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+~~ Licensed under the Apache License, Version 2.0 (the "License");
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+~~ you may not use this file except in compliance with the License.
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+~~ You may obtain a copy of the License at
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+~~
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+~~ http://www.apache.org/licenses/LICENSE-2.0
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+~~
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+~~ Unless required by applicable law or agreed to in writing, software
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+~~ distributed under the License is distributed on an "AS IS" BASIS,
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+~~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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+~~ See the License for the specific language governing permissions and
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+~~ limitations under the License. See accompanying LICENSE file.
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+
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+ ---
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+ Hadoop ${project.version} - Rack Awareness
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+ ---
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+ ---
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+ ${maven.build.timestamp}
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+
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+%{toc|section=1|fromDepth=0}
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+
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+Rack Awareness
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+
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+ Hadoop components are rack-aware. For example, HDFS block placement will
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+ use rack awareness for fault tolerance by placing one block replica on a
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+ different rack. This provides data availability in the event of a network
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+ switch failure or partition within the cluster.
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+
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+ Hadoop master daemons obtain the rack id of the cluster slaves by invoking
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+ either an external script or java class as specified by configuration files.
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+ Using either the java class or external script for topology, output must
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+ adhere to the java <<org.apache.hadoop.net.DNSToSwitchMapping>>
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+ interface. The interface expects a one-to-one correspondence to be
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+ maintained and the topology information in the format of '/myrack/myhost',
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+ where '/' is the topology delimiter, 'myrack' is the rack identifier, and
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+ 'myhost' is the individual host. Assuming a single /24 subnet per rack,
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+ one could use the format of '/192.168.100.0/192.168.100.5' as a
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+ unique rack-host topology mapping.
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+
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+ To use the java class for topology mapping, the class name is specified by
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+ the <<topology.node.switch.mapping.impl>> parameter in the configuration
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+ file. An example, NetworkTopology.java, is included with the hadoop
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+ distribution and can be customized by the Hadoop administrator. Using a
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+ Java class instead of an external script has a performance benefit in
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+ that Hadoop doesn't need to fork an external process when a new slave node
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+ registers itself.
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+
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+ If implementing an external script, it will be specified with the
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+ <<topology.script.file.name>> parameter in the configuration files. Unlike
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+ the java class, the external topology script is not included with the Hadoop
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+ distribution and is provided by the administrator. Hadoop will send
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+ multiple IP addresses to ARGV when forking the topology script. The
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+ number of IP addresses sent to the topology script is controlled with
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+ <<net.topology.script.number.args>> and defaults to 100. If
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+ <<net.topology.script.number.args>> was changed to 1, a topology script
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+ would get forked for each IP submitted by DataNodes and/or NodeManagers.
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+
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+ If <<topology.script.file.name>> or <<topology.node.switch.mapping.impl>> is
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+ not set, the rack id '/default-rack' is returned for any passed IP address.
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+ While this behavior appears desirable, it can cause issues with HDFS block
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+ replication as default behavior is to write one replicated block off rack
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+ and is unable to do so as there is only a single rack named '/default-rack'.
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+
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+ An additional configuration setting is
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+ <<mapreduce.jobtracker.taskcache.levels>> which determines the number of
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+ levels (in the network topology) of caches MapReduce will use. So, for
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+ example, if it is the default value of 2, two levels of caches will be
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+ constructed - one for hosts (host -> task mapping) and another for racks
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+ (rack -> task mapping). Giving us our one-to-one mapping of '/myrack/myhost'.
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+
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+* {python Example}
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+
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++-------------------------------+
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+ #!/usr/bin/python
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+ # this script makes assumptions about the physical environment.
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+ # 1) each rack is its own layer 3 network with a /24 subnet, which
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+ # could be typical where each rack has its own
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+ # switch with uplinks to a central core router.
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+ #
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+ # +-----------+
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+ # |core router|
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+ # +-----------+
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+ # / \
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+ # +-----------+ +-----------+
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+ # |rack switch| |rack switch|
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+ # +-----------+ +-----------+
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+ # | data node | | data node |
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+ # +-----------+ +-----------+
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+ # | data node | | data node |
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+ # +-----------+ +-----------+
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+ #
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+ # 2) topology script gets list of IP's as input, calculates network address, and prints '/network_address/ip'.
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+
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+ import netaddr
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+ import sys
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+ sys.argv.pop(0) # discard name of topology script from argv list as we just want IP addresses
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+
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+ netmask = '255.255.255.0' # set netmask to what's being used in your environment. The example uses a /24
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+
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+ for ip in sys.argv: # loop over list of datanode IP's
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+ address = '{0}/{1}'.format(ip, netmask) # format address string so it looks like 'ip/netmask' to make netaddr work
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+ try:
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+ network_address = netaddr.IPNetwork(address).network # calculate and print network address
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+ print "/{0}".format(network_address)
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+ except:
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+ print "/rack-unknown" # print catch-all value if unable to calculate network address
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++-------------------------------+
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+
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+* {bash Example}
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+
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++-------------------------------+
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+ #!/bin/bash
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+ # Here's a bash example to show just how simple these scripts can be
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+ # Assuming we have flat network with everything on a single switch, we can fake a rack topology.
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+ # This could occur in a lab environment where we have limited nodes,like 2-8 physical machines on a unmanaged switch.
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+ # This may also apply to multiple virtual machines running on the same physical hardware.
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+ # The number of machines isn't important, but that we are trying to fake a network topology when there isn't one.
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+ #
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+ # +----------+ +--------+
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+ # |jobtracker| |datanode|
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+ # +----------+ +--------+
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+ # \ /
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+ # +--------+ +--------+ +--------+
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+ # |datanode|--| switch |--|datanode|
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+ # +--------+ +--------+ +--------+
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+ # / \
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+ # +--------+ +--------+
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+ # |datanode| |namenode|
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+ # +--------+ +--------+
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+ #
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+ # With this network topology, we are treating each host as a rack. This is being done by taking the last octet
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+ # in the datanode's IP and prepending it with the word '/rack-'. The advantage for doing this is so HDFS
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+ # can create its 'off-rack' block copy.
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+ # 1) 'echo $@' will echo all ARGV values to xargs.
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+ # 2) 'xargs' will enforce that we print a single argv value per line
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+ # 3) 'awk' will split fields on dots and append the last field to the string '/rack-'. If awk
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+ # fails to split on four dots, it will still print '/rack-' last field value
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+
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+ echo $@ | xargs -n 1 | awk -F '.' '{print "/rack-"$NF}'
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++-------------------------------+
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+
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Allen Wittenauer