Rack Awareness
当Hadoop组件具有机架感知功能时，例如HDFS块放置将使用机架感知来实现容错，通过在不同机架上放置一个块副本来提高数据的可用性，以应对集群中的网络交换机故障或分区。
Hadoop主节点通过调用配置文件指定的外部脚本或Java类来获取集群工作节点的机架ID。
无论是使用Java类还是外部脚本进行拓扑映射，输出必须符合Java的org.apache.hadoop.net.DNSToSwitchMapping接口的格式。
该接口期望保持一对一的对应关系，并以’/myrack/myhost’的格式提供拓扑信息，其中’/’是拓扑定界符，’myrack’是机架标识符，而’myhost’则是个体主机。假设每个机架有一个/24子网，可以使用’/192.168.100.0/192.168.100.5’的格式作为独特的机架-主机拓扑映射。
要使用Java类进行拓扑映射，需要在配置文件中指定类名，该类名由net.topology.node.switch.mapping.impl参数指定。Hadoop分发中包含了一个名为NetworkTopology.java的示例，可以由Hadoop管理员进行定制。与在新的工作节点注册时无需分叉外部进程相比，使用Java类而不是外部脚本具有性能优势。
如果实施外部脚本，它将由配置文件中的net.topology.script.file.name参数指定。与Java类不同，外部拓扑脚本未随Hadoop分发一起提供，而由管理员提供。Hadoop将在分叉拓扑脚本时将多个IP地址发送到ARGV。控制发送到拓扑脚本的IP地址数量的参数是net.topology.script.number.args，默认为100。如果将net.topology.script.number.args更改为1，拓扑脚本将为DataNodes和/或NodeManagers提交的每个IP地址分叉。
如果未设置net.topology.script.file.name或net.topology.node.switch.mapping.impl，则对于任何传递的IP地址，都将返回机架ID/default-rack。
尽管这种行为似乎是可取的，但它可能会导致HDFS块复制出现问题，因为默认行为是将一个复制块写到机架外，而由于只有一个名为/default-rack的机架，无法实现这一点。
python Example
#!/usr/bin/python3
# this script makes assumptions about the physical environment.
# 1) each rack is its own layer 3 network with a /24 subnet, which
# could be typical where each rack has its own
# switch with uplinks to a central core router.
#
# +-----------+
# |core router|
# +-----------+
# / \
# +-----------+ +-----------+
# |rack switch| |rack switch|
# +-----------+ +-----------+
# | data node | | data node |
# +-----------+ +-----------+
# | data node | | data node |
# +-----------+ +-----------+
#
# 2) topology script gets list of IP's as input, calculates network address, and prints '/network_address/ip'.
import netaddr
import sys
sys.argv.pop(0) # discard name of topology script from argv list as we just want IP addresses
netmask = '255.255.255.0' # set netmask to what's being used in your environment. The example uses a /24
for ip in sys.argv: # loop over list of datanode IP's
address = '{0}/{1}'.format(ip, netmask) # format address string so it looks like 'ip/netmask' to make netaddr work
try:
network_address = netaddr.IPNetwork(address).network # calculate and print network address
print("/{0}".format(network_address))
except:
print("/rack-unknown") # print catch-all value if unable to calculate network address
bash Example
#!/usr/bin/env bash
# Here's a bash example to show just how simple these scripts can be
# Assuming we have flat network with everything on a single switch, we can fake a rack topology.
# This could occur in a lab environment where we have limited nodes,like 2-8 physical machines on a unmanaged switch.
# This may also apply to multiple virtual machines running on the same physical hardware.
# The number of machines isn't important, but that we are trying to fake a network topology when there isn't one.
#
# +----------+ +--------+
# |jobtracker| |datanode|
# +----------+ +--------+
# \ /
# +--------+ +--------+ +--------+
# |datanode|--| switch |--|datanode|
# +--------+ +--------+ +--------+
# / \
# +--------+ +--------+
# |datanode| |namenode|
# +--------+ +--------+
#
# With this network topology, we are treating each host as a rack. This is being done by taking the last octet
# in the datanode's IP and prepending it with the word '/rack-'. The advantage for doing this is so HDFS
# can create its 'off-rack' block copy.
# 1) 'echo $@' will echo all ARGV values to xargs.
# 2) 'xargs' will enforce that we print a single argv value per line
# 3) 'awk' will split fields on dots and append the last field to the string '/rack-'. If awk
# fails to split on four dots, it will still print '/rack-' last field value
echo $@ | xargs -n 1 | awk -F '.' '{print "/rack-"$NF}'
参考资料
https://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-common/RackAwareness.html
