概览

org.apache.dubbo包下META-INF/dubbo/internal/org.apache.dubbo.rpc.cluster.LoadBalance中内部spi实现类有以下几种：

random=org.apache.dubbo.rpc.cluster.loadbalance.RandomLoadBalance
roundrobin=org.apache.dubbo.rpc.cluster.loadbalance.RoundRobinLoadBalance
leastactive=org.apache.dubbo.rpc.cluster.loadbalance.LeastActiveLoadBalance
consistenthash=org.apache.dubbo.rpc.cluster.loadbalance.ConsistentHashLoadBalance
shortestresponse=org.apache.dubbo.rpc.cluster.loadbalance.ShortestResponseLoadBalance

通过ConsumerConfig可设置负载均衡策略：

		...
        //<editor-fold desc="consumer 配置">
        // 负载均衡策略
        // * random - 随机;
        // * roundrobin - 轮询;
        // * leastactive - 最少活跃调用;
        // * consistenthash - 哈希一致 (2.1.0以上版本);
        // * shortestresponse - 最短响应 (2.7.7以上版本);
        ConsumerConfig consumerConfig = new ConsumerConfig();
        consumerConfig.setLoadbalance("roundrobin");
        //</editor-fold>
        ...

另外，对于加权随机，加权轮询等策略都集成自以上的策略中，consumer会检查注册中心中provider是否提供weight参数自动开启加权负载均衡。除了哈希一致策略，其他的均有加权版本（不提供weight参数即权重一样）。

在provider端，我们可以设置ServiceConfig中设置weight，注意：在RegistryConfig中也可以设置weight，不过这是在多注册中心的环境下对该注册中心负载均衡的权重，不是某个服务调用的负载均衡权重。

		//<editor-fold desc="服务配置">
        ServiceConfig<GreetingsService> hiConfig = new ServiceConfig<>();
        hiConfig.setInterface(GreetingsService.class);
        hiConfig.setRef(new GreetingsServiceImpl());
        // 权重
        hiConfig.setWeight(2);
		//</editor-fold>
		
		//<editor-fold desc="registry配置">
        RegistryConfig registryConfig = new RegistryConfig();
        registryConfig.setAddress("zookeeper://127.0.0.1:2181");
        // 多个registry时，该registry的权重
        registryConfig.setWeight(2);
        //</editor-fold>

此时，zk为例注册的服务如下：

该节点名称最后会显示weight参数：

dubbo%3A%2F%2F192.168.247.1%3A20880%2Forg.example.protocol.dubbo.GreetingsService%3Fanyhost%3Dtrue%26application%3Dfirst-dubbo-provider%26application.version%3D1.0.0%26background%3Dfalse%26deprecated%3Dfalse%26dubbo%3D2.0.2%26dynamic%3Dtrue%26generic%3Dfalse%26interface%3Dorg.example.protocol.dubbo.GreetingsService%26methods%3DsayHi%26owner%3Dbty%26pid%3D2716%26release%3D3.1.6%26service-name-mapping%3Dtrue%26side%3Dprovider%26timestamp%3D1677208111268%26weight%3D2

AbstractLoadBalance

该abstract类只有一个作用：获取provider的权重，提升代码复用率。
注：如果provider没有提供weight参数，则默认为100.
其中，random和roundrobin加权时每次都起作用；而leastactive和shortestresponse是在存在符合选取条件的provider有多个时使用加权随机在其中选一个；consistenthash没用到。

RandomLoadBalance

public class RandomLoadBalance extends AbstractLoadBalance {
	
	// invokers代表需要负载均衡的provider列表
	// URL代表该consumer的metadata，和zk中/dubbo/metadata/com.xxx.service/consumer/xxx-consumer节点内容相同
	@Override
    protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
        // Number of invokers
        int length = invokers.size();
        
		// 根据从registry拿到的参数中寻找是否有weight参数或timestamp参数来判断
		// 是否需要加权操作
        if (!needWeightLoadBalance(invokers, invocation)) {
        	// 没有直接random选取
            return invokers.get(ThreadLocalRandom.current().nextInt(length));
        }

        // 标志位，权重是否都一样
        boolean sameWeight = true;
        // weights序列，不过 weights[i]是0到i的provider节点实际weight的和
        // 如：
        // 节点索引i    : 0    1    2
        // 节点i的weight: 2    3    5
        //  weights[i] : 2  2+3=5 2+3+5=10
        // 应该叫weightSum更合适
        // 这是为了后面随机选取节点用，随机数大小落入哪个区间，就选哪个节点
        int[] weights = new int[length];
        // The sum of weights
        int totalWeight = 0;
        for (int i = 0; i < length; i++) {
            int weight = getWeight(invokers.get(i), invocation);
            // Sum
            totalWeight += weight;
            // save for later use
            weights[i] = totalWeight;
            // 如果当前weight不是之前weight的和的索引倍数，则清空same标志
            // 索引:   0    1    2
            //   w:    3    3    2
            // total:  3    6    8
            // w*(i+1) 3    6    6
            //  same?  √    √    ×
            if (sameWeight && totalWeight != weight * (i + 1)) {
                sameWeight = false;
            }
        }
        // 如果权重不等 且 至少有一个invoker的权重大于0
        if (totalWeight > 0 && !sameWeight) {
            // 根据权重和计算随机offset
            int offset = ThreadLocalRandom.current().nextInt(totalWeight);
            // Return a invoker based on the random value.
            for (int i = 0; i < length; i++) {
            	// 选取第一个小于offset的节点i做调用
                if (offset < weights[i]) {
                    return invokers.get(i);
                }
            }
        }
        // 权重相等
        return invokers.get(ThreadLocalRandom.current().nextInt(length));
    }


