hdfs源碼分析第一彈

1. hdfs定義

HDFS is the primary distributed storage used by Hadoop applications. A HDFS cluster primarily consists of a NameNode that manages the file system metadata and DataNodes that store the actual data.

2. hdfs架構

3. hdfs實例

  作為文件系統，文件的讀寫才是核心：

/**
 * 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.
 */

import java.io.File;
import java.io.IOException;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.FSDataInputStream;
import org.apache.hadoop.fs.FSDataOutputStream;
import org.apache.hadoop.fs.Path;

public class HadoopDFSFileReadWrite {
  
  static void usage () {
    System.out.println("Usage : HadoopDFSFileReadWrite <inputfile> <output file>");
    System.exit(1);
  }
  
  static void printAndExit(String str) {
    System.err.println(str);
    System.exit(1);
  }

  public static void main (String[] argv) throws IOException {
    Configuration conf = new Configuration();
    FileSystem fs = FileSystem.get(conf);

    if (argv.length != 2)
      usage();
    
    // Hadoop DFS deals with Path
    Path inFile = new Path(argv[0]);
    Path outFile = new Path(argv[1]);
   
    // Check if input/output are valid
    if (!fs.exists(inFile))
      printAndExit("Input file not found");
    if (!fs.isFile(inFile))
      printAndExit("Input should be a file");
    if (fs.exists(outFile))
      printAndExit("Output already exists");

    // Read from and write to new file
    FSDataInputStream in = fs.open(inFile);
    FSDataOutputStream out = fs.create(outFile);
    byte buffer[] = new byte[256];
    try {
      int bytesRead = 0;
      while ((bytesRead = in.read(buffer)) > 0) {
        out.write(buffer, 0, bytesRead);
      }
    } catch (IOException e) {
      System.out.println("Error while copying file");
    } finally {
      in.close();
      out.close();
    }
  }
}

上述示例，將一個文件的內容復制到另一個文件中，具體步驟如下：

第一步：創建一個文件系統實例，給該實例傳遞新的配置。

Configuration conf = new Configuration();
FileSystem fs = FileSystem.get(conf);

第二步：獲取文件路徑

// Hadoop DFS deals with Path
    Path inFile = new Path(argv[0]);
    Path outFile = new Path(argv[1]);
   
    // Check if input/output are valid
    if (!fs.exists(inFile))
      printAndExit("Input file not found");
    if (!fs.isFile(inFile))
      printAndExit("Input should be a file");
    if (fs.exists(outFile))
      printAndExit("Output already exists");

第三步：打開文件輸入輸出流，將輸入流寫到輸出流中：

    // Read from and write to new file
    FSDataInputStream in = fs.open(inFile);
    FSDataOutputStream out = fs.create(outFile);
    byte buffer[] = new byte[256];
    try {
      int bytesRead = 0;
      while ((bytesRead = in.read(buffer)) > 0) {
        out.write(buffer, 0, bytesRead);
      }
    } catch (IOException e) {
      System.out.println("Error while copying file");
    } finally {
      in.close();
      out.close();
    }

上面文件讀寫功能涉及到了文件系統FileSystem、配置文件Configuration、輸入流/輸出流FSDataInputStream/FSDataOutputStream

 4. 基本概念分析

    4.1 文件系統

　　文件系統的層次結構如下所示：

　　文件系統有兩個重要的分支，一個是分布式文件系統，另一個是“本地”（映射到本地連接的磁盤）文件系統，本地磁盤適用於比較少的hadoop實例和測試。絕大部分情況下使用分布式文件系統，hadoop 分布式文件系統使用多個機器的系統，但對用戶來說只有一個磁盤。它的容錯性和大容量性使它非常有用。

　　4.2 配置文件

　　配置文件的層次結構如下：

我們關注的是HdfsConfiguration，其涉及到的配置文件有hdfs-default.xml和hdfs-site.xml：

  static {
    addDeprecatedKeys();

    // adds the default resources
    Configuration.addDefaultResource("hdfs-default.xml");
    Configuration.addDefaultResource("hdfs-site.xml");

  }

 

　　4.3 輸入/輸出流

　　　　輸入/輸出流和文件系統相對應，先看一下輸入流：

　　

其中，HdfsDataInputStream是FSDataInputStream的實現，其構造函數為：

  public HdfsDataInputStream(DFSInputStream in) throws IOException {
    super(in);
  }

DFSInputStream層次結構如下圖所示：

　　在了解一下輸出流：

　　

其中，重點是HdfsDataOutputStream，其構造函數為：

  public HdfsDataOutputStream(DFSOutputStream out, FileSystem.Statistics stats,
      long startPosition) throws IOException {
    super(out, stats, startPosition);
  }

DFSOutputStream 的層次結構為：

 

參考文獻：

【1】http://hadoop.apache.org/docs/r2.6.0/hadoop-project-dist/hadoop-hdfs/HdfsUserGuide.html

【2】http://hadoop.apache.org/docs/r2.6.0/hadoop-project-dist/hadoop-hdfs/HdfsDesign.html

【3】http://wiki.apache.org/hadoop/HadoopDfsReadWriteExample

【4】http://blog.csdn.net/gaoxingnengjisuan/article/details/11177049