loadBalance是现在服务端框架的标配，dubbo作为在互联网端流行这么多年的rpc框架，自然也少不了。现在的服务端一般都是集群部署，dubbo 需要对服务消费者的调用请求进行分配，避免少数服务提供者负载过大。
一、四种负载均衡策略
dubbo 提供了四种负载均衡策略，分别是，
-
Random LoadBalance 随机（默认负载均衡策略），可以完全随机，也可以按权重设置随机概率。
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。 -
RoundRobin LoadBalance 轮循，按公约后的权重设置轮循比率。
存在响应慢的提供者会累积请求的问题，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。 -
LeastActive LoadBalance 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。
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。 -
ConsistentHash LoadBalance 一致性Hash，相同参数的请求总是发到同一提供者。
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。
二、负载均衡配置
1、服务端（提供者）
@Service(interfaceClass = IBookService.class, loadbalance = "roundrobin", retries = 3, version = "1.0")
public class BookServiceImpl implements IBookService {
// 省略实现
}<dubbo:service interface="..." loadbalance="roundrobin" />2、调用端（消费者）
@Service
public class DubboService {
@Reference(interfaceClass = IBookService.class, loadbalance = "random", retries = 3, version = "1.0")
IBookService bookService;
// 省略逻辑实现
}<dubbo:reference interface="..." loadbalance="roundrobin" />三、源码解析
负载均衡类的依赖
AbstractLoadBalance 抽象类是所有负载均衡策略实现类的父类，实现了LoadBalance接口 的方法，同时提供抽象方法交由子类实现。
@Override
public <T> Invoker<T> select(List<Invoker<T>> invokers, URL url, Invocation invocation) {
if (CollectionUtils.isEmpty(invokers)) {
return null;
}
if (invokers.size() == 1) {
return invokers.get(0);
}
return doSelect(invokers, url, invocation);
}
protected abstract <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation);1、RandomLoadBalance，随机
RandomLoadBalance 的实现逻辑分为权重相同和权重不同的情况。如果每个提供者的权重都相同，那么根据列表长度直接随机选取一个，如果权重不同，累加权重值。根据0~累加的权重值 选取一个随机数，然后判断该随机数落在那个提供者上。这种策略大概率能够让权重大的provider获取更多机会同时兼顾了权重小的接口。
@Override
protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
// Number of invokers
int length = invokers.size();
// Every invoker has the same weight?
boolean sameWeight = true;
// the weight of every invokers
int[] weights = new int[length];
// the first invoker's weight
int firstWeight = getWeight(invokers.get(0), invocation);
weights[0] = firstWeight;
// The sum of weights
int totalWeight = firstWeight;
for (int i = 1; i < length; i++) {
int weight = getWeight(invokers.get(i), invocation);
// save for later use
weights[i] = weight;
// Sum
totalWeight += weight;
if (sameWeight && weight != firstWeight) {
sameWeight = false;
}
}
if (totalWeight > 0 && !sameWeight) {
// If (not every invoker has the same weight & at least one invoker's weight>0), select randomly based on totalWeight.
int offset = ThreadLocalRandom.current().nextInt(totalWeight);
// Return a invoker based on the random value.
for (int i = 0; i < length; i++) {
offset -= weights[i];
if (offset < 0) {
return invokers.get(i);
}
}
}
// If all invokers have the same weight value or totalWeight=0, return evenly.
return invokers.get(ThreadLocalRandom.current().nextInt(length));
}2、RoundRobinLoadBalance，轮询
轮询的实现逻辑分为权重相同和权重不同两种情况。先判断权重是否一致，如果一致，通过维护一个 AtomicLong 的增长 进行取模来轮询。如果权重不一致，通过维护一个 AtomicLong 的增长与最大权重取模作为当前权重，然后获取大于当前权重的列表作为调用者列表，然后进行取模轮询。
@Override
protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
String key = invokers.get(0).getUrl().getServiceKey() + "." + invocation.getMethodName();
ConcurrentMap<String, WeightedRoundRobin> map = methodWeightMap.get(key);
if (map == null) {
methodWeightMap.putIfAbsent(key, new ConcurrentHashMap<String, WeightedRoundRobin>());
map = methodWeightMap.get(key);
}
int totalWeight = 0;
long maxCurrent = Long.MIN_VALUE;
long now = System.currentTimeMillis();
Invoker<T> selectedInvoker = null;
WeightedRoundRobin selectedWRR = null;
for (Invoker<T> invoker : invokers) {
String identifyString = invoker.getUrl().toIdentityString();
WeightedRoundRobin weightedRoundRobin = map.get(identifyString);
int weight = getWeight(invoker, invocation);
if (weightedRoundRobin == null) {
weightedRoundRobin = new WeightedRoundRobin();
weightedRoundRobin.setWeight(weight);
map.putIfAbsent(identifyString, weightedRoundRobin);
}
if (weight != weightedRoundRobin.getWeight()) {
//weight changed
weightedRoundRobin.setWeight(weight);
}
long cur = weightedRoundRobin.increaseCurrent();
weightedRoundRobin.setLastUpdate(now);
if (cur > maxCurrent) {
maxCurrent = cur;
selectedInvoker = invoker;
selectedWRR = weightedRoundRobin;
}
totalWeight += weight;
}
if (!updateLock.get() && invokers.size() != map.size()) {
if (updateLock.compareAndSet(false, true)) {
try {
// copy -> modify -> update reference
