在集群负载均衡时，Dubbo 提供了多种均衡策略，缺省为 random 随机调用。我们还可以扩展自己的负责均衡策略，前提是你已经从一个小白变成了大牛，嘻嘻
1、Random LoadBalance
1.1 随机，按权重设置随机概率。
1.2 在一个截面上碰撞的概率高，但调用量越大分布越均匀，而且按概率使用权重后也比较均匀，有利于动态调整提供者权重。
1.3 源码分析
package com.alibaba.dubbo.rpc.cluster.loadbalance; import java.util.List; import java.util.Random; import com.alibaba.dubbo.common.URL; import com.alibaba.dubbo.rpc.Invocation; import com.alibaba.dubbo.rpc.Invoker; /** * random load balance. * * @author qianlei * @author william.liangf */ public class RandomLoadBalance extends AbstractLoadBalance { public static final String NAME = "random"; private final Random random = new Random(); protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) { int length = invokers.size(); // 总个数 int totalWeight = 0; // 总权重 boolean sameWeight = true; // 权重是否都一样 for (int i = 0; i < length; i++) { int weight = getWeight(invokers.get(i), invocation); totalWeight += weight; // 累计总权重 if (sameWeight && i > 0 && weight != getWeight(invokers.get(i - 1), invocation)) { sameWeight = false; // 计算所有权重是否一样 } } if (totalWeight > 0 && ! sameWeight) { // 如果权重不相同且权重大于0则按总权重数随机 int offset = random.nextInt(totalWeight); // 并确定随机值落在哪个片断上 for (int i = 0; i < length; i++) { offset -= getWeight(invokers.get(i), invocation); if (offset < 0) { return invokers.get(i); } } } // 如果权重相同或权重为0则均等随机 return invokers.get(random.nextInt(length)); } }
说明：从源码可以看出随机负载均衡的策略分为两种情况
a. 如果总权重大于0并且权重不相同，就生成一个1~totalWeight(总权重数)的随机数，然后再把随机数和所有的权重值一一相减得到一个新的随机数，直到随机 数小于0，那么此时访问的服务器就是使得随机数小于0的权重所在的机器
b. 如果权重相同或者总权重数为0，就生成一个1~length(权重的总个数)的随机数，此时所访问的机器就是这个随机数对应的权重所在的机器
2、RoundRobin LoadBalance
2.1 轮循，按公约后的权重设置轮循比率。
2.2 存在慢的提供者累积请求的问题，比如：第二台机器很慢，但没挂，当请求调到第二台时就卡在那，久而久之，所有请求都卡在调到第二台上。
2.3 源码分析
package com.alibaba.dubbo.rpc.cluster.loadbalance; import java.util.ArrayList; import java.util.List; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ConcurrentMap; import com.alibaba.dubbo.common.URL; import com.alibaba.dubbo.common.utils.AtomicPositiveInteger; import com.alibaba.dubbo.rpc.Invocation; import com.alibaba.dubbo.rpc.Invoker; /** * Round robin load balance. * * @author qian.lei * @author william.liangf */ public class RoundRobinLoadBalance extends AbstractLoadBalance { public static final String NAME = "roundrobin"; private final ConcurrentMap<String, AtomicPositiveInteger> sequences = new ConcurrentHashMap<String, AtomicPositiveInteger>(); private final ConcurrentMap<String, AtomicPositiveInteger> weightSequences = new ConcurrentHashMap<String, AtomicPositiveInteger>(); protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) { String key = invokers.get(0).getUrl().getServiceKey() + "." + invocation.getMethodName(); int length = invokers.size(); // 总个数 int maxWeight = 0; // 最大权重 int minWeight = Integer.MAX_VALUE; // 最小权重 for (int i = 0; i < length; i++) { int weight = getWeight(invokers.get(i), invocation); maxWeight = Math.max(maxWeight, weight); // 累计最大权重 minWeight = Math.min(minWeight, weight); // 累计最小权重 } if (maxWeight > 0 && minWeight < maxWeight) { // 权重不一样 AtomicPositiveInteger weightSequence = weightSequences.get(key); if (weightSequence == null) { weightSequences.putIfAbsent(key, new AtomicPositiveInteger()); weightSequence = weightSequences.get(key); } int currentWeight = weightSequence.getAndIncrement() % maxWeight; List<Invoker<T>> weightInvokers = new ArrayList<Invoker<T>>(); for (Invoker<T> invoker : invokers) { // 筛选权重大于当前权重基数的Invoker if (getWeight(invoker, invocation) > currentWeight) { weightInvokers.add(invoker); } } int weightLength = weightInvokers.size(); if (weightLength == 1) { return weightInvokers.get(0); } else if (weightLength > 1) { invokers = weightInvokers; length = invokers.size(); } } AtomicPositiveInteger sequence = sequences.get(key); if (sequence == null) { sequences.putIfAbsent(key, new AtomicPositiveInteger()); sequence = sequences.get(key); } // 取模轮循 return invokers.get(sequence.getAndIncrement() % length); } }
