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  1. Getting Started
  2. Steps to Configure a Single-Node YARN Cluster
  3. Run Sample MapReduce Examples
  4. Wrap-up
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This chapter is from the book

Steps to Configure a Single-Node YARN Cluster

The following type of installation is often referred to as “pseudo-distributed” because it mimics some of the functionality of a distributed Hadoop cluster. A single machine is, of course, not practical for any production use, nor is it parallel. A small-scale Hadoop installation can provide a simple method for learning Hadoop basics, however.

The recommended minimal installation hardware is a dual-core processor with 2 GB of RAM and 2 GB of available hard drive space. The system will need a recent Linux distribution with Java installed (e.g., Red Hat Enterprise Linux or rebuilds, Fedora, Suse Linux Enterprise, OpenSuse, Ubuntu). Red Hat Enterprise Linux 6.3 is used for this installation example. A bash shell environment is also assumed. The first step is to download Apache Hadoop.

Note that the following commands and files are available for download from the book repository; see Appendix A for details.

Step 1: Download Apache Hadoop

Download the latest distribution from the Hadoop website (http://hadoop.apache.org/). For example, as root do the following:

# cd /root
# wget http://mirrors.ibiblio.org/apache/hadoop/common/hadoop-2.2.0/hadoop-2.2.0.tar.gz

Next create and extract the package in /opt/yarn:

# mkdir –p /opt/yarn
# cd /opt/yarn
# tar xvzf /root/hadoop-2.2.0.tar.gz

Step 2: Set JAVA_HOME

For Hadoop 2, the recommended version of Java can be found at http://wiki.apache.org/hadoop/HadoopJavaVersions. In general, a Java Development Kit 1.6 (or greater) should work. For this install, we will use Open Java 1.6.0_24, which is part of Red Hat Enterprise Linux 6.3. Make sure you have a working Java JDK installed; in this case, it is the Java-1.6.0-openjdk RPM. To include JAVA_HOME for all bash users (other shells must be set in a similar fashion), make an entry in /etc/profile.d as follows:

# echo "export JAVA_HOME=/usr/lib/jvm/java-1.6.0-openjdk-1.6.0.0.x86_64/" > /etc/ profile.d/java.sh

To make sure JAVA_HOME is defined for this session, source the new script:

# source /etc/profile.d/java.sh

Step 3: Create Users and Groups

It is best to run the various daemons with separate accounts. Three accounts (yarn, hdfs, mapred) in the group hadoop can be created as follows:

# groupadd hadoop
# useradd -g hadoop yarn
# useradd -g hadoop hdfs
# useradd -g hadoop mapred

Step 4: Make Data and Log Directories

Hadoop needs various data and log directories with various permissions. Enter the following lines to create these directories:

# mkdir -p /var/data/hadoop/hdfs/nn
# mkdir -p /var/data/hadoop/hdfs/snn
# mkdir -p /var/data/hadoop/hdfs/dn
# chown hdfs:hadoop /var/data/hadoop/hdfs –R
# mkdir -p /var/log/hadoop/yarn
# chown yarn:hadoop /var/log/hadoop/yarn -R

Next, move to the YARN installation root and create the log directory and set the owner and group as follows:

# cd /opt/yarn/hadoop-2.2.0
# mkdir logs
# chmod g+w logs
# chown yarn:hadoop . -R

Step 5: Configure core-site.xml

From the base of the Hadoop installation path (e.g., /opt/yarn/hadoop-2.2.0), edit the etc/hadoop/core-site.xml file. The original installed file will have no entries other than the <configuration> </configuration> tags. Two properties need to be set. The first is the fs.default.name property, which sets the host and request port name for the NameNode (metadata server for HDFS). The second is hadoop.http.staticuser.user, which will set the default user name to hdfs. Copy the following lines to the Hadoop etc/hadoop/core-site.xml file and remove the original empty <configuration> </configuration> tags.

<configuration>
       <property>
               <name>fs.default.name</name>
               <value>hdfs://localhost:9000</value>
       </property>
       <property>
               <name>hadoop.http.staticuser.user</name>
               <value>hdfs</value>
       </property>
</configuration>

Step 6: Configure hdfs-site.xml

From the base of the Hadoop installation path, edit the etc/hadoop/hdfs-site.xml file. In the single-node pseudo-distributed mode, we don’t need or want the HDFS to replicate file blocks. By default, HDFS keeps three copies of each file in the file system for redundancy. There is no need for replication on a single machine; thus the value of dfs.replication will be set to 1.

