A68 random subsetting

api/src/test/java/io/grpc/LoadBalancerRegistryTest.java

@@ -40,7 +40,7 @@ public void getClassesViaHardcoded_classesPresent() throws Exception {
   @Test
   public void stockProviders() {
     LoadBalancerRegistry defaultRegistry = LoadBalancerRegistry.getDefaultRegistry();
-    assertThat(defaultRegistry.providers()).hasSize(3);
+    assertThat(defaultRegistry.providers()).hasSize(4);
 
     LoadBalancerProvider pickFirst = defaultRegistry.getProvider("pick_first");
     assertThat(pickFirst).isInstanceOf(PickFirstLoadBalancerProvider.class);
@@ -56,6 +56,11 @@ public void stockProviders() {
     assertThat(outlierDetection.getClass().getName()).isEqualTo(
         "io.grpc.util.OutlierDetectionLoadBalancerProvider");
     assertThat(roundRobin.getPriority()).isEqualTo(5);
+
+    LoadBalancerProvider randomSubsetting = defaultRegistry.getProvider("random_subsetting");
+    assertThat(randomSubsetting.getClass().getName()).isEqualTo(
+        "io.grpc.util.RandomSubsettingLoadBalancerProvider");
+    assertThat(randomSubsetting.getPriority()).isEqualTo(5);
   }
 
   @Test

util/src/main/java/io/grpc/util/RandomSubsettingLoadBalancer.java

@@ -0,0 +1,168 @@
+/*
+ * Copyright 2025 The gRPC Authors
+ *
+ * Licensed 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 io.grpc.util;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import com.google.common.hash.HashCode;
+import com.google.common.hash.HashFunction;
+import com.google.common.hash.Hashing;
+import com.google.common.primitives.UnsignedBytes;
+import io.grpc.EquivalentAddressGroup;
+import io.grpc.LoadBalancer;
+import io.grpc.Status;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.Random;
+
+
+/**
+ * Wraps a child {@code LoadBalancer}, separating the total set of backends into smaller subsets for
+ * the child balancer to balance across.
+ *
+ * <p>This implements random subsetting gRFC:
+ * https://https://github.com/grpc/proposal/blob/master/A68-random-subsetting.md
+ */
+final class RandomSubsettingLoadBalancer extends LoadBalancer {
+  private static final Comparator<byte[]> BYTE_ARRAY_COMPARATOR =
+      UnsignedBytes.lexicographicalComparator();
+
+  private final GracefulSwitchLoadBalancer switchLb;
+  private final HashFunction hashFunc;
+
+  public RandomSubsettingLoadBalancer(Helper helper) {
+    switchLb = new GracefulSwitchLoadBalancer(checkNotNull(helper, "helper"));
+    int seed = new Random().nextInt();
+    hashFunc = Hashing.murmur3_128(seed);
+  }
+
+  @Override
+  public Status acceptResolvedAddresses(ResolvedAddresses resolvedAddresses) {
+    RandomSubsettingLoadBalancerConfig config =
+        (RandomSubsettingLoadBalancerConfig)
+            resolvedAddresses.getLoadBalancingPolicyConfig();
+
+    ResolvedAddresses subsetAddresses = filterEndpoints(resolvedAddresses, config.subsetSize);
+
+    return switchLb.acceptResolvedAddresses(
+        subsetAddresses.toBuilder()
+            .setLoadBalancingPolicyConfig(config.childConfig)
+            .build());
+  }
+
+  // implements the subsetting algorithm, as described in A68:
+  // https://github.com/grpc/proposal/pull/423
+  private ResolvedAddresses filterEndpoints(ResolvedAddresses resolvedAddresses, long subsetSize) {
+    // configured subset sizes in the range [Integer.MAX_VALUE, Long.MAX_VALUE] will always fall
+    // into this if statement due to collection indexing limitations in JVM
+    if (subsetSize >= resolvedAddresses.getAddresses().size()) {
+      return resolvedAddresses;
+    }
+
+    ArrayList<EndpointWithHash> endpointWithHashList =
+        new ArrayList<>(resolvedAddresses.getAddresses().size());
+
+    for (EquivalentAddressGroup addressGroup : resolvedAddresses.getAddresses()) {
+      endpointWithHashList.add(
+          new EndpointWithHash(
+              addressGroup,
+              hashFunc.hashString(
+                  addressGroup.getAddresses().get(0).toString(),
+                  StandardCharsets.UTF_8)));
+    }
+
+    Collections.sort(endpointWithHashList, new HashAddressComparator());
+
+    // array is constructed for subset sizes in range [0, Integer.MAX_VALUE), therefore casting
+    // from long to int is not going to overflow here
+    ArrayList<EquivalentAddressGroup> addressGroups = new ArrayList<>((int) subsetSize);
+
+    // for loop is executed for subset sizes in range [0, Integer.MAX_VALUE), therefore indexing
+    // variable is not going to overflow here
+    for (int idx = 0; idx < subsetSize; ++idx) {
+      addressGroups.add(endpointWithHashList.get(idx).addressGroup);
+    }
+
+    return resolvedAddresses.toBuilder().setAddresses(addressGroups).build();
+  }
+
+  @Override
+  public void handleNameResolutionError(Status error) {
+    switchLb.handleNameResolutionError(error);
+  }
+
+  @Override
+  public void shutdown() {
+    switchLb.shutdown();
+  }
+
+  private static final class EndpointWithHash {
+    public final EquivalentAddressGroup addressGroup;
+    public final HashCode hashCode;
+
+    public EndpointWithHash(EquivalentAddressGroup addressGroup, HashCode hashCode) {
+      this.addressGroup = addressGroup;
+      this.hashCode = hashCode;
+    }
+  }
+
+  private static final class HashAddressComparator implements Comparator<EndpointWithHash> {
+    @Override
+    public int compare(EndpointWithHash lhs, EndpointWithHash rhs) {
+      return BYTE_ARRAY_COMPARATOR.compare(lhs.hashCode.asBytes(), rhs.hashCode.asBytes());
+    }
+  }
+
+  public static final class RandomSubsettingLoadBalancerConfig {
+    public final long subsetSize;
+    public final Object childConfig;
+
+    private RandomSubsettingLoadBalancerConfig(long subsetSize, Object childConfig) {
+      this.subsetSize = subsetSize;
+      this.childConfig = childConfig;
+    }
+
+    public static class Builder {
+      Long subsetSize;
+      Object childConfig;
+
+      public Builder setSubsetSize(Integer subsetSize) {
+        checkNotNull(subsetSize, "subsetSize");
+        // {@code Integer.toUnsignedLong(int)} is not part of Android API level 21, therefore doing
+        // it manually
+        Long subsetSizeAsLong = ((long) subsetSize) & 0xFFFFFFFFL;
+        checkArgument(subsetSizeAsLong > 0L, "Subset size must be greater than 0");
+        this.subsetSize = subsetSizeAsLong;
+        return this;
+      }
+
+      public Builder setChildConfig(Object childConfig) {
+        this.childConfig = checkNotNull(childConfig, "childConfig");
+        return this;
+      }
+
+      public RandomSubsettingLoadBalancerConfig build() {
+        return new RandomSubsettingLoadBalancerConfig(
+            checkNotNull(subsetSize, "subsetSize"),
+            checkNotNull(childConfig, "childConfig"));
+      }
+    }
+  }
+}

