xds: implement the top-level LB policy (grpc#7203)

The top-level LB policy, which is an aggregator for CDS policies. It maintains the lifecycle of CDS LB policy instances. The pick argument taken from the Channel contains the information to determine which child CDS policy instance should the picking operation be delegated to.

The implementation is similar to the action part of what we currently have in the routing policy. The existing routing policy will be refactored to two parts, with the route match part moved into ConfigSelector and action part being this top-level LB policy.

xds/src/main/java/io/grpc/xds/ClusterManagerLoadBalancer.java

@@ -0,0 +1,265 @@
+/*
+ * Copyright 2020 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.xds;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+import static io.grpc.ConnectivityState.CONNECTING;
+import static io.grpc.ConnectivityState.IDLE;
+import static io.grpc.ConnectivityState.READY;
+import static io.grpc.ConnectivityState.TRANSIENT_FAILURE;
+import static io.grpc.xds.XdsSubchannelPickers.BUFFER_PICKER;
+
+import com.google.common.annotations.VisibleForTesting;
+import io.grpc.CallOptions;
+import io.grpc.ConnectivityState;
+import io.grpc.InternalLogId;
+import io.grpc.LoadBalancer;
+import io.grpc.LoadBalancerProvider;
+import io.grpc.Status;
+import io.grpc.SynchronizationContext;
+import io.grpc.SynchronizationContext.ScheduledHandle;
+import io.grpc.internal.ServiceConfigUtil.PolicySelection;
+import io.grpc.util.ForwardingLoadBalancerHelper;
+import io.grpc.util.GracefulSwitchLoadBalancer;
+import io.grpc.xds.ClusterManagerLoadBalancerProvider.ClusterManagerConfig;
+import io.grpc.xds.XdsLogger.XdsLogLevel;
+import io.grpc.xds.XdsSubchannelPickers.ErrorPicker;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.TimeUnit;
+import javax.annotation.Nullable;
+
+/**
+ * The top-level load balancing policy.
+ */
+class ClusterManagerLoadBalancer extends LoadBalancer {
+
+  @VisibleForTesting
+  static final int DELAYED_CHILD_DELETION_TIME_MINUTES = 15;
+  @VisibleForTesting
+  static final CallOptions.Key<String> ROUTING_CLUSTER_NAME_KEY =
+      CallOptions.Key.create("io.grpc.xds.ROUTING_CLUSTER_NAME_KEY");
+
+  private final Map<String, ChildLbState> childLbStates = new HashMap<>();
+  private final Helper helper;
+  private final SynchronizationContext syncContext;
+  private final ScheduledExecutorService timeService;
+  private final XdsLogger logger;
+
+  ClusterManagerLoadBalancer(Helper helper) {
+    this.helper = checkNotNull(helper, "helper");
+    this.syncContext = checkNotNull(helper.getSynchronizationContext(), "syncContext");
+    this.timeService = checkNotNull(helper.getScheduledExecutorService(), "timeService");
+    logger = XdsLogger.withLogId(
+        InternalLogId.allocate("cluster_manager-lb", helper.getAuthority()));
+    logger.log(XdsLogLevel.INFO, "Created");
+  }
+
+  @Override
+  public void handleResolvedAddresses(ResolvedAddresses resolvedAddresses) {
+    logger.log(XdsLogLevel.DEBUG, "Received resolution result: {0}", resolvedAddresses);
+    ClusterManagerConfig config = (ClusterManagerConfig)
+        resolvedAddresses.getLoadBalancingPolicyConfig();
+    Map<String, PolicySelection> newChildPolicies = config.childPolicies;
+    logger.log(
+        XdsLogLevel.INFO,
+        "Received cluster_manager lb config: child names={0}", newChildPolicies.keySet());
+    for (Map.Entry<String, PolicySelection> entry : newChildPolicies.entrySet()) {
+      final String name = entry.getKey();
+      LoadBalancerProvider childPolicyProvider = entry.getValue().getProvider();
+      Object childConfig = entry.getValue().getConfig();
+      if (!childLbStates.containsKey(name)) {
+        childLbStates.put(name, new ChildLbState(name, childPolicyProvider));
+      } else {
+        childLbStates.get(name).reactivate(childPolicyProvider);
+      }
+      final LoadBalancer childLb = childLbStates.get(name).lb;
+      final ResolvedAddresses childAddresses =
+          resolvedAddresses.toBuilder().setLoadBalancingPolicyConfig(childConfig).build();
+      syncContext.execute(new Runnable() {
+        @Override
+        public void run() {
+          childLb.handleResolvedAddresses(childAddresses);
+        }
+      });
+    }
+    for (String name : childLbStates.keySet()) {
+      if (!newChildPolicies.containsKey(name)) {
+        childLbStates.get(name).deactivate();
+      }
+    }
+    updateOverallBalancingState();
+  }
+
+  @Override
+  public void handleNameResolutionError(Status error) {
+    logger.log(XdsLogLevel.WARNING, "Received name resolution error: {0}", error);
+    boolean gotoTransientFailure = true;
+    for (ChildLbState state : childLbStates.values()) {
+      if (!state.deactivated) {