说明：从源码可以看出轮循负载均衡的算法是：
a. 如果权重不一样时，获取一个当前的权重基数，然后从权重集合中筛选权重大于当前权重基数的集合，如果筛选出的集合的长度为1，此时所访问的机器就是集合里面的权重对应的机器
b. 如果权重一样时就取模轮循
3、LeastActive LoadBalance
3.1 最少活跃调用数，相同活跃数的随机，活跃数指调用前后计数差(调用前的时刻减去响应后的时刻的值)。
3.2 使慢的提供者收到更少请求，因为越慢的提供者的调用前后计数差会越大
3.3 对应的源码
package com.alibaba.dubbo.rpc.cluster.loadbalance; import java.util.List; import java.util.Random; import com.alibaba.dubbo.common.Constants; import com.alibaba.dubbo.common.URL; import com.alibaba.dubbo.rpc.Invocation; import com.alibaba.dubbo.rpc.Invoker; import com.alibaba.dubbo.rpc.RpcStatus; /** * LeastActiveLoadBalance * * @author william.liangf */ public class LeastActiveLoadBalance extends AbstractLoadBalance { public static final String NAME = "leastactive"; private final Random random = new Random(); protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) { int length = invokers.size(); // 总个数 int leastActive = -1; // 最小的活跃数 int leastCount = 0; // 相同最小活跃数的个数 int[] leastIndexs = new int[length]; // 相同最小活跃数的下标 int totalWeight = 0; // 总权重 int firstWeight = 0; // 第一个权重，用于于计算是否相同 boolean sameWeight = true; // 是否所有权重相同 for (int i = 0; i < length; i++) { Invoker<T> invoker = invokers.get(i); int active = RpcStatus.getStatus(invoker.getUrl(), invocation.getMethodName()).getActive(); // 活跃数 int weight = invoker.getUrl().getMethodParameter(invocation.getMethodName(), Constants.WEIGHT_KEY, Constants.DEFAULT_WEIGHT); // 权重 if (leastActive == -1 || active < leastActive) { // 发现更小的活跃数，重新开始 leastActive = active; // 记录最小活跃数 leastCount = 1; // 重新统计相同最小活跃数的个数 leastIndexs[0] = i; // 重新记录最小活跃数下标 totalWeight = weight; // 重新累计总权重 firstWeight = weight; // 记录第一个权重 sameWeight = true; // 还原权重相同标识 } else if (active == leastActive) { // 累计相同最小的活跃数 leastIndexs[leastCount ++] = i; // 累计相同最小活跃数下标 totalWeight += weight; // 累计总权重 // 判断所有权重是否一样 if (sameWeight && i > 0 && weight != firstWeight) { sameWeight = false; } } } // assert(leastCount > 0) if (leastCount == 1) { // 如果只有一个最小则直接返回 return invokers.get(leastIndexs[0]); } if (! sameWeight && totalWeight > 0) { // 如果权重不相同且权重大于0则按总权重数随机 int offsetWeight = random.nextInt(totalWeight); // 并确定随机值落在哪个片断上 for (int i = 0; i < leastCount; i++) { int leastIndex = leastIndexs[i]; offsetWeight -= getWeight(invokers.get(leastIndex), invocation); if (offsetWeight <= 0) return invokers.get(leastIndex); } } // 如果权重相同或权重为0则均等随机 return invokers.get(leastIndexs[random.nextInt(leastCount)]); } }
说明：源码里面的注释已经很清晰了，大致的意思就是活跃数越小的的机器分配到的请求越多
4、ConsistentHash LoadBalance
4.1 一致性 Hash，相同参数的请求总是发到同一提供者。
4.2 当某一台提供者挂时，原本发往该提供者的请求，基于虚拟节点，平摊到其它提供者，不会引起剧烈变动。
4.3 缺省只对第一个参数 Hash，如果要修改，请配置 <dubbo:parameter key="hash.arguments" value="0,1" />
4.4 缺省用 160 份虚拟节点，如果要修改，请配置 <dubbo:parameter key="hash.nodes" value="320" />
4.5 源码分析
/* * Copyright 1999-2012 Alibaba Group. * * 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.alibaba.dubbo.rpc.cluster.loadbalance; import java.io.UnsupportedEncodingException; import java.security.MessageDigest; import java.security.NoSuchAlgorithmException; import java.util.List; import java.util.SortedMap; import java.util.TreeMap; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ConcurrentMap; import com.alibaba.dubbo.common.Constants; import com.alibaba.dubbo.common.URL; import com.alibaba.dubbo.rpc.Invocation; import com.alibaba.dubbo.rpc.Invoker; /** * ConsistentHashLoadBalance * * @author william.liangf */ public class ConsistentHashLoadBalance extends AbstractLoadBalance { private final ConcurrentMap<String, ConsistentHashSelector<?