ZooKeeper应用之负载均衡

负载均衡

  服务器端负载均衡分为算法包括：轮循算法（Round Robin）、哈希算法（HASH）、响应速度算法（Response Time）、加权法（Weighted ）等。网上有文章提到“最少连接算法（Least Connection）”也是负载均衡算法的一种，当然，这样说是需要有前提条件的，就是当服务器性能良好，可以及时处理所有请求的时候，最少连接算法才能达到负载均衡的目的，否则最少连接的服务器可能正在处理比较耗时的操作，继续将请求分发到该服务器可能导致该服务器load值升高，性能下降。参见https://devcentral.f5.com/articles/back-to-basics-least-connections-is-not-least-loaded 。之后我会写关于负载均衡各类算法的博客。

ZK实现负载均衡
  使用zk做负载均衡，需要在zk上指定path下注册提供服务的集群中每个节点的ip地址，消费者需要在该path上注册watcher，当服务提供者集群中增删几点时，zk会发起watcher通知，所有消费者都会收到该watcher回调，此时，消费者可以及时知晓服务提供者集群中节点状况，在可用服务器列表中进行负载均衡计算。

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package org.zk.loadbalance;

import com.java.roundrrobin.RoundRobin;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooKeeper;
import org.junit.Before;

import java.io.IOException;
import java.util.List;
import java.util.concurrent.CountDownLatch;

/**
 * @auth zhengzhi.ren
 * @date 2015/7/27.
 */
public class LoadBalance {

    private ZooKeeper zkConn;

    private String path = "/dubbo/org.zk.naming.HelloWorldService/provider";

    private String connectionString = "127.0.0.1:2181";

    private int timeout = 5000;

    private List<String> hosts;

    private RoundRobin<String> roundRobin = new RoundRobin<String>(hosts);

    @Before
    public void connect() {
        try {
            final CountDownLatch semaphore = new CountDownLatch(1);
            zkConn = new ZooKeeper(connectionString, timeout, new Watcher() {
                @Override
                public void process(WatchedEvent e) {
                    if (e.getState() == Event.KeeperState.SyncConnected){
                        if (e.getType() == Event.EventType.None && e.getPath() == null){
                            System.out.println("Connect success!");
                            semaphore.countDown();
                        }
                    }
                }
            });
            semaphore.await();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void registry() {
        try {
            hosts = zkConn.getChildren(this.path, new NodeChildrenChangedWatcher());
        } catch (KeeperException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public String get(){
        return roundRobin.iterator().next();
    }

    class NodeChildrenChangedWatcher implements Watcher {

        @Override
        public void process(WatchedEvent watchedEvent) {
            if (watchedEvent.getType() == Event.EventType.NodeChildrenChanged) {
                registry();
            }
        }
    }
}

负载均衡之轮询算法

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package com.java.roundrrobin;

import java.util.Iterator;
import java.util.List;

/**
 * @auth zhengzhi.ren
 * @date 2015/7/27.
 * round robin algorithm of load balance
 */
public class RoundRobin<T>{
    private List<T> collection;

    public RoundRobin(List<T> collection) {
        this.collection = collection;
    }

    public Iterator<T> iterator() {
        return new Iterator<T>() {
            private int index = 0;
            @Override
            public boolean hasNext() {
                return false;
            }

            @Override
            public T next() {
                if (collection == null || collection.size() <= 0){
                    return null;
                }
                T e = collection.get(index);
                index = (index + 1) % collection.size();
                return e;
            }

            @Override
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
    }

    public List<T> getCollection() {
        return collection;
    }

    public void setCollection(List<T> collection) {
        this.collection = collection;
    }
}

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