In hdfs-site.xml, we specify the NameNode, Secondary NameNode, and DataNode data directories that we created in Step 4. These are the directories used by the various components of HDFS to store data. Copy the following lines into Hadoop etc/hadoop/hdfs-site.xml and remove the original empty <configuration> </configuration> tags.

<configuration>
 <property>
   <name>dfs.replication</name>
   <value>1</value>
 </property>
 <property>
   <name>dfs.namenode.name.dir</name>
   <value>file:/var/data/hadoop/hdfs/nn</value>
 </property>
 <property>
   <name>fs.checkpoint.dir</name>
   <value>file:/var/data/hadoop/hdfs/snn</value>
 </property>
 <property>
   <name>fs.checkpoint.edits.dir</name>
   <value>file:/var/data/hadoop/hdfs/snn</value>
 </property>
 <property>
   <name>dfs.datanode.data.dir</name>
   <value>file:/var/data/hadoop/hdfs/dn</value>
 </property>
</configuration>

Step 7: Configure mapred-site.xml

From the base of the Hadoop installation, edit the etc/hadoop/mapred-site.xml file. A new configuration option for Hadoop 2 is the capability to specify a framework name for MapReduce, setting the mapreduce.framework.name property. In this install, we will use the value of “yarn” to tell MapReduce that it will run as a YARN application. First, copy the template file to the mapred-site.xml.

# cp mapred-site.xml.template mapred-site.xml

Next, copy the following lines into Hadoop etc/hadoop/mapred-site.xml file and remove the original empty <configuration> </configuration> tags.

<configuration>
<property>
   <name>mapreduce.framework.name</name>
   <value>yarn</value>
 </property>
</configuration>

Step 8: Configure yarn-site.xml

From the base of the Hadoop installation, edit the etc/hadoop/yarn-site.xml file. The yarn.nodemanager.aux-services property tells NodeManagers that there will be an auxiliary service called mapreduce.shuffle that they need to implement. After we tell the NodeManagers to implement that service, we give it a class name as the means to implement that service. This particular configuration tells MapReduce how to do its shuffle. Because NodeManagers won’t shuffle data for a non-MapReduce job by default, we need to configure such a service for MapReduce. Copy the following lines to the Hadoop etc/hadoop/yarn-site.xml file and remove the original empty <configuration> </configuration> tags.

<configuration>
<property>
   <name>yarn.nodemanager.aux-services</name>
   <value>mapreduce_shuffle</value>
 </property>
 <property>
   <name>yarn.nodemanager.aux-services.mapreduce.shuffle.class</name>
   <value>org.apache.hadoop.mapred.ShuffleHandler</value>
 </property>
</configuration>

Step 9: Modify Java Heap Sizes

The Hadoop installation uses several environment variables that determine the heap sizes for each Hadoop process. These are defined in the etc/hadoop/*-env.sh files used by Hadoop. The default for most of the processes is a 1 GB heap size; because we’re running on a workstation that will probably have limited resources compared to a standard server, however, we need to adjust the heap size settings. The values that follow are adequate for a small workstation or server.

Edit the etc/hadoop/hadoop-env.sh file to reflect the following (don’t forget to remove the “#” at the beginning of the line):

HADOOP_HEAPSIZE="500"
HADOOP_NAMENODE_INIT_HEAPSIZE="500"

Next, edit mapred-env.sh to reflect the following:

HADOOP_JOB_HISTORYSERVER_HEAPSIZE=250

Finally, edit yarn-env.sh to reflect the following:

JAVA_HEAP_MAX=-Xmx500m

The following line will need to be added to yarn-env.sh:

YARN_HEAPSIZE=500

Step 10: Format HDFS

For the HDFS NameNode to start, it needs to initialize the directory where it will hold its data. The NameNode service tracks all the metadata for the file system. The format process will use the value assigned to dfs.namenode.name.dir in etc/hadoop/hdfs-site.xml earlier (i.e., /var/data/hadoop/hdfs/nn). Formatting destroys everything in the directory and sets up a new file system. Format the NameNode directory as the HDFS superuser, which is typically the “hdfs” user account.

