【SpringCloud】Ribbon默认负载均衡轮询算法原理

负载均衡算法：rest接口第几次请求数 % 服务器集群总数量 = 实际调用服务器位置下标 ，每次服务重启动后rest接口计数从1开始。

List instances = discoveryClient.getInstances(“CLOUD-PAYMENT-SERVICE”);

如： List [0] instances = 127.0.0.1:8002
　　　List [1] instances = 127.0.0.1:8001

8001+ 8002 组合成为集群，它们共计2台机器，集群总数为2， 按照轮询算法原理：

当总请求数为1时： 1 % 2 =1 对应下标位置为1 ，则获得服务地址为127.0.0.1:8001
当总请求数位2时： 2 % 2 =0 对应下标位置为0 ，则获得服务地址为127.0.0.1:8002
当总请求数位3时： 3 % 2 =1 对应下标位置为1 ，则获得服务地址为127.0.0.1:8001
当总请求数位4时： 4 % 2 =0 对应下标位置为0 ，则获得服务地址为127.0.0.1:8002
如此类推…

如果中途重启了一台，计数重头开始！！！

RoundRobinRule源码分析

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package com.netflix.loadbalancer;

import com.netflix.client.config.IClientConfig;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class RoundRobinRule extends AbstractLoadBalancerRule {
    
    
    private AtomicInteger nextServerCyclicCounter;
    private static final boolean AVAILABLE_ONLY_SERVERS = true;
    private static final boolean ALL_SERVERS = false;
    private static Logger log = LoggerFactory.getLogger(RoundRobinRule.class);

    public RoundRobinRule() {
    
    
        this.nextServerCyclicCounter = new AtomicInteger(0);
    }

    public RoundRobinRule(ILoadBalancer lb) {
    
    
        this();
        this.setLoadBalancer(lb);
    }

    public Server choose(ILoadBalancer lb, Object key) {
    
    
        if (lb == null) {
    
    
            log.warn("no load balancer");
            return null;
        } else {
    
    
            Server server = null;
            int count = 0;

            while(true) {
    
    
                if (server == null && count++ < 10) {
    
    
                    List<Server> reachableServers = lb.getReachableServers();
                    List<Server> allServers = lb.getAllServers();
                    int upCount = reachableServers.size();
                    int serverCount = allServers.size();
                    if (upCount != 0 && serverCount != 0) {
    
    
                        int nextServerIndex = this.incrementAndGetModulo(serverCount);
                        server = (Server)allServers.get(nextServerIndex);
                        if (server == null) {
    
    
                            Thread.yield();
                        } else {
    
    
                            if (server.isAlive() && server.isReadyToServe()) {
    
    
                                return server;
                            }

                            server = null;
                        }
                        continue;
                    }

                    log.warn("No up servers available from load balancer: " + lb);
                    return null;
                }

                if (count >= 10) {
    
    
                    log.warn("No available alive servers after 10 tries from load balancer: " + lb);
                }

                return server;
            }
        }
    }

    private int incrementAndGetModulo(int modulo) {
    
    
        int current;
        int next;
        do {
    
    
            current = this.nextServerCyclicCounter.get();
            next = (current + 1) % modulo;
        } while(!this.nextServerCyclicCounter.compareAndSet(current, next));

        return next;
    }

    public Server choose(Object key) {
    
    
        return this.choose(this.getLoadBalancer(), key);
    }

    public void initWithNiwsConfig(IClientConfig clientConfig) {
    
    
    }
}