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Hadoop
Hadoop is a distributed computing platform.
Hadoop primarily consists of the Hadoop Distributed FileSystem
(HDFS) and an
implementation of the
Map-Reduce programming paradigm.
Hadoop is a software framework that lets one easily write and run applications
that process vast amounts of data. Here's what makes Hadoop especially useful:
-
Scalable: Hadoop can reliably store and process petabytes.
-
Economical: It distributes the data and processing across clusters
of commonly available computers. These clusters can number into the thousands
of nodes.
-
Efficient: By distributing the data, Hadoop can process it in parallel
on the nodes where the data is located. This makes it extremely rapid.
-
Reliable: Hadoop automatically maintains multiple copies of data and
automatically redeploys computing tasks based on failures.
Requirements
Platforms
-
Hadoop has been demonstrated on GNU/Linux clusters with more than 4000 nodes.
-
Windows is also a supported platform.
Requisite Software
-
Java 1.6.x, preferably from
Sun.
Set JAVA_HOME to the root of your Java installation.
-
ssh must be installed and sshd must be running to use Hadoop's
scripts to manage remote Hadoop daemons.
-
rsync may be installed to use Hadoop's scripts to manage remote
Hadoop installations.
Installing Required Software
If your platform does not have the required software listed above, you
will have to install it.
For example on Ubuntu Linux:
$ sudo apt-get install ssh
$ sudo apt-get install rsync
Getting Started
First, you need to get a copy of the Hadoop code.
Edit the file conf/hadoop-env.sh to define at least
JAVA_HOME.
Try the following command:
bin/hadoop
This will display the documentation for the Hadoop command script.
Standalone operation
By default, Hadoop is configured to run things in a non-distributed
mode, as a single Java process. This is useful for debugging, and can
be demonstrated as follows:
mkdir input
cp conf/*.xml input
bin/hadoop jar hadoop-*-examples.jar grep input output 'dfs[a-z.]+'
cat output/*
This will display counts for each match of the
regular expression.
Note that input is specified as a directory containing input
files and that output is also specified as a directory where parts are
written.
Distributed operation
To configure Hadoop for distributed operation you must specify the
following:
- The NameNode (Distributed Filesystem master) host. This is
specified with the configuration property fs.default.name.
- The org.apache.hadoop.mapred.JobTracker (MapReduce master)
host and port. This is specified with the configuration property
mapred.job.tracker.
- A slaves file that lists the names of all the hosts in
the cluster. The default slaves file is conf/slaves.
Pseudo-distributed configuration
You can in fact run everything on a single host. To run things this
way, put the following in:
conf/core-site.xml:
<configuration>
<property>
<name>fs.default.name</name>
<value>hdfs://localhost/</value>
</property>
</configuration>
conf/hdfs-site.xml:
<configuration>
<property>
<name>dfs.replication</name>
<value>1</value>
</property>
</configuration>
conf/mapred-site.xml:
<configuration>
<property>
<name>mapred.job.tracker</name>
<value>localhost:9001</value>
</property>
</configuration>
(We also set the HDFS replication level to 1 in order to
reduce warnings when running on a single node.)
Now check that the command
ssh localhost
does not
require a password. If it does, execute the following commands:
ssh-keygen -t dsa -P '' -f ~/.ssh/id_dsa
cat ~/.ssh/id_dsa.pub >> ~/.ssh/authorized_keys
Bootstrapping
A new distributed filesystem must be formatted with the following
command, run on the master node:
bin/hadoop namenode -format
The Hadoop daemons are started with the following command:
bin/start-all.sh
Daemon log output is written to the logs/ directory.
Input files are copied into the distributed filesystem as follows:
bin/hadoop fs -put input input
Distributed execution
Things are run as before, but output must be copied locally to
examine it:
bin/hadoop jar hadoop-*-examples.jar grep input output 'dfs[a-z.]+'
bin/hadoop fs -get output output
cat output/*
When you're done, stop the daemons with:
bin/stop-all.sh
Fully-distributed operation
Fully distributed operation is just like the pseudo-distributed operation
described above, except, specify:
- The hostname or IP address of your master server in the value
for fs.default.name,
as hdfs://master.example.com/ in conf/core-site.xml.
- The host and port of the your master server in the value
of mapred.job.tracker
as master.example.com:port in conf/mapred-site.xml.
- Directories for dfs.name.dir and
dfs.data.dir
in conf/hdfs-site.xml.
These are local directories used to hold distributed filesystem
data on the master node and slave nodes respectively. Note
that dfs.data.dir may contain a space- or comma-separated
list of directory names, so that data may be stored on multiple local
devices.
- mapred.local.dir
in conf/mapred-site.xml, the local directory where temporary
MapReduce data is stored. It also may be a list of directories.
- mapred.map.tasks
and mapred.reduce.tasks
in conf/mapred-site.xml.
As a rule of thumb, use 10x the
number of slave processors for mapred.map.tasks, and 2x the
number of slave processors for mapred.reduce.tasks.
Finally, list all slave hostnames or IP addresses in your
conf/slaves file, one per line. Then format your filesystem
and start your cluster on your master node, as above.