chore(spanner): add epsilon-greedy strategy to ReplicaSelector

Add an epsilon-greedy strategy to the standard ReplicaSelector to ensure that all
endpoints are used from time to time. This ensures that new endpoints that show up
are also used.

Author: olavloite

java-spanner/google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/PowerOfTwoReplicaSelector.java

@@ -19,6 +19,7 @@
 import com.google.api.core.BetaApi;
 import com.google.api.core.InternalApi;
 import com.google.common.base.MoreObjects;
+import com.google.common.base.Preconditions;
 import java.util.List;
 import java.util.Random;
 import java.util.concurrent.ThreadLocalRandom;
@@ -29,6 +30,19 @@
 @BetaApi
 public class PowerOfTwoReplicaSelector implements ReplicaSelector {
 
+  public static final double DEFAULT_EPSILON = 0.1;
+
+  private final double epsilon;
+
+  public PowerOfTwoReplicaSelector() {
+    this(DEFAULT_EPSILON);
+  }
+
+  public PowerOfTwoReplicaSelector(double epsilon) {
+    Preconditions.checkArgument(epsilon >= 0.0 && epsilon <= 1.0, "epsilon must be in [0, 1]");
+    this.epsilon = epsilon;
+  }
+
   @Override
   public ChannelEndpoint select(
       List<ChannelEndpoint> candidates, Function<ChannelEndpoint, Double> scoreLookup) {
@@ -40,6 +54,11 @@ public ChannelEndpoint select(
     }
 
     Random random = ThreadLocalRandom.current();
+
+    // Epsilon-greedy exploration: with probability epsilon, pick a random candidate.
+    if (random.nextDouble() < epsilon) {
+      return candidates.get(random.nextInt(candidates.size()));
+    }
     int index1 = random.nextInt(candidates.size());
     int index2 = random.nextInt(candidates.size() - 1);
     if (index2 >= index1) {

java-spanner/google-cloud-spanner/src/test/java/com/google/cloud/spanner/spi/v1/PowerOfTwoReplicaSelectorTest.java

@@ -75,7 +75,8 @@ public void testSingleElement() {
 
   @Test
   public void testTwoElementsPicksBetter() {
-    PowerOfTwoReplicaSelector selector = new PowerOfTwoReplicaSelector();
+    // Use epsilon=0.0 to test pure Po2RC behavior
+    PowerOfTwoReplicaSelector selector = new PowerOfTwoReplicaSelector(0.0);
     ChannelEndpoint better = new TestEndpoint("better");
     ChannelEndpoint worse = new TestEndpoint("worse");
 
@@ -92,7 +93,8 @@ public void testTwoElementsPicksBetter() {
 
   @Test
   public void testThreeElementsNeverPicksWorst() {
-    PowerOfTwoReplicaSelector selector = new PowerOfTwoReplicaSelector();
+    // Use epsilon=0.0 to test pure Po2RC behavior
+    PowerOfTwoReplicaSelector selector = new PowerOfTwoReplicaSelector(0.0);
     ChannelEndpoint best = new TestEndpoint("best");
     ChannelEndpoint middle = new TestEndpoint("middle");
     ChannelEndpoint worst = new TestEndpoint("worst");
@@ -112,7 +114,8 @@ public void testThreeElementsNeverPicksWorst() {
 
   @Test
   public void testNullScoresTreatedAsMax() {
-    PowerOfTwoReplicaSelector selector = new PowerOfTwoReplicaSelector();
+    // Use epsilon=0.0 to test pure Po2RC behavior
+    PowerOfTwoReplicaSelector selector = new PowerOfTwoReplicaSelector(0.0);
     ChannelEndpoint withScore = new TestEndpoint("withScore");
     ChannelEndpoint withoutScore = new TestEndpoint("withoutScore");
 
@@ -125,4 +128,27 @@ public void testNullScoresTreatedAsMax() {
       assertEquals(withScore, selector.select(candidates, scores::get));
     }
   }
+
+  @Test
+  public void testEpsilonExploration() {
+    // Set epsilon to 1.0 to force 100% exploration
+    PowerOfTwoReplicaSelector selector = new PowerOfTwoReplicaSelector(1.0);
+    ChannelEndpoint best = new TestEndpoint("best");
+    ChannelEndpoint worst = new TestEndpoint("worst");
+
+    Map<ChannelEndpoint, Double> scores = new HashMap<>();
+    scores.put(best, 10.0);
+    scores.put(worst, 20.0);
+
+    List<ChannelEndpoint> candidates = Arrays.asList(best, worst);
+
+    boolean pickedWorst = false;
+    for (int i = 0; i < 100; i++) {
+      if (selector.select(candidates, scores::get) == worst) {
+        pickedWorst = true;
+        break;
+      }
+    }
+    assertTrue("Should occasionally pick worst with epsilon=1.0", pickedWorst);
+  }
 }