util/src/main/java/io/grpc/util/RandomSubsettingLoadBalancerProvider.java

@@ -0,0 +1,86 @@
+/*
+ * Copyright 2025 The gRPC Authors
+ *
+ * Licensed 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 io.grpc.util;
+
+import io.grpc.Internal;
+import io.grpc.LoadBalancer;
+import io.grpc.LoadBalancerProvider;
+import io.grpc.NameResolver.ConfigOrError;
+import io.grpc.Status;
+import io.grpc.internal.JsonUtil;
+import java.util.Map;
+
+@Internal
+public final class RandomSubsettingLoadBalancerProvider extends LoadBalancerProvider {
+  private static final String POLICY_NAME = "random_subsetting";
+
+  @Override
+  public LoadBalancer newLoadBalancer(LoadBalancer.Helper helper) {
+    return new RandomSubsettingLoadBalancer(helper);
+  }
+
+  @Override
+  public boolean isAvailable() {
+    return true;
+  }
+
+  @Override
+  public int getPriority() {
+    return 5;
+  }
+
+  @Override
+  public String getPolicyName() {
+    return POLICY_NAME;
+  }
+
+  @Override
+  public ConfigOrError parseLoadBalancingPolicyConfig(Map<String, ?> rawConfig) {
+    try {
+      return parseLoadBalancingPolicyConfigInternal(rawConfig);
+    } catch (RuntimeException e) {
+      return ConfigOrError.fromError(
+          Status.UNAVAILABLE
+              .withCause(e)
+              .withDescription("Failed parsing configuration for " + getPolicyName()));
+    }
+  }
+
+  private ConfigOrError parseLoadBalancingPolicyConfigInternal(Map<String, ?> rawConfig) {
+    Integer subsetSize = JsonUtil.getNumberAsInteger(rawConfig, "subsetSize");
+    if (subsetSize == null) {
+      return ConfigOrError.fromError(
+          Status.INTERNAL.withDescription(
+              "Subset size missing in " + getPolicyName() + ", LB policy config=" + rawConfig));
+    }
+
+    ConfigOrError childConfig = GracefulSwitchLoadBalancer.parseLoadBalancingPolicyConfig(
+        JsonUtil.getListOfObjects(rawConfig, "childPolicy"));
+    if (childConfig.getError() != null) {
+      return ConfigOrError.fromError(Status.INTERNAL
+          .withDescription(
+              "Failed to parse child in " + getPolicyName() + ", LB policy config=" + rawConfig)
+          .withCause(childConfig.getError().asRuntimeException()));
+    }
+
+    return ConfigOrError.fromConfig(
+        new RandomSubsettingLoadBalancer.RandomSubsettingLoadBalancerConfig.Builder()
+            .setSubsetSize(subsetSize)
+            .setChildConfig(childConfig.getConfig())
+            .build());
+  }
+}

util/src/main/resources/META-INF/services/io.grpc.LoadBalancerProvider

@@ -1,2 +1,3 @@
 io.grpc.util.SecretRoundRobinLoadBalancerProvider$Provider
 io.grpc.util.OutlierDetectionLoadBalancerProvider
+io.grpc.util.RandomSubsettingLoadBalancerProvider