RandomStrategy.java

// =================================================================================================
// Copyright 2011 Twitter, Inc.
// -------------------------------------------------------------------------------------------------
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this work except in compliance with the License.
// You may obtain a copy of the License in the LICENSE file, or 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 com.twitter.common.net.loadbalancing;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Lists;
import com.twitter.common.net.pool.ResourceExhaustedException;

import java.util.Collection;
import java.util.List;
import java.util.Random;
import java.util.Set;

/**
 * A load balancer that selects a random backend each time a request is made..
 *
 * @author William Farner
 */
public class RandomStrategy<S> extends StaticLoadBalancingStrategy<S> {

  private List<S> targets = Lists.newArrayList();
  private final Random random;

  public RandomStrategy() {
    this(new Random());
  }

  @VisibleForTesting
  RandomStrategy(Random random) {
    this.random = Preconditions.checkNotNull(random);
  }

  @Override
  protected Collection<S> onBackendsOffered(Set<S> targets) {
    this.targets = ImmutableList.copyOf(targets);
    return this.targets;
  }

  @Override
  public S nextBackend() throws ResourceExhaustedException {
    if (targets.isEmpty()) throw new ResourceExhaustedException("No backends.");
    return targets.get(random.nextInt(targets.size()));
  }
}