kfaraz commented on code in PR #18860:
URL: https://github.com/apache/druid/pull/18860#discussion_r2639012911


##########
embedded-tests/src/test/java/org/apache/druid/testing/embedded/indexing/TaskScaleDownRolloverTest.java:
##########
@@ -0,0 +1,232 @@
+/*
+ * 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.druid.testing.embedded.indexing;
+
+import org.apache.druid.data.input.impl.TimestampSpec;
+import org.apache.druid.indexing.kafka.KafkaIndexTaskModule;
+import org.apache.druid.indexing.kafka.simulate.KafkaResource;
+import org.apache.druid.indexing.kafka.supervisor.KafkaSupervisorSpec;
+import 
org.apache.druid.indexing.seekablestream.supervisor.autoscaler.CostBasedAutoScalerConfig;
+import org.apache.druid.java.util.common.StringUtils;
+import org.apache.druid.query.DruidMetrics;
+import org.apache.druid.testing.embedded.EmbeddedBroker;
+import org.apache.druid.testing.embedded.EmbeddedClusterApis;
+import org.apache.druid.testing.embedded.EmbeddedCoordinator;
+import org.apache.druid.testing.embedded.EmbeddedDruidCluster;
+import org.apache.druid.testing.embedded.EmbeddedHistorical;
+import org.apache.druid.testing.embedded.EmbeddedIndexer;
+import org.apache.druid.testing.embedded.EmbeddedOverlord;
+import org.apache.druid.testing.embedded.EmbeddedRouter;
+import org.apache.druid.testing.embedded.junit5.EmbeddedClusterTestBase;
+import org.apache.kafka.clients.producer.ProducerRecord;
+import org.hamcrest.Matchers;
+import org.joda.time.DateTime;
+import org.joda.time.DateTimeZone;
+import org.joda.time.Seconds;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.Timeout;
+
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+
+import static 
org.apache.druid.indexing.seekablestream.supervisor.autoscaler.CostBasedAutoScaler.OPTIMAL_TASK_COUNT_METRIC;
+
+@SuppressWarnings("SameParameterValue")
+public class TaskScaleDownRolloverTest extends EmbeddedClusterTestBase
+{
+  private static final String TOPIC = 
EmbeddedClusterApis.createTestDatasourceName();
+  private static final String EVENT_TEMPLATE = 
"{\"timestamp\":\"%s\",\"dimension\":\"value%d\",\"metric\":%d}";
+  private static final int PARTITION_COUNT = 50;
+
+  private final EmbeddedIndexer indexer = new EmbeddedIndexer();
+  private final EmbeddedOverlord overlord = new EmbeddedOverlord();
+  private KafkaResource kafkaServer;
+
+  @Override
+  public EmbeddedDruidCluster createCluster()
+  {
+    final EmbeddedDruidCluster cluster = 
EmbeddedDruidCluster.withEmbeddedDerbyAndZookeeper();
+
+    kafkaServer = new KafkaResource()
+    {
+      @Override
+      public void start()
+      {
+        super.start();
+        createTopicWithPartitions(TOPIC, PARTITION_COUNT);
+        produceRecordsToKafka(500, 1);
+      }
+
+      @Override
+      public void stop()
+      {
+        deleteTopic(TOPIC);
+        super.stop();
+      }
+    };
+
+    // Increase worker capacity to handle more tasks
+    indexer.addProperty("druid.segment.handoff.pollDuration", "PT0.1s")
+           .addProperty("druid.worker.capacity", "60");
+
+    overlord.addProperty("druid.indexer.task.default.context", 
"{\"useConcurrentLocks\": true}")
+            .addProperty("druid.manager.segments.pollDuration", "PT0.1s");
+
+    cluster.useLatchableEmitter()
+           .useDefaultTimeoutForLatchableEmitter(20)
+           .addServer(new EmbeddedCoordinator())
+           .addServer(overlord)
+           .addServer(indexer)
+           .addServer(new EmbeddedRouter())
+           .addServer(new EmbeddedBroker())
+           .addServer(new EmbeddedHistorical())
+           .addExtension(KafkaIndexTaskModule.class)
+           .addCommonProperty("druid.monitoring.emissionPeriod", "PT1s")
+           .addResource(kafkaServer);
+
+    return cluster;
+  }
+
+  /**
+   * Tests that scale down happen during task rollover via checkTaskDuration().
+   *
+   * <p>Test flow:</p>
+   * <ol>
+   *   <li>Start supervisor with 20 tasks and 50 partitions, minimal data (500 
records)</li>
+   *   <li>Wait for initial tasks to start running</li>
+   *   <li>Wait for the first task rollover to complete (task duration is 10 
seconds)</li>
+   *   <li>Verify that after rollover, fewer tasks are running due to 
cost-based autoscaler (no ingestion at all)</li>
+   * </ol>
+   *
+   * <p>Scale down during rollover is triggered in {@code 
SeekableStreamSupervisor.checkTaskDuration()}
+   * when all task groups have rolled over and the autoscaler recommends a 
lower task count.</p>
+   */
+  @SuppressWarnings("resource")
+  @Test
+  @Timeout(300)
+  void test_scaleDownDuringTaskRollover()

Review Comment:
   > because we're testing abstract autoscaler actions during the task 
rollover, no specifically the cost-based one.
   
   I am not sure if this is entirely true, since cost-based auto-scaler is the 
only one that supports task count change on rollover right now.
   
   The new test should be in the existing `CostBasedAutoScalerIntegrationTest` 
since it uses the same cluster setup (AFAICT) and verifies an important aspect 
of the cost-based auto-scaler.
   
   In the future, when we add more auto-scalers, we can add separate tests for 
them.



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