大数据笔记（十）——Shuffle与MapReduce编程案例（A）

一.什么是Shuffle

yarn-site.xml文件配置的时候有这个参数：yarn.nodemanage.aux-services:mapreduce_shuffle

因为mapreduce程序运行在nodemanager上，nodemanager运行mapreduce程序的方式就是shuffle。

1.首先，数据在HDFS上是以数据块的形式保存，默认大小128M。

2.数据块对应成数据切片送到Mapper。默认一个数据块对应一个数据切块。

3.Mapper阶段

4.Mapper处理完，写到内存中作缓冲（环形缓冲区，默认100M）

5.内存满80%就发生溢写，进行一次IO操作，写到HDFS的文件系统上。

6.作一个处理，将小文件合成一个大文件

7.Combiner:在Mapper端先做一次Reducer，做一个合并操作

8.将Combiner的数据放到Reducer

9.输出到HDFS

图解：

Maprecue的缺点：发生的IO次数太多（图示标号），严重影响性能。

解决方式：Spark（基于内存）

二.MapReduce编程案例

1.多表查询：等值连接

查询员工信息：部门名称、员工姓名

实现SQL语句：在emp表，dept表联合查询，查询每个部门下面的员工

select d.dname,e.ename
from emp e,dept d
where e.deptno=d.deptno;

分析：

使用MR实现等值连接的分析流程：

程序：

MultiTableQueryMapper.java

package demo.multiTable;

import java.io.IOException;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
//k2 部门号 v2 部门名称
public class MultiTableQueryMapper extends Mapper<LongWritable, Text, LongWritable, Text> {

    @Override
    protected void map(LongWritable key1, Text value1, Context context)
            throws IOException, InterruptedException {
        //数据：可能是部门，也可能是员工
        String data = value1.toString();
        //分词
        String[] words = data.split(",");
        //判断数组的长度
        if (words.length == 3) {
            //部门表：部门号 部门名称
            context.write(new LongWritable(Long.parseLong(words[0])), new Text("*"+words[1]));
        }else {
            //员工表：部门号 员工名称
            context.write(new LongWritable(Long.parseLong(words[7])), new Text(words[1]));
        }
    }

}

MultiTableQueryReducer.java

package demo.multiTable;

import java.io.IOException;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

public class MultiTableQueryReducer extends Reducer<LongWritable, Text, Text, Text> {

    @Override
    protected void reduce(LongWritable k3, Iterable<Text> v3, Context context)
            throws IOException, InterruptedException {
        //定义变量：保存 部门名称和员工姓名
        String dname = "";
        String empNameList = "";

        for (Text text : v3) {
            String string = text.toString();
            //找到* 号的位置
            int index = string.indexOf("*");
            if (index >= 0) {
                //代表的是部门名称
                dname = string.substring(1);
            }else {
                //代表的是员工姓名
                empNameList = string + ";" + empNameList;
            }
        }

        //输出 部门名字 员工姓名字符串
        context.write(new Text(dname), new Text(empNameList));
    }

}

MultiTableQueryMain.java

package demo.multiTable;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.DoubleWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

public class MultiTableQueryMain {

    public static void main(String[] args) throws Exception {
        Job job = Job.getInstance(new Configuration());

        job.setJarByClass(MultiTableQueryMain.class);

        job.setMapperClass(MultiTableQueryMapper.class);
        job.setMapOutputKeyClass(LongWritable.class);
        job.setMapOutputValueClass(Text.class);

        job.setReducerClass(MultiTableQueryReducer.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);

        FileInputFormat.setInputPaths(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));

        job.waitForCompletion(true);
    }

}

结果：

2.多表查询：自连接

自连接：通过表的别名，将同一张表看成多张表

需求：查询一个表内老板姓名和对应的员工姓名

实现SQL语句：

select b.ename,e.ename
from emp b,emp e
where b.empno=e.mgr;

分析：

实现：

SelfJoinMapper.java

package demo.selfJoin;

import java.io.IOException;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;

public class SelfJoinMapper extends Mapper<LongWritable, Text, LongWritable, Text> {

    @Override
    protected void map(LongWritable key1, Text value1, Context context)
            throws IOException, InterruptedException {
        //7698,BLAKE,MANAGER,7839,1981/5/1,2850,30
        String data = value1.toString();

        //分词
        String[] words = data.split(",");

        //输出
        //1.作为老板表
        context.write(new LongWritable(Long.parseLong(words[0])), new Text("*"+words[1]));

        //2.作为员工表
        try{
            context.write(new LongWritable(Long.parseLong(words[3])), new Text(words[1]));
        }catch(Exception e){
            //老板号为空值
            context.write(new LongWritable(-1), new Text(words[1]));
        }
    }

}

SelfJoinReducer.java

package demo.selfJoin;

import java.io.IOException;

import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;

public class SelfJoinReducer extends Reducer<LongWritable, Text, Text, Text> {

    @Override
    protected void reduce(LongWritable k3, Iterable<Text> v3, Context context)
            throws IOException, InterruptedException {
        //定义变量：保存老板姓名 员工姓名
        String bossName = "";
        String empNameList = "";

        for (Text text : v3) {
            String string = text.toString();
            //判断是否存在*号
            //*号的作用为了区分是哪张表
            int index = string.indexOf("*");
            if (index >= 0) {
                //老板姓名 去掉*号
                bossName = string.substring(1);
            }else {
                //员工姓名
                empNameList = string + ";" + empNameList;
            }
        }

        //输出
        //如果存在老板和员工 才输出
        if (bossName.length() > 0 && empNameList.length() > 0) {
            context.write(new Text(bossName), new Text(empNameList));
        }
    }

}

SelfJoinMain.java

package demo.selfJoin;

import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import demo.multiTable.MultiTableQueryMain;
import demo.multiTable.MultiTableQueryMapper;
import demo.multiTable.MultiTableQueryReducer;

public class SelfJoinMain {

    public static void main(String[] args) throws Exception {

        Job job = Job.getInstance(new Configuration());

        job.setJarByClass(SelfJoinMain.class);

        job.setMapperClass(SelfJoinMapper.class);
        job.setMapOutputKeyClass(LongWritable.class);
        job.setMapOutputValueClass(Text.class);

        job.setReducerClass(SelfJoinReducer.class);
        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);

        FileInputFormat.setInputPaths(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));

        job.waitForCompletion(true);

    }

}

结果：