Distributed File Systems (DFS) are a new type of file systems which provides some extra features over normal file systems and are used for storing and sharing files across wide area network and provide easy programmatic access. File Systems like HDFS from Hadoop and many others falls in the category of distributed file systems and has been widely used and are quite popular.

This tutorial provides a step by step guide for accessing and using distributed file system for storing and retrieving data using j\Java. Hadoop Distributed File System has been used for this tutorial because it is freely available, easy to setup and is one of the most popular and well known Distributed file system. The tutorial demonstrates how to access Hadoop distributed file system using java showing all the basic operations.

Introduction
Distributed File Systems (DFS) are a new type of file systems which provides some extra features over normal file systems and are used for storing and sharing files across wide area network and provide easy programmatic access.

Distributed file system is used to make files distributed across multiple servers appear to users as if they reside in one place on the network. Distributed file system allows administrators to consolidate file shares that may exist on multiple servers to appear as if they all are in the same location so that users can access them from a single point on the network.
HDFS stands for Hadoop Distributed File System and is a distributed file system designed to run on commodity hardware. Some of the features provided by Hadoop are:
•    Fault tolerance: Data can be replicated, so if any of the servers goes down, resources still will be available for user.
•    Resource management and accessibility: Users does not require knowing the physical location of the data; they can access all the resources through a single point. HDFS also provides web browser interface to view the contents of the file.
•    It provides high throughput access to application data.

This tutorial will demonstrate how to use HDFS for basic distributed file system operations using Java. Java 1.6 version and Hadoop driver has been used (link is given in Pre-requisites section). The development environment consists of Eclipse 3.4.2 and Hadoop 0.19.1 on Microsoft Windows XP – SP3.

Pre-requisites
1.      Hadoop-0.19.1 installation - here and here -

2.      Hadoop-0.19.1-core.jar file

3.      Commons-logging-1.1.jar file

4.      Java 1.6

5.      Eclipse 3.4.2

Creating New Project and FileSystem Object

First step is to create a new project in Eclipse and then create a new class in that project.
Now add all the jar files to the project, as mentioned in the pre-requisites.
First step in using or accessing Hadoop Distributed File System (HDFS) is to create file system object.
Without creating an object you cannot perform any operations on the HDFS, so file system object is always required to be created.
Two input parameters are required to create object. They are “Host name” and “Port”.
Code below shows how to create file system object to access HDFS.

Configuration config = new Configuration();

config.set("fs.default.name","hdfs://127.0.0.1:9000/");

FileSystem dfs = FileSystem.get(config);

Here Host name = “127.0.0.1” & Port = “9000”.

Various HDFS operations

Now we will see various operations that can be performed on HDFS.

Creating Directory

Now we will start with creating a directory.
First step for using HDFS is to create a directory where we will store our data.
Now let us create a directory named “TestDirectory”.

String dirName = "TestDirectory";

Path src = new Path(dfs.getWorkingDirectory()+"/"+dirName);

dfs.mkdirs(src);

Here dfs.getWorkingDirectory() function will return the path of the working directory which is the basic working directory and all the data will be stored inside this directory. mkdirs() function accepts object of the type Path, so as shown above Path object is created first. Directory is required to be created inside basic working directory, so Path object is created accordingly. dfs.mkdirs(src) function will create a directory in the working folder with name “TestDirectory”.

Sub directories can also be created inside the “TestDirectory”; in that case path specified during creation of Path object will change. For example a directory named “subDirectory” can be created inside directory “TestDirectory” as shown in below code.

String subDirName = "subDirectory";

Path src = new Path(dfs.getWorkingDirectory()+"/TestDirectory/"+ subDirName);

dfs.mkdirs(src);

Deleting Directory or file

Existing directory in the HDFS can be deleted. Below code shows how to delete the existing directory.

String dirName = "TestDirectory";

Path src = new Path(dfs.getWorkingDirectory()+"/"+dirName);

Dfs.delete(src);

Please note that delete() method can also be used to delete files. What needs to be deleted should be specified in the Path object.

Copying file to/from HDFS from/to Local file system

Basic aim of using HDFS is to store data, so now we will see how to put data in HDFS.
Once directory is created, required data can be stored in HDFS from the local file system.
So consider that a file named “file1.txt” is located at “E:\HDFS” in the local file system, and it is required to be copied under the folder “subDirectory” (that was created earlier) in HDFS.
Code below shows how to copy file from local file system to HDFS.

