core/src/main/java/io/grpc/util/RoundRobinLoadBalancerFactory.java

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package io.grpc.util;

import com.google.common.base.Supplier;
import io.grpc.Attributes;
import io.grpc.EquivalentAddressGroup;
import io.grpc.ExperimentalApi;
import io.grpc.LoadBalancer;
import io.grpc.NameResolver;
import io.grpc.ResolvedServerInfoGroup;
import io.grpc.Status;
import io.grpc.TransportManager;
import io.grpc.TransportManager.InterimTransport;
import io.grpc.internal.RoundRobinServerList;
import java.util.ArrayList;
import java.util.List;
import javax.annotation.concurrent.GuardedBy;


/**
 * A {@link LoadBalancer} that provides round-robin load balancing mechanism over the
 * addresses from the {@link NameResolver}.  The sub-lists received from the name resolver
 * are considered to be an {@link EquivalentAddressGroup} and each of these sub-lists is
 * what is then balanced across.
 */
@ExperimentalApi("https://github.com/grpc/grpc-java/issues/1771")
public final class RoundRobinLoadBalancerFactory extends LoadBalancer.Factory {

  private static final RoundRobinLoadBalancerFactory instance = new RoundRobinLoadBalancerFactory();

  private RoundRobinLoadBalancerFactory() {
  }

  public static RoundRobinLoadBalancerFactory getInstance() {
    return instance;
  }

  @Override
  public <T> LoadBalancer<T> newLoadBalancer(String serviceName, TransportManager<T> tm) {
    return new RoundRobinLoadBalancer<T>(tm);
  }

  private static class RoundRobinLoadBalancer<T> extends LoadBalancer<T> {
    private static final Status SHUTDOWN_STATUS =
        Status.UNAVAILABLE.augmentDescription("RoundRobinLoadBalancer has shut down");

    private final Object lock = new Object();

    @GuardedBy("lock")
    private RoundRobinServerList<T> addresses;
    @GuardedBy("lock")
    private InterimTransport<T> interimTransport;
    @GuardedBy("lock")
    private Status nameResolutionError;
    @GuardedBy("lock")
    private boolean closed;

    private final TransportManager<T> tm;

    private RoundRobinLoadBalancer(TransportManager<T> tm) {
      this.tm = tm;
    }

    @Override
    public T pickTransport(Attributes affinity) {
      final RoundRobinServerList<T> addressesCopy;
      synchronized (lock) {
        if (closed) {
          return tm.createFailingTransport(SHUTDOWN_STATUS);
        }
        if (addresses == null) {
          if (nameResolutionError != null) {
            return tm.createFailingTransport(nameResolutionError);
          }
          if (interimTransport == null) {
            interimTransport = tm.createInterimTransport();
          }
          return interimTransport.transport();
        }
        addressesCopy = addresses;
      }
      return addressesCopy.getTransportForNextServer();
    }

    @Override
    public void handleResolvedAddresses(List<ResolvedServerInfoGroup> updatedServers,
        Attributes attributes) {
      final InterimTransport<T> savedInterimTransport;
      final RoundRobinServerList<T> addressesCopy;
      synchronized (lock) {
        if (closed) {
          return;
        }
        addresses = new RoundRobinServerList.Builder<T>(tm).addAll(
            resolvedServerInfoGroupToEquivalentAddressGroup(updatedServers)).build();
        addressesCopy = addresses;
        nameResolutionError = null;
        savedInterimTransport = interimTransport;
        interimTransport = null;
      }
      if (savedInterimTransport != null) {
        savedInterimTransport.closeWithRealTransports(new Supplier<T>() {
            @Override public T get() {
              return addressesCopy.getTransportForNextServer();
            }
          });
      }
    }

    @Override
    public void handleNameResolutionError(Status error) {
      InterimTransport<T> savedInterimTransport;
      synchronized (lock) {
        if (closed) {
          return;
        }
        error = error.augmentDescription("Name resolution failed");
        savedInterimTransport = interimTransport;
        interimTransport = null;
        nameResolutionError = error;
      }
      if (savedInterimTransport != null) {
        savedInterimTransport.closeWithError(error);
      }
    }

    @Override
    public void shutdown() {
      InterimTransport<T> savedInterimTransport;
      synchronized (lock) {
        if (closed) {
          return;
        }
        closed = true;
        savedInterimTransport = interimTransport;
        interimTransport = null;
      }
      if (savedInterimTransport != null) {
        savedInterimTransport.closeWithError(SHUTDOWN_STATUS);
      }
    }

    private static List<EquivalentAddressGroup> resolvedServerInfoGroupToEquivalentAddressGroup(
        List<ResolvedServerInfoGroup> groupList) {
      List<EquivalentAddressGroup> addrs = new ArrayList<EquivalentAddressGroup>(groupList.size());
      for (ResolvedServerInfoGroup group : groupList) {
        addrs.add(group.toEquivalentAddressGroup());
      }
      return addrs;
    }
  }
}