/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.dubbo.rpc.cluster.loadbalance;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.rpc.Invocation;
import org.apache.dubbo.rpc.Invoker;

import java.util.List;
import java.util.concurrent.ThreadLocalRandom;

/**
 * This class select one provider from multiple providers randomly.
 * You can define weights for each provider:
 * If the weights are all the same then it will use random.nextInt(number of invokers).
 * If the weights are different then it will use random.nextInt(w1 + w2 + ... + wn)
 * Note that if the performance of the machine is better than others, you can set a larger weight.
 * If the performance is not so good, you can set a smaller weight.
 */
public class RandomLoadBalance extends AbstractLoadBalance {

    public static final String NAME = "random";

    /**
     * Select one invoker between a list using a random criteria
     * @param invokers List of possible invokers
     * @param url URL
     * @param invocation Invocation
     * @param <T>
     * @return The selected invoker
     */
    @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 maxWeight of every invokers, the minWeight = 0 or the maxWeight of the last invoker
        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;
            if (sameWeight && totalWeight != weight * (i + 1)) {
                sameWeight = false;
            }
        }
        // 带权重的随机算法
        if (totalWeight > 0 && !sameWeight) {
            // If (not every invoker has the same weight & at least one invoker's weight>0), select randomly based on totalWeight.
            // 如果权重之和大于0，且权重不一样，就将权重之和作为产生随机数的bound
            int offset = ThreadLocalRandom.current().nextInt(totalWeight);
            // Return a invoker based on the random value.
            for (int i = 0; i < length; i++) {
                if (offset < weights[i]) {// 权重越大，越容易被取到
                    return invokers.get(i);
                }
            }
        }
        // If all invokers have the same weight value or totalWeight=0, return evenly.
        // 如果没设置权重，或者权重相同，就随机返回一个服务提供者
        return invokers.get(ThreadLocalRandom.current().nextInt(length));
    }

}

    /**
     * Get the weight of the invoker's invocation which takes warmup time into account
     * if the uptime is within the warmup time, the weight will be reduce proportionally
     * 
     * 获取服务的权重，考虑预热因素，如果服务正在预热中，权重相应的减少
     *
     * @param invoker    the invoker
     * @param invocation the invocation of this invoker
     * @return weight
     */
    int getWeight(Invoker<?> invoker, Invocation invocation) {
        int weight;
        URL url = invoker.getUrl();
        // Multiple registry scenario, load balance among multiple registries.
        if (REGISTRY_SERVICE_REFERENCE_PATH.equals(url.getServiceInterface())) {
            weight = url.getParameter(REGISTRY_KEY + "." + WEIGHT_KEY, DEFAULT_WEIGHT);
        } else {
            weight = url.getMethodParameter(invocation.getMethodName(), WEIGHT_KEY, DEFAULT_WEIGHT);
            if (weight > 0) {
                long timestamp = invoker.getUrl().getParameter(TIMESTAMP_KEY, 0L);
                if (timestamp > 0L) {
                    long uptime = System.currentTimeMillis() - timestamp;
                    if (uptime < 0) {
                        return 1;
                    }
                    int warmup = invoker.getUrl().getParameter(WARMUP_KEY, DEFAULT_WARMUP);
                    if (uptime > 0 && uptime < warmup) {
                        weight = calculateWarmupWeight((int)uptime, warmup, weight);
                    }
                }
            }
        }
        return Math.max(weight, 0);
    }

获取服务的权重，考虑预热因素，如果服务正在预热中，权重相应的减少

    /**
     * Calculate the weight according to the uptime proportion of warmup time
     * the new weight will be within 1(inclusive) to weight(inclusive)
     *
     * @param uptime the uptime in milliseconds
     * @param warmup the warmup time in milliseconds
     * @param weight the weight of an invoker
     * @return weight which takes warmup into account
     */
    static int calculateWarmupWeight(int uptime, int warmup, int weight) {
        int ww = (int) ( uptime / ((float) warmup / weight));
        return ww < 1 ? 1 : (Math.min(ww, weight));
    }

int DEFAULT_WEIGHT = 100;

int DEFAULT_WARMUP = 10 * 60 * 1000;

 int ww = (int) ( uptime / ((float) warmup / weight))

      // 带权重的随机算法
        if (totalWeight > 0 && !sameWeight) {
            // If (not every invoker has the same weight & at least one invoker's weight>0), select randomly based on totalWeight.
            // 如果权重之和大于0，且权重不一样，就将权重之和作为产生随机数的bound
            int offset = ThreadLocalRandom.current().nextInt(totalWeight);
            // Return a invoker based on the random value.
            for (int i = 0; i < length; i++) {
                if (offset < weights[i]) {// 权重越大，越容易被取到
                    return invokers.get(i);
                }
            }
        }