清理（​​上下文）方法有什么作用？

Question

我无法理解Hadoop中的清理方法究竟是什么，它是如何运作的？我有以下Map-Reduce代码来计算一堆数字的最大值，最小值和平均值。

public class Statistics 
{
    public static class Map extends Mapper<LongWritable, Text, Text, Text>
    {
        public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException 
        {
            /* code to calculate min, max, and mean from among a bunch of numbers */
        }
        public void cleanup(Context context) throws IOException, InterruptedException
        {
            Text key_min = new Text();
            key_min.set("min");
            Text value_min = new Text();
            value_min.set(String.valueOf(min));
            context.write(key_min,value_min);

            Text key_max = new Text();
            key_max.set("max");
            Text value_max = new Text();
            value_max.set(String.valueOf(max));
            context.write(key_max,value_max);

            Text key_avg = new Text();
            key_avg.set("avg");
            Text value_avg = new Text();
            value_avg.set(String.valueOf(linear_sum)+","+count);
            context.write(key_avg,value_avg);

            Text key_stddev = new Text();
            key_stddev.set("stddev");
            Text value_stddev = new Text();
            value_stddev.set(String.valueOf(linear_sum)+","+count+","+String.valueOf(quadratic_sum));
            context.write(key_stddev,value_stddev);
        }
    }
    public static class Reduce extends Reducer<Text,Text,Text,Text>
    {
        public void reduce(Text key, Iterable<Text> values,Context context) throws IOException, InterruptedException 
        {
            /* code to further find min, max and mean from among the outputs of different mappers */
        }
    }
    public static void main(String[] args) throws Exception 
    {
        /* driver program */
    }
}

那么cleanup(Context context)方法到底在做什么呢？我假设它从一堆映射器收集输出（键，值）对并将其传递给reducer。在其他网站上，我已经读过在MapReduce中运行的订单是：setup - ＆gt;地图 - ＆gt;清理然后设置 - ＆gt;减少 - ＆gt;清理。为什么这个程序没有使用设置方法？

Answer 1

这些值必须不在Mapper中计算，必须在Reduce步骤上计算。 https://hadoop.apache.org/docs/current/hadoop-mapreduce-client/hadoop-mapreduce-client-core/MapReduceTutorial.html#Reducer