Dubbo的几个负载均衡类--最少活跃数

-----------------看过之前一致性哈希的可以跳过-----------------?

链接在此：Dubbo的几个负载均衡类--一致性哈希

消费者发起调用过程中涉及如下几步

1：接口调用，比如DemoService.demoMethod

2：InvokerInvocationHandler.invoker：消费端启动时，通过JavassistProxyFactory.getProxy反射获取代理类，之后服务调用就直接调用这个Handler

3：MigrationInvoker.invoke：Dubbo 发起调用非常重要的一步，如果失败了，通过这个invoker做切换

4：其他

5：FailoverClusterInvoker.invoke（目前我们使用的，实际在AbstractClusterInvoker里面invoke逻辑是固定的）

6：其他

@Override
    public Result invoke(final Invocation invocation) throws RpcException {
        checkWhetherDestroyed();

        // binding attachments into invocation.
        Map<String, Object> contextAttachments = RpcContext.getContext().getObjectAttachments();
        if (contextAttachments != null && contextAttachments.size() != 0) {
            ((RpcInvocation) invocation).addObjectAttachments(contextAttachments);
        }
        //如果设置了标签规则，则通过list方法过滤出来符合标签的几个invoker
        List<Invoker<T>> invokers = list(invocation);
        LoadBalance loadbalance = initLoadBalance(invokers, invocation);
        RpcUtils.attachInvocationIdIfAsync(getUrl(), invocation);
        //请求负载并且做好灾备降级
        return doInvoke(invocation, invokers, loadbalance);
    }

public Result doInvoke(Invocation invocation, final List<Invoker<T>> invokers, LoadBalance loadbalance) throws RpcException {
        List<Invoker<T>> copyInvokers = invokers;
        checkInvokers(copyInvokers, invocation);
        String methodName = RpcUtils.getMethodName(invocation);
        int len = calculateInvokeTimes(methodName);
        // retry loop.
        RpcException le = null; // last exception.
        List<Invoker<T>> invoked = new ArrayList<Invoker<T>>(copyInvokers.size()); // invoked invokers.
        Set<String> providers = new HashSet<String>(len);
        for (int i = 0; i < len; i++) {
            //Reselect before retry to avoid a change of candidate `invokers`.
            //NOTE: if `invokers` changed, then `invoked` also lose accuracy.
            if (i > 0) {
                checkWhetherDestroyed();
                copyInvokers = list(invocation);
                // check again
                checkInvokers(copyInvokers, invocation);
            }
            //这里通过loadBalance做负载
            Invoker<T> invoker = select(loadbalance, invocation, copyInvokers, invoked);
            invoked.add(invoker);
            RpcContext.getContext().setInvokers((List) invoked);
            try {
                Result result = invoker.invoke(invocation);
                if (le != null && logger.isWarnEnabled()) {
                    logger.warn("Although retry the method " + methodName
                            + " in the service " + getInterface().getName()
                            + " was successful by the provider " + invoker.getUrl().getAddress()
                            + ", but there have been failed providers " + providers
                            + " (" + providers.size() + "/" + copyInvokers.size()
                            + ") from the registry " + directory.getUrl().getAddress()
                            + " on the consumer " + NetUtils.getLocalHost()
                            + " using the dubbo version " + Version.getVersion() + ". Last error is: "
                            + le.getMessage(), le);
                }
                return result;
            } catch (RpcException e) {
                if (e.isBiz()) { // biz exception.
                    throw e;
                }
                le = e;
            } catch (Throwable e) {
                le = new RpcException(e.getMessage(), e);
            } finally {
                providers.add(invoker.getUrl().getAddress());
            }
        }
        throw new RpcException(le.getCode(), "Failed to invoke the method "
                + methodName + " in the service " + getInterface().getName()
                + ". Tried " + len + " times of the providers " + providers
                + " (" + providers.size() + "/" + copyInvokers.size()
                + ") from the registry " + directory.getUrl().getAddress()
                + " on the consumer " + NetUtils.getLocalHost() + " using the dubbo version "
                + Version.getVersion() + ". Last error is: "
                + le.getMessage(), le.getCause() != null ? le.getCause() : le);
    }

-----------------看过之前一致性哈希的可以跳过-----------------?

源码如下，

@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 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;


        // 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 requests on the invoker
            int active = RpcStatus.getStatus(invoker.getUrl(), invocation.getMethodName()).getActive();
            // Get the weight of the invoker's 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 && 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)]);
    }

首先Dubbo通过RpcStatus记录每个接口的调用次数，记录的工具是一个AtomicInteger

方法getWeight是处理那些启动在10分钟内的服务，做一定的服务负载降级。超过十分钟，那么返回的负载都是100，如果小于1分钟那么就是1,1分钟到10分钟之间分别的是分钟数的平方值。

基本针对的就是我们在版本发布的时候，需要启动某个服务的所有实例，这时候越早启动的权重值越大，这么做的原因其实也很合理，越早启动的越稳定，包括各种缓存热点的加载，连接数/链接池的初始化等等

然后就开始比active的大小了，大概意思就是把当前最小active的往数组前面放

一轮遍历后的leastIndexs就是一个有序的递增数组，也就是第一个元素的active最小，所以会有下面一个if

if (leastCount == 1) {
            // If we got exactly one invoker having the least active value, return this invoker directly.
            return invokers.get(leastIndexes[0]);
        }

但如果有多个最小的，那么就要看权重了。如果有权重不一样，就执行如下，意思就是在总权重范围内选一个随机值，然后看这个随机值能够到哪个，那么就用哪个。

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);
                }
            }
        }

如果权重一样，就随机选一个

return invokers.get(leastIndexes[ThreadLocalRandom.current().nextInt(leastCount)]);

李梓萌：

[1,2,3]：出来的leastIndexes数组只有一个：[1,0,0]，直接取第一个。（只走一次第一个if，后面两不符合条件）

[1,1,1]：出来的leastIndexes数组有三个：[1,1,1]，如果权重再一样，随机取一个；如果权重不一样，3内随机一个权重值比如2，那么就选第三个（注意条件是：offsetWeight<0）（第一个1走的第一个if，后面两1，都是走的第二个if）

[2,1,2]：出来的leastIndexes数组有一个：[1,0,0]，直接取第一个（走了两次第一个if，最后那个2不符合条件）

[2,2,1]：出来的leastIndexes数组有两个：[1,2,0]，直接取第一个（先走了第一个if，再走第二个if，第三个1又走了第一个if）

[2,3,3,2,1]：出来的leastIndexes数组有两个：[1,2,0,0,0]，直接取第一个（同上，中间3不符合条件）

总结：最小活跃数真的就会取最少活跃的那个。

不过我咋没找到这个active在调用后怎么递增的呢

这是我问“文文”的回答