[grid] Add GreedySlotSelector as a built-in slot-selector option

java/src/org/openqa/selenium/grid/distributor/selector/GreedySlotSelector.java

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+// Licensed to the Software Freedom Conservancy (SFC) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The SFC 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.openqa.selenium.grid.distributor.selector;
+
+import static com.google.common.collect.ImmutableSet.toImmutableSet;
+import static org.openqa.selenium.grid.data.Availability.UP;
+
+import java.util.Comparator;
+import java.util.Set;
+import org.openqa.selenium.Capabilities;
+import org.openqa.selenium.grid.config.Config;
+import org.openqa.selenium.grid.data.NodeStatus;
+import org.openqa.selenium.grid.data.SemanticVersionComparator;
+import org.openqa.selenium.grid.data.Slot;
+import org.openqa.selenium.grid.data.SlotId;
+import org.openqa.selenium.grid.data.SlotMatcher;
+
+/**
+ * A greedy slot selector that aims to maximize node utilization by minimizing the number of
+ * partially filled nodes. The algorithm works as follows: 1. Sort nodes by their utilization load
+ * (descending). 2. Among nodes with the same utilization, prefer those with fewer total slots. 3.
+ * Then sort by the last session created (oldest first). This approach helps to: - Fill up nodes
+ * that are already partially utilized - Minimize the number of nodes that are partially filled -
+ * Distribute load evenly across nodes
+ */
+public class GreedySlotSelector implements SlotSelector {
+
+  public static SlotSelector create(Config config) {
+    return new GreedySlotSelector();
+  }
+
+  @Override
+  public Set<SlotId> selectSlot(
+      Capabilities capabilities, Set<NodeStatus> nodes, SlotMatcher slotMatcher) {
+    return nodes.stream()
+        .filter(node -> node.hasCapacity(capabilities, slotMatcher) && node.getAvailability() == UP)
+        .sorted(
+            // First and foremost, sort by utilization ratio (descending)
+            // This ensures we ALWAYS try to fill nodes that are already partially utilized first
+            Comparator.comparingDouble(NodeStatus::getLoad)
+                .reversed()
+                // Then sort by total number of slots (ascending)
+                // Among nodes with same utilization, prefer those with fewer total slots
+                .thenComparingLong(node -> node.getSlots().size())
+                // Then last session created (oldest first)
+                .thenComparingLong(NodeStatus::getLastSessionCreated)
+                // Then sort by stereotype browserVersion (descending order)
+                .thenComparing(
+                    Comparator.comparing(
+                        NodeStatus::getBrowserVersion, new SemanticVersionComparator().reversed())))
+        .flatMap(
+            node ->
+                node.getSlots().stream()
+                    .filter(slot -> slot.getSession() == null)
+                    .filter(slot -> slot.isSupporting(capabilities, slotMatcher))
+                    .map(Slot::getId))
+        .collect(toImmutableSet());
+  }
+}