From the base of the Hadoop distribution, change directories to the “bin” directory and execute the following commands:

# su - hdfs
$ cd /opt/yarn/hadoop-2.2.0/bin
$ ./hdfs namenode -format

If the command worked, you should see the following near the end of a long list of messages:

INFO common.Storage: Storage directory /var/data/hadoop/hdfs/nn has been successfully formatted.

Step 11: Start the HDFS Services

Once formatting is successful, the HDFS services must be started. There is one service for the NameNode (metadata server), a single DataNode (where the actual data is stored), and the SecondaryNameNode (checkpoint data for the NameNode). The Hadoop distribution includes scripts that set up these commands as well as name other values such as PID directories, log directories, and other standard process configurations. From the bin directory in Step 10, execute the following as user hdfs:

$ cd ../sbin
$ ./hadoop-daemon.sh start namenode

The command should show the following:

starting namenode, logging to /opt/yarn/hadoop-2.2.0/logs/hadoop-hdfs-namenode-limulus.out

The secondarynamenode and datanode services can be started in the same way:

$ ./hadoop-daemon.sh start secondarynamenode
starting secondarynamenode, logging to /opt/yarn/hadoop-2.2.0/logs/hadoop-hdfs-secondarynamenode-limulus.out
$ ./hadoop-daemon.sh start datanode
starting datanode, logging to /opt/yarn/hadoop-2.2.0/logs/hadoop-hdfs-datanode-limulus.out

If the daemon started successfully, you should see responses that will point to the log file. (Note that the actual log file is appended with “.log,” not “.out.”). As a sanity check, issue a jps command to confirm that all the services are running. The actual PID (Java Process ID) values will be different than shown in this listing:

$ jps
15140 SecondaryNameNode
15015 NameNode
15335 Jps
15214 DataNode

If the process did not start, it may be helpful to inspect the log files. For instance, examine the log file for the NameNode. (Note that the path is taken from the preceding command.)

vi /opt/yarn/hadoop-2.2.0/logs/hadoop-hdfs-namenode-limulus.log

All Hadoop services can be stopped using the hadoop-daemon.sh script. For example, to stop the datanode service, enter the following (as user hdfs in the /opt/yarn/hadoop-2.2.0/sbin directory):

$ ./hadoop-daemon.sh stop datanode

The same can be done for the NameNode and SecondaryNameNode.

Step 12: Start YARN Services

As with HDFS services, the YARN services need to be started. One Resource-Manager and one NodeManager must be started as user yarn (exiting from user hdfs first):

$ exit
logout
# su - yarn
$ cd /opt/yarn/hadoop-2.2.0/sbin
$ ./yarn-daemon.sh start resourcemanager
starting resourcemanager, logging to /opt/yarn/hadoop-2.2.0/logs/yarn-yarn-resourcemanager-limulus.out
$ ./yarn-daemon.sh start nodemanager
starting nodemanager, logging to /opt/yarn/hadoop-2.2.0/logs/yarn-yarn-nodemanager-limulus.out

As when the HDFS daemons were started in Step 1, the status of the running daemons is sent to their respective log files. To check whether the services are running, issue a jps command. The following shows all the services necessary to run YARN on a single server:

$ jps
15933 Jps
15567 ResourceManager
15785 NodeManager

If there are missing services, check the log file for the specific service. Similar to the case with HDFS services, the services can be stopped by issuing a stop argument to the daemon script:

./yarn-daemon.sh stop nodemanager

Step 13: Verify the Running Services Using the Web Interface

Both HDFS and the YARN ResourceManager have a web interface. These interfaces are a convenient way to browse many of the aspects of your Hadoop installation. To monitor HDFS, enter the following (or use your favorite web browser):

$ firefox  http://localhost:50070

Connecting to port 50070 will bring up a web interface similar to Figure 2.1.

Figure 2.1

Figure 2.1 Webpage for HDFS file system

A web interface for the ResourceManager can be viewed by entering the following:

$ firefox http://localhost:8088

A webpage similar to that shown in Figure 2.2 will be displayed.

Figure 2.2

Figure 2.2 Webpage for YARN ResourceManager

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