At the time of this writing, Git is (seemingly) not installed by default on any GNU/Linux distribution or any other operating system. So, before you can use Git, you must install it. The steps to install Git depend greatly on the vendor and version of your operating system. This chapter describes how to install Git on Linux and Microsoft Windows and within Cygwin.
This excerpt is from Hadoop: The Definitive Guide.Hadoop: The Definitive Guide is a comprehensive resource for using Hadoop to build reliable, scalable, distributed systems. Programmers will find details for analyzing large datasets with Hadoop, and administrators will learn how to set up and run Hadoop clusters. The book includes case studies that illustrate how Hadoop is used to solve specific problems.
It’s easy to install Hadoop on a single machine to try it out. (For installation on a cluster, please refer to Chapter 9, Setting Up a Hadoop Cluster.) The quickest way is to download and run a binary release from an Apache Software Foundation Mirror.
Hadoop runs on Unix and on Windows. Linux is the only supported production platform, but other flavors of Unix (including Mac OS X) can be used to run Hadoop for development. Windows is only supported as a development platform, and additionally requires Cygwin to run. During the Cygwin installation process, you should include the openssh package if you plan to run Hadoop in pseudo-distributed mode (see following explanation).
Download a stable release, which is packaged as a gzipped tar file, from the Apache Hadoop releases page (http://hadoop.apache.org/core/releases.html) and unpack it somewhere on your filesystem:
tar xzf hadoop-
Before you can run Hadoop, you need to tell it where Java is
located on your system. If you have the
environment variable set to point to a suitable Java installation, that
will be used, and you don’t have to configure anything further.
Otherwise, you can set the Java installation that Hadoop uses by editing
conf/hadoop-env.sh, and specifying the
JAVA_HOME variable. For example, on my Mac I changed
the line to read:
to point to version 1.6.0 of Java. On Ubuntu, the equivalent line is:
It’s very convenient to create an environment variable that points
to the Hadoop installation directory (
say) and to put the Hadoop binary directory on your command-line path.
Check that Hadoop runs by typing:
hadoop versionHadoop 0.20.0 Subversion https://svn.apache.org/repos/asf/hadoop/core/branches/branch-0.20 -r 763504 Compiled by ndaley on Thu Apr 9 05:18:40 UTC 2009
Each component in Hadoop is configured using an XML file. Core
properties go in
core-site.xml, HDFS properties go
hdfs-site.xml, and MapReduce properties go in
mapred-site.xml. These files are all located in the
In earlier versions of Hadoop, there was a single site
configuration file for the Core, HDFS, and MapReduce components,
hadoop-site.xml. From release 0.20.0
onward this file has been split into three: one for each component.
The property names have not changed, just the configuration file they
have to go in. You can see the default settings for all the properties
that are governed by these configuration files by looking in the
docs directory of your Hadoop installation for
HTML files called
There are no daemons running and everything runs in a single JVM. Standalone mode is suitable for running MapReduce programs during development, since it is easy to test and debug them.
The Hadoop daemons run on a cluster of machines. This setup is described in Chapter 9, Setting Up a Hadoop Cluster.
To run Hadoop in a particular mode, you need to do two things: set the appropriate properties, and start the Hadoop daemons. Table A.1, “Key configuration properties for different modes” shows the minimal set of properties to configure each mode. In standalone mode, the local filesystem and the local MapReduce job runner are used, while in the distributed modes the HDFS and MapReduce daemons are started.
Table A.1. Key configuration properties for different modes
You can read more about configuration in the section called “Hadoop Configuration”.
The configuration files should be created with the following
contents, and placed in the
(although you can place configuration files in any directory as long
as you start the daemons with the
<?xml version="1.0"?> <!-- core-site.xml --> <configuration> <property> <name>fs.default.name</name> <value>hdfs://localhost/</value> </property> </configuration>
<?xml version="1.0"?> <!-- hdfs-site.xml --> <configuration> <property> <name>dfs.replication</name> <value>1</value> </property> </configuration>
<?xml version="1.0"?> <!-- mapred-site.xml --> <configuration> <property> <name>mapred.job.tracker</name> <value>localhost:8021</value> </property> </configuration>
In pseudo-distributed mode, we have to start daemons, and to
do that, we need to have SSH installed. Hadoop doesn’t actually
distinguish between pseudo-distributed and fully distributed modes:
it merely starts daemons on the set of hosts in the cluster (defined
slaves file) by SSH-ing to each host and
starting a daemon process. Pseudo-distributed mode is just a special
case of fully distributed mode in which the (single) host is
localhost, so we need to make sure that we can SSH to localhost and
log in without having to enter a password.
First, make sure that SSH is installed and a server is running. On Ubuntu, for example, this is achieved with:
sudo apt-get install ssh
On Windows with Cygwin, you can set up an SSH server (after
having installed the
openssh package) by
Then to enable password-less login, generate a new SSH key with an empty passphrase:
ssh-keygen -t rsa -P '' -f ~/.ssh/id_rsa
%cat ~/.ssh/id_rsa.pub >> ~/.ssh/authorized_keys
Test this with:
You should be logged in without having to type a password.
Before it can be used, a brand-new HDFS installation needs to be formatted. The formatting process creates an empty filesystem by creating the storage directories and the initial versions of the namenode’s persistent data structures. Datanodes are not involved in the initial formatting process, since the namenode manages all of the filesystem’s metadata, and datanodes can join or leave the cluster dynamically. For the same reason, you don’t need to say how large a filesystem to create, since this is determined by the number of datanodes in the cluster, which can be increased as needed, long after the filesystem was formatted.
Formatting HDFS is quick to do. Just type the following:
hadoop namenode -format
If you have placed configuration files outside the default
conf directory, start the daemons with the
--config option, which takes an absolute path
to the configuration directory:
Three daemons will be started on your local machine: a
namenode, a secondary namenode, and a datanode. You can check
whether the daemons started successfully by looking at the logfiles
logs directory (in the Hadoop
installation directory), or by looking at the web UIs, at
http://localhost:50030/ for the jobtracker, and at
http://localhost:50070/ for the namenode. You can also
jps command to see whether they are
Stopping the daemons is done in the obvious way:
Setting up a cluster of machines brings many additional considerations, so this mode is covered in Chapter 9, Setting Up a Hadoop Cluster.
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