Create a rack topology script

HDFS uses topology scripts to determine the rack location of nodes and uses this information to replicate block data to redundant racks.

1. Create an executable topology script and a topology data file.
   Consider the following examples:
   The following is an example topology script named rack-topology.sh.

   
   #!/bin/bash
   # Adjust/Add the property "net.topology.script.file.name"
   # to core-site.xml with the "absolute" path the this
   # file. ENSURE the file is "executable".
   
   # Supply appropriate rack prefix
   RACK_PREFIX=default
   
   # To test, supply a hostname as script input:
   if [ $# -gt 0 ]; then
   
   CTL_FILE=${CTL_FILE:-"rack_topology.data"}
   
   HADOOP_CONF=${HADOOP_CONF:-"/etc/hadoop/conf"} 
   
   if [ ! -f ${HADOOP_CONF}/${CTL_FILE} ]; then
    echo -n "/$RACK_PREFIX/rack "
    exit 0
   fi
   
   while [ $# -gt 0 ] ; do
    nodeArg=$1
    exec< ${HADOOP_CONF}/${CTL_FILE}
    result=""
    while read line ; do
    ar=( $line )
    if [ "${ar[0]}" = "$nodeArg" ] ; then
    result="${ar[1]}"
    fi
    done
    shift
    if [ -z "$result" ] ; then
    echo -n "/$RACK_PREFIX/rack "
    else
    echo -n "/$RACK_PREFIX/rack_$result "
    fi
   done
   
   else
    echo -n "/$RACK_PREFIX/rack "
   fi

   The following is an example topology data file named rack_topology.data.

   
   # This file should be:
   # - Placed in the /etc/hadoop/conf directory
   # - On the Namenode (and backups IE: HA, Failover, etc)
   # - On the Job Tracker OR Resource Manager (and any Failover JT's/RM's) 
   # This file should be placed in the /etc/hadoop/conf directory.
   
   # Add Hostnames to this file. Format <host ip> <rack_location>
   192.168.2.10 01
   192.168.2.11 02
   192.168.2.12 03

2. Copy the topology script and the data file to the /etc/hadoop/conf directory on all cluster nodes.
3. Run the topology script to ensure that it returns the correct rack information for each host.