Path src = new Path("E://HDFS/file1.txt");

Path dst = new Path(dfs.getWorkingDirectory()+"/TestDirectory/subDirectory/");

dfs.copyFromLocalFile(src, dst);

Here src and dst are the Path objects created for specifying the local file system path where file is located and HDFS path where file is required to be copied respectively. copyFromLocalFile() method is used for copying file from local file system to HDFS.

Similarly, file can also be copied from HDFS to local file system. Code below shows how to copy file from HDFS to local file system.

Path src = new Path(dfs.getWorkingDirectory()+"/TestDirectory/subDirectory/file1.txt");

Path dst = new Path("E://HDFS/");

dfs.copyToLocalFile(src, dst);

Here copyToLocalFile() method is used for copying file from HDFS to local file system.

Creating a file and writing data in it

It is also possible to create a file in HDFS and write data in it. So if required instead of directly copying the file from the local file system, a file can be first created and then data can be written in it.
Code below shows how to create a file name “file2.txt” in HDFS directory.

Path src = new Path(dfs.getWorkingDirectory()+"/TestDirectory/subDirectory/file2.txt");

dfs.createNewFile(src);

Here createNewFile() method will create the file in HDFS based on the input provided in src object.

Now as the file is created, data can be written in it. Code below shows how to write data present in the “file1.txt” of local file system to “file2.txt” of HDFS.

Path src = new Path(dfs.getWorkingDirectory()+"/TestDirectory/subDirectory/file2.txt");

FileInputStream fis = new FileInputStream("E://HDFS/file1.txt");

int len = fis.available();

byte[] btr = new byte[len];

fis.read(btr);

FSDataOutputStream fs = dfs.create(src);

fs.write(btr);

fs.close();

Here write() method of FSDataOutputStream is used to write data in file located in HDFS.

Reading data from a file

It is always necessary to read the data from file for performing various operations on data. It is possible to read data from the file which is stored in HDFS.
Code below shows how to retrieve data from the file present in the HDFS. Here data is read from the file (file1.txt) which is present in the directory (subDirectory) that was created earlier.

Path src = new Path(dfs.getWorkingDirectory()+"/TestDirectory/subDirectory/file1.txt");

FSDataInputStream fs = dfs.open(src);

String str = null;

while ((str = fs.readline())!= null)
{
System.out.println(str);
}

Here readline() method of FSDataInputStream is used to read data from the file located in HDFS. Also src is the Path object used to specify the path of the file in HDFS which has to be read.

Miscellaneous operations that can be performed on HDFS

Below are some of the basic operations that can be performed on HDFS.

Below is the code that can be used to check whether particular file or directory exists in HDFS. If it exists, it returns true and if it doesn’t exists it returns false. dfs.exists() method is used for this.

Path src = new Path(dfs.getWorkingDirectory()+"/TestDirectory/HDFS/file1.txt");

System.out.println(dfs.exists(src));

Below is the code that can be used to check the default block size in which file would be split. It returns block size in terms of Number of Bytes. dfs.getDefaultBlockSize() method is used for this.

System.out.println(dfs.getDefaultBlockSize());

To check for the default replication factor, as shown below dfs.getDefaultReplication() method can be used.

System.out.println(dfs.getDefaultReplication());

To check whether given path is HDFS directory or file, as shown below dfs.isDirectory() or dfs.isFile() methods can be used.

Path src = new Path(dfs.getWorkingDirectory()+"/TestDirectory/subDirectory/file1.txt");
System.out.println(dfs.isDirectory(src));
System.out.println(dfs.isFile(src));

Conclusion
So we just learned some of the basics about Hadoop Distributed File System, how to create and delete directory, how to copy file to/from HDFS from/to local file system, how to create and delete file into directory, how to write data in file, and how to read data from file. We also learned various other operations that can be performed on HDFS. Thus from what we have done we can say that, HDFS is easy to use for data storage and retrieval.

References:
http://hadoop.apache.org/common/docs/current/hdfs_design.html
http://en.wikipedia.org/wiki/Hadoop