+        gotoTransientFailure = false;
+        state.lb.handleNameResolutionError(error);
+      }
+    }
+    if (gotoTransientFailure) {
+      helper.updateBalancingState(TRANSIENT_FAILURE, new ErrorPicker(error));
+    }
+  }
+
+  @Override
+  public boolean canHandleEmptyAddressListFromNameResolution() {
+    return true;
+  }
+
+  @Override
+  public void shutdown() {
+    logger.log(XdsLogLevel.INFO, "Shutdown");
+    for (ChildLbState state : childLbStates.values()) {
+      state.shutdown();
+    }
+  }
+
+  private void updateOverallBalancingState() {
+    ConnectivityState overallState = null;
+    final Map<String, SubchannelPicker> childPickers = new HashMap<>();
+    for (ChildLbState childLbState : childLbStates.values()) {
+      if (childLbState.deactivated) {
+        continue;
+      }
+      childPickers.put(childLbState.name, childLbState.currentPicker);
+      overallState = aggregateState(overallState, childLbState.currentState);
+    }
+    if (overallState != null) {
+      SubchannelPicker picker = new SubchannelPicker() {
+        @Override
+        public PickResult pickSubchannel(PickSubchannelArgs args) {
+          String clusterName = args.getCallOptions().getOption(ROUTING_CLUSTER_NAME_KEY);
+          SubchannelPicker delegate = childPickers.get(clusterName);
+          if (delegate == null) {
+            return
+                PickResult.withError(
+                    Status.UNAVAILABLE.withDescription("Unable to find cluster " + clusterName));
+          }
+          return delegate.pickSubchannel(args);
+        }
+      };
+      helper.updateBalancingState(overallState, picker);
+    }
+  }
+
+  @Nullable
+  private static ConnectivityState aggregateState(
+      @Nullable ConnectivityState overallState, ConnectivityState childState) {
+    if (overallState == null) {
+      return childState;
+    }
+    if (overallState == READY || childState == READY) {
+      return READY;
+    }
+    if (overallState == CONNECTING || childState == CONNECTING) {
+      return CONNECTING;
+    }
+    if (overallState == IDLE || childState == IDLE) {
+      return IDLE;
+    }
+    return overallState;
+  }
+
+  private final class ChildLbState {
+    private final String name;
+    private final GracefulSwitchLoadBalancer lb;
+    private LoadBalancerProvider policyProvider;
+    private ConnectivityState currentState = CONNECTING;
+    private SubchannelPicker currentPicker = BUFFER_PICKER;
+    private boolean deactivated;
+    @Nullable
+    ScheduledHandle deletionTimer;
+
+    ChildLbState(String name, LoadBalancerProvider policyProvider) {
+      this.name = name;
+      this.policyProvider = policyProvider;
+      lb = new GracefulSwitchLoadBalancer(new ChildLbStateHelper());
+      lb.switchTo(policyProvider);
+    }
+
+    void deactivate() {
+      if (deactivated) {
+        return;
+      }
+
+      class DeletionTask implements Runnable {
+        @Override
+        public void run() {
+          shutdown();
+          childLbStates.remove(name);
+        }
+      }
+
+      deletionTimer =
+          syncContext.schedule(
+              new DeletionTask(),
+              DELAYED_CHILD_DELETION_TIME_MINUTES,
+              TimeUnit.MINUTES,
+              timeService);
+      deactivated = true;
+      logger.log(XdsLogLevel.DEBUG, "Child balancer {0} deactivated", name);
+    }
+
+    void reactivate(LoadBalancerProvider policyProvider) {
+      if (deletionTimer != null && deletionTimer.isPending()) {
+        deletionTimer.cancel();
+        deactivated = false;
+        logger.log(XdsLogLevel.DEBUG, "Child balancer {0} reactivated", name);
+      }
+      if (!this.policyProvider.getPolicyName().equals(policyProvider.getPolicyName())) {
+        logger.log(
+            XdsLogLevel.DEBUG,
+            "Child balancer {0} switching policy from {1} to {2}",
+            name, this.policyProvider.getPolicyName(), policyProvider.getPolicyName());
+        lb.switchTo(policyProvider);
+        this.policyProvider = policyProvider;
+      }
+    }
+
+    void shutdown() {
+      deactivated = true;
+      if (deletionTimer != null && deletionTimer.isPending()) {
+        deletionTimer.cancel();
+      }
+      lb.shutdown();
+      logger.log(XdsLogLevel.DEBUG, "Child balancer {0} deleted", name);
+    }
+
+    private final class ChildLbStateHelper extends ForwardingLoadBalancerHelper {
+
+      @Override
+      public void updateBalancingState(ConnectivityState newState, SubchannelPicker newPicker) {
+        currentState = newState;
+        currentPicker = newPicker;
+        if (!deactivated) {
+          updateOverallBalancingState();
+        }
+      }
+
+      @Override
+      protected Helper delegate() {
+        return helper;
+      }
+    }
+  }
+}