| /* |
| * Licensed to the Apache Software Foundation (ASF) under one |
| * or more contributor license agreements. See the NOTICE file |
| * distributed with this work for additional information |
| * regarding copyright ownership. The ASF licenses this file |
| * to you 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 org.apache.tinkerpop.gremlin.driver; |
| |
| import org.apache.tinkerpop.gremlin.driver.message.RequestMessage; |
| |
| import java.util.ArrayList; |
| import java.util.Collection; |
| import java.util.Iterator; |
| import java.util.List; |
| import java.util.Random; |
| import java.util.concurrent.CopyOnWriteArrayList; |
| import java.util.concurrent.atomic.AtomicInteger; |
| |
| /** |
| * Provides a method for selecting the host from a {@link Cluster}. |
| * |
| * @author Stephen Mallette (http://stephen.genoprime.com) |
| */ |
| public interface LoadBalancingStrategy extends Host.Listener { |
| |
| /** |
| * Initialize the strategy with the {@link Cluster} instance and the expected host list. |
| */ |
| public void initialize(final Cluster cluster, final Collection<Host> hosts); |
| |
| /** |
| * Provide an ordered list of hosts to send the the given {@link RequestMessage} to. |
| */ |
| public Iterator<Host> select(final RequestMessage msg); |
| |
| /** |
| * A simple round-robin strategy that simply selects the next host in the {@link Cluster} to send the |
| * {@link RequestMessage} to. |
| */ |
| public static class RoundRobin implements LoadBalancingStrategy { |
| |
| private final CopyOnWriteArrayList<Host> availableHosts = new CopyOnWriteArrayList<>(); |
| private final AtomicInteger index = new AtomicInteger(); |
| |
| @Override |
| public void initialize(final Cluster cluster, final Collection<Host> hosts) { |
| this.availableHosts.addAll(hosts); |
| this.index.set(new Random().nextInt(Math.max(hosts.size(), 1))); |
| } |
| |
| @Override |
| public Iterator<Host> select(final RequestMessage msg) { |
| final List<Host> hosts = new ArrayList<>(); |
| |
| // a host could be marked as dead in which case we dont need to send messages to it - just skip it for |
| // now. it might come back online later |
| availableHosts.iterator().forEachRemaining(host -> { |
| if (host.isAvailable()) hosts.add(host); |
| }); |
| |
| final int startIndex = index.getAndIncrement(); |
| |
| if (startIndex > Integer.MAX_VALUE - 10000) |
| index.set(0); |
| |
| return new Iterator<Host>() { |
| |
| private int currentIndex = startIndex; |
| private int remainingHosts = hosts.size(); |
| |
| @Override |
| public boolean hasNext() { |
| return remainingHosts > 0; |
| } |
| |
| @Override |
| public Host next() { |
| remainingHosts--; |
| int c = currentIndex++ % hosts.size(); |
| if (c < 0) |
| c += hosts.size(); |
| return hosts.get(c); |
| } |
| }; |
| } |
| |
| @Override |
| public void onAvailable(final Host host) { |
| this.availableHosts.addIfAbsent(host); |
| } |
| |
| @Override |
| public void onUnavailable(final Host host) { |
| this.availableHosts.remove(host); |
| } |
| |
| @Override |
| public void onNew(final Host host) { |
| onAvailable(host); |
| } |
| |
| @Override |
| public void onRemove(final Host host) { |
| onUnavailable(host); |
| } |
| } |
| } |