|NO.Z.00022|——————————|BigDataEnd|——|Hadoop&HDFS.V07|——|Hadoop.v07|HDFS读写解析|

一、HDFS读写解析：HDFS读数据流程

### --- HDFS读数据流程

~~~     客户端通过Distributed FileSystem向NameNode请求下载文件，
~~~     NameNode通过查询元数据，找到文件块所在的DataNode地址。
~~~     挑选一台DataNode（就近原则，然后随机）服务器，请求读取数据。
~~~     DataNode开始传输数据给客户端（从磁盘里面读取数据输入流，以Packet为单位来做校验）。
~~~     客户端以Packet为单位接收，先在本地缓存，然后写入目标文件。

二、HDFS写数据流程

### --- HDFS写数据流程

~~~     客户端通过Distributed FileSystem模块向NameNode请求上传文件，
~~~     NameNode检查目标文件是否已存在，父目录是否存在。
~~~     NameNode返回是否可以上传。
~~~     客户端请求第一个 Block上传到哪几个DataNode服务器上。
~~~     NameNode返回3个DataNode节点，分别为dn1、dn2、dn3。
~~~     客户端通过FSDataOutputStream模块请求dn1上传数据，dn1收到请求会继续调用dn2，
~~~     然后dn2调用dn3，将这个通信管道建立完成。
~~~     dn1、dn2、dn3逐级应答客户端。
~~~     客户端开始往dn1上传第一个Block（先从磁盘读取数据放到一个本地内存缓存），
~~~     以Packet为单位，dn1收到一个Packet就会传给dn2，dn2传给dn3；
~~~     dn1每传一个packet会放入一个确认队列等待确认。
~~~     当一个Block传输完成之后，客户端再次请求NameNode上传第二个Block的服务器。（重复执行3-7步）。

三、验证Packet代码

    @Test
    public void testUploadPacket() throws IOException {
        //1 准备读取本地文件的输入流
        final FileInputStream in = new FileInputStream(new
File("e:/yanqi.txt"));
        //2 准备好写出数据到hdfs的输出流
        final FSDataOutputStream out = fs.create(new Path("/yanqi.txt"), new
Progressable() {
            public void progress() { //这个progress方法就是每传输64KB（packet）就会执行一次，

                System.out.println("&");
            }
        });
        //3 实现流拷贝
        IOUtils.copyBytes(in, out, configuration); //默认关闭流选项是true，所以会自动关闭

        //4 关流 可以再次关闭也可以不关了
    }

四、编程代码

### --- 编程代码：验证packet代码

package cn.yanqi.hdfs;


import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.*;
import org.apache.hadoop.fs.permission.FsPermission;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.util.Progressable;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;

public class HdfsClientDemo12 {
    FileSystem fs = null;
    Configuration configuration = null;
    @Before
    public void init() throws URISyntaxException, IOException, InterruptedException {
        //1、获取Hadoop集群的configuration对象
        configuration = new Configuration();
        //configuration.set( "fs.defaultFS", "hdfs://linux121:9000" );
        //configuration.set( "dfs.replication", "2" );

        //2、根据configuration获取Filesystem对象
        fs = FileSystem.get( new URI( "hdfs://linux121:9000" ), configuration, "root" );
    }


    @After
    public void destory() throws IOException {
        //4、释放filesystem对象(类似数据库连接)
        fs.close();
    }


    //验证packet代码
    @Test
    public void testUploadPacket() throws IOException {
        //1 准备读取本地文件的输入流
        final FileInputStream in = new FileInputStream(new  File("e:/yanqi.txt"));
        //2 准备好写出数据到hdfs的输出流
        final FSDataOutputStream out = fs.create(new Path("/yanqi.txt"), new Progressable() {
            public void progress() { //这个progress方法就是每传输64KB（packet）就会执行一次，
                System.out.println("&");
            }
        });
        //3 实现流拷贝
        IOUtils.copyBytes(in, out, configuration); //默认关闭流选项是true，所以会自动关闭

        //4 关流 可以再次关闭也可以不关了
    }

}

### --- 打印输出

D:\JAVA\jdk1.8.0_231\bin\java.exe -ea -Didea.test.cyclic.buffer.size=1048576 "-javaagent:D:\IntelliJIDEA\IntelliJ IDEA 2019.3.3\lib\idea_rt.jar=53127:D:\IntelliJIDEA\IntelliJ IDEA 2019.3.3\bin" -Dfile.encoding=UTF-8 -classpath "D:\IntelliJIDEA\IntelliJ IDEA 2019.3.3\lib\idea_rt.jar;D:\IntelliJIDEA\IntelliJ IDEA 2019.3.3\plugins\junit\lib\junit5-rt.jar;D:\IntelliJIDEA\IntelliJ IDEA 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com.intellij.rt.junit.JUnitStarter -ideVersion5 -junit4 cn.yanqi.hdfs.HdfsClientDemo12
&
&

Process finished with exit code 0

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Walter Savage Landor:strove with none,for none was worth my strife.Nature I loved and, next to Nature, Art:I warm'd both hands before the fire of life.It sinks, and I am ready to depart

                                                                                                                                                   ——W.S.Landor

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