}

ThreadLocalRandom随机数生成

RandomLoadBalance中随机数生成使用ThreadLocalRandom，该类始于JDK1.7，由Doug Lea操刀编写。

优点：When applicable, use of ThreadLocalRandom rather than shared Random objects in concurrent programs will typically encounter much less overhead and contention.
用法：ThreadLocalRandom.current().nextX(…) (where X is Int, Long, etc).
缺点：Instances of ThreadLocalRandom are not cryptographically secure. Consider instead using java.security.SecureRandom in security-sensitive applications. Additionally, default-constructed instances do not use a cryptographically random seed unless the system property java.util.secureRandomSeed is set to true.

RoundRobinLoadBalance

public class RoundRobinLoadBalance extends AbstractLoadBalance {
	
	 protected static class WeightedRoundRobin {
	 	private int weight;
        private AtomicLong current = new AtomicLong(0);
        private long lastUpdate;
		...
		// 每次加一个权值
		public long increaseCurrent() {
            return current.addAndGet(weight);
        }
	}
	...
	// 接口名称:<providerId:WeightedRoundRobin>
	// 如：
	// org.example.protocol.dubbo.GreetingsService.sayHi : < dubbo://192.168.247.1:20882/org.example.protocol.dubbo.GreetingsService , WeightedRoundRobin >
	 private ConcurrentMap<String, ConcurrentMap<String, WeightedRoundRobin>> methodWeightMap = new ConcurrentHashMap<String, ConcurrentMap<String, WeightedRoundRobin>>();

	
	...
	
	@Override
    protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
    	// <接口.方法>，如org.example.protocol.dubbo.GreetingsService.sayHi
        String key = invokers.get(0).getUrl().getServiceKey() + "." + invocation.getMethodName();
        // 初始化
        ConcurrentMap<String, WeightedRoundRobin> map = methodWeightMap.computeIfAbsent(key, k -> new ConcurrentHashMap<>());
        // 权重和
        int totalWeight = 0;
        // 记录当前最大值
        long maxCurrent = Long.MIN_VALUE;
        // 记录更新时间，一个接口的所有provider的时间都一样。
        long now = System.currentTimeMillis();
        // 选中的
        Invoker<T> selectedInvoker = null;
        // 选中的
        WeightedRoundRobin selectedWRR = null;
        // 每次都会循环一遍，不会中途退出
        for (Invoker<T> invoker : invokers) {
        	// dubbo://192.168.247.1:20882/org.example.protocol.dubbo.GreetingsService
            String identifyString = invoker.getUrl().toIdentityString();
            // 获取权重，如果没有权重，则默认权重都一样，均为100
            int weight = getWeight(invoker, invocation);
            // 没有缓存，则为GreetingsService的这个provider设置weight，生成WeightedRoundRobin对象
            // 缓存了直接获取
            WeightedRoundRobin weightedRoundRobin = map.computeIfAbsent(identifyString, k -> {			
                WeightedRoundRobin wrr = new WeightedRoundRobin();
                wrr.setWeight(weight);
                return wrr;
            });
			// 缓存的WeightedRoundRobin可能会过时，这里判定以下，保持最新的weight
            if (weight != weightedRoundRobin.getWeight()) {
                //weight changed
                weightedRoundRobin.setWeight(weight);
            }
            // 更新
            // 每次每个provider都会先增加自己权重的值
            long cur = weightedRoundRobin.increaseCurrent();
            // 标记更新时间
            weightedRoundRobin.setLastUpdate(now);
            // 如果当前值大于最大值，则选取
            if (cur > maxCurrent) {
                maxCurrent = cur;
                selectedInvoker = invoker;
                selectedWRR = weightedRoundRobin;
            }
            // 
            totalWeight += weight;
        }
        if (invokers.size() != map.size()) {
            map.entrySet().removeIf(item -> now - item.getValue().getLastUpdate() > RECYCLE_PERIOD);
        }
        if (selectedInvoker != null) {
        	// 注意，选中的weightedRoundRobin的current会减去totalWeight;
            selectedWRR.sel(totalWeight);
            return selectedInvoker;
        }
        // should not happen here
        return invokers.get(0);
    }
}

