Fix deterministic server selection in balancer by adding tie-breaking logic

server/src/main/java/org/apache/druid/client/selector/ConnectionCountServerSelectorStrategy.java

@@ -30,11 +30,15 @@
 import java.util.Comparator;
 import java.util.List;
 import java.util.Set;
+import java.util.concurrent.ThreadLocalRandom;
 
 public class ConnectionCountServerSelectorStrategy implements ServerSelectorStrategy
 {
   private static final Comparator<QueryableDruidServer> COMPARATOR =
-      Comparator.comparingInt(s -> ((DirectDruidClient) s.getQueryRunner()).getNumOpenConnections());
+      Comparator.comparingDouble(s ->
+                                     ((DirectDruidClient) s.getQueryRunner()).getNumOpenConnections()
+                                     + ThreadLocalRandom.current().nextDouble()
+      );
 
   @Nullable
   @Override

server/src/test/java/org/apache/druid/client/selector/ConnectionCountServerSelectorStrategyTest.java

@@ -0,0 +1,115 @@
+/*
+ * 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.client.selector;
+
+import org.apache.druid.client.DirectDruidClient;
+import org.apache.druid.client.DruidServer;
+import org.apache.druid.client.QueryableDruidServer;
+import org.apache.druid.java.util.common.DateTimes;
+import org.apache.druid.java.util.common.Intervals;
+import org.apache.druid.server.coordination.ServerType;
+import org.apache.druid.timeline.DataSegment;
+import org.apache.druid.timeline.partition.NumberedShardSpec;
+import org.easymock.EasyMock;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+public class ConnectionCountServerSelectorStrategyTest
+{
+  @Test
+  public void testDifferentConnectionCount()
+  {
+    QueryableDruidServer s1 = mockServer("test1", 2);
+    QueryableDruidServer s2 = mockServer("test2", 1);
+    QueryableDruidServer s3 = mockServer("test3", 4);
+    ServerSelector serverSelector = initSelector(s1, s2, s3);
+
+    for (int i = 0; i < 100; ++i) {
+      Assert.assertEquals(s2, serverSelector.pick(null));
+    }
+  }
+
+  @Test
+  public void testBalancerTieBreaking()
+  {
+    QueryableDruidServer s1 = mockServer("test1", 100);
+    QueryableDruidServer s2 = mockServer("test2", 100);
+    ServerSelector serverSelector = initSelector(s1, s2);
+
+    Set<String> pickedServers = new HashSet<>();
+    for (int i = 0; i < 100; ++i) {
+      pickedServers.add(serverSelector.pick(null).getServer().getName());
+    }
+    Assert.assertTrue(
+        "Multiple servers should be selected when the number of connections is equal.",
+        pickedServers.size() > 1
+    );
+  }
+
+  private QueryableDruidServer mockServer(String name, int openConnections)
+  {
+    DirectDruidClient client = EasyMock.createMock(DirectDruidClient.class);
+    EasyMock.expect(client.getNumOpenConnections()).andReturn(openConnections).anyTimes();
+    EasyMock.replay(client);
+    return new QueryableDruidServer(
+        new DruidServer(
+            name,
+            "localhost",
+            null,
+            0,
+            ServerType.HISTORICAL,
+            DruidServer.DEFAULT_TIER,
+            0
+        ), client
+    );
+  }
+
+  private ServerSelector initSelector(QueryableDruidServer... servers)
+  {
+    TierSelectorStrategy strategy = new HighestPriorityTierSelectorStrategy(new ConnectionCountServerSelectorStrategy());
+    ServerSelector selector = new ServerSelector(
+        new DataSegment(
+            "test",
+            Intervals.of("2025-01-01/2025-01-02"),
+            DateTimes.of("2025-01-01").toString(),
+            new HashMap<>(),
+            new ArrayList<>(),
+            new ArrayList<>(),
+            new NumberedShardSpec(0, 0),
+            0,
+            0L
+        ), strategy
+    );
+    List<QueryableDruidServer> serverList = new ArrayList<>(Arrays.asList(servers));
+    Collections.shuffle(serverList);
+    for (QueryableDruidServer server : serverList) {
+      selector.addServerAndUpdateSegment(server, selector.getSegment());
+    }
+    return selector;
+  }
+}