+
+      // Sort groups by total invalidDocs in descending order, if invalidDocs 
count are same, prefer group with
+      // higher number of small segments in them
+      // remove the groups having only 1 segments in them -- this check can be 
later removed if we want single-segment

Review Comment:
   `--` meant a TODO here?



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