>> selectors = new ConcurrentHashMap<String, ConsistentHashSelector<?>>(); @SuppressWarnings("unchecked") @Override protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) { String key = invokers.get(0).getUrl().getServiceKey() + "." + invocation.getMethodName(); int identityHashCode = System.identityHashCode(invokers); ConsistentHashSelector<T> selector = (ConsistentHashSelector<T>) selectors.get(key); if (selector == null || selector.getIdentityHashCode() != identityHashCode) { selectors.put(key, new ConsistentHashSelector<T>(invokers, invocation.getMethodName(), identityHashCode)); selector = (ConsistentHashSelector<T>) selectors.get(key); } return selector.select(invocation); } private static final class ConsistentHashSelector<T> { private final TreeMap<Long, Invoker<T>> virtualInvokers; private final int replicaNumber; private final int identityHashCode; private final int[] argumentIndex; public ConsistentHashSelector(List<Invoker<T>> invokers, String methodName, int identityHashCode) { this.virtualInvokers = new TreeMap<Long, Invoker<T>>(); this.identityHashCode = System.identityHashCode(invokers); URL url = invokers.get(0).getUrl(); this.replicaNumber = url.getMethodParameter(methodName, "hash.nodes", 160); String[] index = Constants.COMMA_SPLIT_PATTERN.split(url.getMethodParameter(methodName, "hash.arguments", "0")); argumentIndex = new int[index.length]; for (int i = 0; i < index.length; i ++) { argumentIndex[i] = Integer.parseInt(index[i]); } for (Invoker<T> invoker : invokers) { for (int i = 0; i < replicaNumber / 4; i++) { byte[] digest = md5(invoker.getUrl().toFullString() + i); for (int h = 0; h < 4; h++) { long m = hash(digest, h); virtualInvokers.put(m, invoker); } } } } public int getIdentityHashCode() { return identityHashCode; } public Invoker<T> select(Invocation invocation) { String key = toKey(invocation.getArguments()); byte[] digest = md5(key); Invoker<T> invoker = sekectForKey(hash(digest, 0)); return invoker; } private String toKey(Object[] args) { StringBuilder buf = new StringBuilder(); for (int i : argumentIndex) { if (i >= 0 && i < args.length) { buf.append(args[i]); } } return buf.toString(); } private Invoker<T> sekectForKey(long hash) { Invoker<T> invoker; Long key = hash; if (!virtualInvokers.containsKey(key)) { SortedMap<Long, Invoker<T>> tailMap = virtualInvokers.tailMap(key); if (tailMap.isEmpty()) { key = virtualInvokers.firstKey(); } else { key = tailMap.firstKey(); } } invoker = virtualInvokers.get(key); return invoker; } private long hash(byte[] digest, int number) { return (((long) (digest[3 + number * 4] & 0xFF) << 24) | ((long) (digest[2 + number * 4] & 0xFF) << 16) | ((long) (digest[1 + number * 4] & 0xFF) << 8) | (digest[0 + number * 4] & 0xFF)) & 0xFFFFFFFFL; } private byte[] md5(String value) { MessageDigest md5; try { md5 = MessageDigest.getInstance("MD5"); } catch (NoSuchAlgorithmException e) { throw new IllegalStateException(e.getMessage(), e); } md5.reset(); byte[] bytes = null; try { bytes = value.getBytes("UTF-8"); } catch (UnsupportedEncodingException e) { throw new IllegalStateException(e.getMessage(), e); } md5.update(bytes); return md5.digest(); } } }
说明：根据传递的参数进行hash然后调用服务，如果两次传递的参数一样就调用的是同一个机器上的服务
5、dubbo官方的文档的负载均衡配置示例
服务端服务级别
<dubbo:service interface="..." loadbalance="roundrobin" />
客户端服务级别
<dubbo:reference interface="..." loadbalance="roundrobin" />
服务端方法级别
<dubbo:service interface="...">
<dubbo:method name="..." loadbalance="roundrobin"/>
</dubbo:service>
客户端方法级别
<dubbo:reference interface="...">
<dubbo:method name="..." loadbalance="roundrobin"/>
</dubbo:reference>