ConcurrentMap<String, WeightedRoundRobin> newMap = new ConcurrentHashMap<String, WeightedRoundRobin>();
newMap.putAll(map);
Iterator<Entry<String, WeightedRoundRobin>> it = newMap.entrySet().iterator();
while (it.hasNext()) {
Entry<String, WeightedRoundRobin> item = it.next();
if (now - item.getValue().getLastUpdate() > RECYCLE_PERIOD) {
it.remove();
}
}
methodWeightMap.put(key, newMap);
} finally {
updateLock.set(false);
}
}
}
if (selectedInvoker != null) {
selectedWRR.sel(totalWeight);
return selectedInvoker;
}
// should not happen here
return invokers.get(0);
}3、LeastActiveLoadBalance，最少活跃调用数
最少活跃调用数的思想就是：获取最小的活跃数，把活跃数等于最小活跃数的调用者维护成一个数组。如果权重一致随机取出，如果不同则跟 RandomLoadBalance 一致，累加权重，然后随机取出。
@Override
protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
// Number of invokers
int length = invokers.size();
// The least active value of all invokers
int leastActive = -1;
// The number of invokers having the same least active value (leastActive)
int leastCount = 0;
// The index of invokers having the same least active value (leastActive)
int[] leastIndexes = new int[length];
// the weight of every invokers
int[] weights = new int[length];
// The sum of the warmup weights of all the least active invokes
int totalWeight = 0;
// The weight of the first least active invoke
int firstWeight = 0;
// Every least active invoker has the same weight value?
boolean sameWeight = true;
// Filter out all the least active invokers
for (int i = 0; i < length; i++) {
Invoker<T> invoker = invokers.get(i);
// Get the active number of the invoke
int active = RpcStatus.getStatus(invoker.getUrl(), invocation.getMethodName()).getActive();
// Get the weight of the invoke configuration. The default value is 100.
int afterWarmup = getWeight(invoker, invocation);
// save for later use
weights[i] = afterWarmup;
// If it is the first invoker or the active number of the invoker is less than the current least active number
if (leastActive == -1 || active < leastActive) {
// Reset the active number of the current invoker to the least active number
leastActive = active;
// Reset the number of least active invokers
leastCount = 1;
// Put the first least active invoker first in leastIndexes
leastIndexes[0] = i;
// Reset totalWeight
totalWeight = afterWarmup;
// Record the weight the first least active invoker
firstWeight = afterWarmup;
// Each invoke has the same weight (only one invoker here)
sameWeight = true;
// If current invoker's active value equals with leaseActive, then accumulating.
} else if (active == leastActive) {
// Record the index of the least active invoker in leastIndexes order
leastIndexes[leastCount++] = i;
// Accumulate the total weight of the least active invoker
totalWeight += afterWarmup;
// If every invoker has the same weight?
if (sameWeight && i > 0
&& afterWarmup != firstWeight) {
sameWeight = false;
}
}
}
// Choose an invoker from all the least active invokers
if (leastCount == 1) {
// If we got exactly one invoker having the least active value, return this invoker directly.
return invokers.get(leastIndexes[0]);
}
if (!sameWeight && totalWeight > 0) {
// If (not every invoker has the same weight & at least one invoker's weight>0), select randomly based on
// totalWeight.
int offsetWeight = ThreadLocalRandom.current().nextInt(totalWeight);
// Return a invoker based on the random value.
for (int i = 0; i < leastCount; i++) {
int leastIndex = leastIndexes[i];
offsetWeight -= weights[leastIndex];
if (offsetWeight < 0) {
return invokers.get(leastIndex);
}
}
}
// If all invokers have the same weight value or totalWeight=0, return evenly.
return invokers.get(leastIndexes[ThreadLocalRandom.current().nextInt(leastCount)]);
}4、ConsistentHashLoadBalance，一致性hash
一致性hash，主要是通过内部类 ConsistentHashSelector 来实现（ConsistentHashSelector 的源码在这里就不列出了）：
- 首先根据invokers的url获取分片个数，创建相同大小的虚拟节点。
- 然后根据参数的MD5值 获取对应的提供者。
@Override
protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
String methodName = RpcUtils.getMethodName(invocation);
String key = invokers.get(0).getUrl().getServiceKey() + "." + methodName;
int identityHashCode = System.identityHashCode(invokers);
ConsistentHashSelector<T> selector = (ConsistentHashSelector<T>) selectors.get(key);
if (selector == null || selector.identityHashCode != identityHashCode) {
selectors.put(key, new ConsistentHashSelector<T>(invokers, methodName, identityHashCode));
selector = (ConsistentHashSelector<T>) selectors.get(key);
}
return selector.select(invocation);
}