轮询算法流程

轮次	结果
0
1
2
3
4
5
6

可以看到调用顺序为：A$\to$B$\to$A$\to$C$\to$B$\to$A。

LeastActiveLoadBalance

按照最小调用次数优先的方式选provider，如果存在多个则加权随机选取。
Active等参数由RpcStatus提供，记录RpcStatus的工作由ActiveLimitFilter完成

public class LeastActiveLoadBalance extends AbstractLoadBalance {

	 @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;
        // 存储所有具有相同least active value的invoker的index，数组长度==leastCount
        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 invokers
        int totalWeight = 0;
        // The weight of the first least active invoker
        int firstWeight = 0;
        // Every least active invoker has the same weight value?
        boolean sameWeight = true;


        // 过滤找到调用次数最少的provider，可能存在多个provider
        for (int i = 0; i < length; i++) {
            Invoker<T> invoker = invokers.get(i);
            // Get the active number of the invoker
            int active = RpcStatus.getStatus(invoker.getUrl(), invocation.getMethodName()).getActive();
            // 获取权重参数，provider没有提供则默认100
            int afterWarmup = getWeight(invoker, invocation);
            weights[i] = afterWarmup;
            // 如果是第一次调用，或当前active小于最小的，设置新的leastActive
            if (leastActive == -1 || active < leastActive) {
              ...
            } else 
            // 多个leastActive
            if (active == leastActive) {
              ...
            }
        }
        // 如果符合最少调用次数的provider只有一个，直接返回
        if (leastCount == 1) {
            return invokers.get(leastIndexes[0]);
        }
        // 
        if (!sameWeight && totalWeight > 0) {
            // 权重不等且totalWeight>0则利用权重随机选取
            int offsetWeight = ThreadLocalRandom.current().nextInt(totalWeight);
            // Return a invoker based on the random value.
            for (int i = 0; i < leastCount; i++) {
           		// 找到相同权重的provider第一个比随机选取的offsetWeight大的provider
                int leastIndex = leastIndexes[i];
                offsetWeight -= weights[leastIndex];
                if (offsetWeight < 0) {
                    return invokers.get(leastIndex);
                }
            }
        }
        // 如果所有provider的weight相同或totalWeight==0则随机选取.
        return invokers.get(leastIndexes[ThreadLocalRandom.current().nextInt(leastCount)]);
    }


}

ShortestResponseLoadBalance

同上，利用RpcStatus中获取的参数选取响应最低的provider(滑动时间窗口内)，存在多个则加权随机选取。

ConsistentHashLoadBalance

public class ConsistentHashLoadBalance extends AbstractLoadBalance {

	// 对每一个接口方法缓存一个ConsistentHashSelector
	// key为interface.methodName
    private final ConcurrentMap<String, ConsistentHashSelector<?>> selectors = new ConcurrentHashMap<String, ConsistentHashSelector<?>>();

    @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;
        // 用来代表List<Invoker<T>>，如果内容变化，则该hashcode也变化，为了保持缓存一致性
        int invokersHashCode = invokers.hashCode();
        ConsistentHashSelector<T> selector = (ConsistentHashSelector<T>) selectors.get(key);
        // 缓存的ConsistentHashSelector为空，或已经过期
        if (selector == null || selector.identityHashCode != invokersHashCode) {
            selectors.put(key, new ConsistentHashSelector<T>(invokers, methodName, invokersHashCode));
            selector = (ConsistentHashSelector<T>) selectors.get(key);
        }
        return selector.select(invocation);
    }

	 private static final class ConsistentHashSelector<T> {
		
		// 虚拟节点和Invoker对应关系
        private final TreeMap<Long, Invoker<T>> virtualInvokers;
		
		// 一致性哈希，哈希环虚拟节点个数
        private final int replicaNumber;
		
        private final int identityHashCode;
		// select时候以方法的那个参数为key进行hash映射到hash环，默认第一个参数
        private final int[] argumentIndex;

		ConsistentHashSelector(List<Invoker<T>> invokers, String methodName, int identityHashCode) {
            this.virtualInvokers = new TreeMap<Long, Invoker<T>>();
            this.identityHashCode = identityHashCode;
            URL url = invokers.get(0).getUrl();
            // 如果hash.nodes没有指定，默认每个provider160个节点
            this.replicaNumber = url.getMethodParameter(methodName, HASH_NODES, 160);
            
            String[] index = 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]);
            }
            // 为每个invoker分配哈希环160个虚拟节点
            for (Invoker<T> invoker : invokers) {
                String address = invoker.getUrl().getAddress();
                for (int i = 0; i < replicaNumber / 4; i++) {
                    byte[] digest = Bytes.getMD5(address + i);
                    for (int h = 0; h < 4; h++) {
                        long m = hash(digest, h);
                        virtualInvokers.put(m, invoker);
                    }
                }
            }
        }
		 public Invoker<T> select(Invocation invocation) {
            boolean isGeneric = invocation.getMethodName().equals($INVOKE);
            // 获取此次invocation的hash的key
            String key = toKey(invocation.getArguments(),isGeneric); 
            byte[] digest = Bytes.getMD5(key);
            // 映射
            return selectForKey(hash(digest, 0));
        }
	}
}