core/src/main/java/io/grpc/LoadBalancer.java

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package io.grpc;

import java.util.List;

import javax.annotation.concurrent.ThreadSafe;

/**
 * A pluggable component that receives resolved addresses from {@link NameResolver} and provides the
 * channel a usable transport when asked.
 *
 * <p>Note to implementations: all methods are expected to return quickly. Any work that may block
 * should be done asynchronously.
 *
 * @param <T> the transport type to balance
 */
// TODO(zhangkun83): since it's also used for non-loadbalancing cases like pick-first,
// "RequestRouter" might be a better name.
@ExperimentalApi("https://github.com/grpc/grpc-java/issues/1771")
@ThreadSafe
public abstract class LoadBalancer<T> {
  /**
   * Pick a transport that Channel will use for next RPC.
   *
   * <p>If called after {@link #shutdown} has been called, this method will return
   * a transport that would fail all requests.
   *
   * @param affinity for affinity-based routing
   */
  @ExperimentalApi("https://github.com/grpc/grpc-java/issues/1766")
  public abstract T pickTransport(Attributes affinity);

  /**
   * Shuts down this {@code LoadBalancer}.
   */
  public void shutdown() { }

  /**
   * Handles newly resolved server groups and metadata attributes from name resolution system.
   * {@code servers} contained in {@link ResolvedServerInfoGroup} should be considered equivalent
   * but may be flattened into a single list if needed.
   *
   * <p>Implementations should not modify the given {@code servers}.
   *
   * @param servers the resolved server addresses, never empty.
   * @param attributes extra metadata from naming system.
   */
  public void handleResolvedAddresses(List<ResolvedServerInfoGroup> servers,
      Attributes attributes) { }

  /**
   * Handles an error from the name resolution system.
   *
   * @param error a non-OK status
   */
  public void handleNameResolutionError(Status error) { }

  /**
   * Called when a transport is fully connected and ready to accept traffic.
   */
  public void handleTransportReady(EquivalentAddressGroup addressGroup) { }

  /**
   * Called when a transport is shutting down.
   */
  public void handleTransportShutdown(EquivalentAddressGroup addressGroup, Status s) { }

  public abstract static class Factory {
    /**
     * Creates a {@link LoadBalancer} that will be used inside a channel.
     *
     * @param serviceName the DNS-style service name, which is also the authority
     * @param tm the interface where an {@code LoadBalancer} implementation gets connected
     *               transports from
     */
    public abstract <T> LoadBalancer<T> newLoadBalancer(String serviceName, TransportManager<T> tm);
  }
}