com.hotels.styx.client.loadbalancing.strategies.PowerOfTwoStrategy Maven / Gradle / Ivy

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/*
  Copyright (C) 2013-2020 Expedia Inc.

  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 com.hotels.styx.client.loadbalancing.strategies;


import com.google.common.annotations.VisibleForTesting;
import com.hotels.styx.api.Environment;
import com.hotels.styx.api.extension.ActiveOrigins;
import com.hotels.styx.api.extension.RemoteHost;
import com.hotels.styx.api.extension.loadbalancing.spi.LoadBalancer;
import com.hotels.styx.api.extension.loadbalancing.spi.LoadBalancerFactory;
import com.hotels.styx.api.configuration.Configuration;

import java.util.Optional;
import java.util.Random;

import static com.google.common.collect.Iterables.toArray;
import static java.util.Objects.requireNonNull;


/**
 * A load balancing strategy that selects two hosts randomly and chooses the one with fewer ongoing connections.
 *
 */
public class PowerOfTwoStrategy implements LoadBalancer {
    private final ActiveOrigins activeOrigins;
    private final Random rng;

    @VisibleForTesting
    PowerOfTwoStrategy(ActiveOrigins activeOrigins, Random rng) {
        this.activeOrigins = requireNonNull(activeOrigins);
        this.rng = requireNonNull(rng);
    }

    public PowerOfTwoStrategy(ActiveOrigins activeOrigins) {
        this(activeOrigins, new Random());
    }

    /**
     * A load balancing strategy that favours the origin with the least response time.
     */
    public static class Factory implements LoadBalancerFactory {
        @Override
        public LoadBalancer create(Environment environment, Configuration strategyConfiguration, ActiveOrigins activeOrigins) {
            return new PowerOfTwoStrategy(activeOrigins);
        }
    }

    @Override
    public Optional choose(LoadBalancer.Preferences preferences) {
        RemoteHost[] hosts = toArray(activeOrigins.snapshot(), RemoteHost.class);

        if (hosts.length == 0) {
            return Optional.empty();
        } else if (hosts.length == 1) {
            return Optional.of(hosts[0]);
        } else {
            int i1 = rng.nextInt(hosts.length);
            int i2 = drawFromRemaining(hosts.length, i1);

            return Optional.of(betterOf(hosts[i1], hosts[i2]));
        }
    }

    private int drawFromRemaining(int bound, int otherIndex) {
        int i = rng.nextInt(bound - 1);
        return (i < otherIndex) ? i : i + 1;
    }

    private RemoteHost betterOf(RemoteHost host1, RemoteHost host2) {
        return host1.metric().ongoingConnections() < host2.metric().ongoingConnections() ? host1 : host2;
    }

}