Add a topology mapper that reads hostname to rack mappings from a Java
properties file
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Key: HADOOP-8091
URL: https://issues.apache.org/jira/browse/HADOOP-8091
Project: Hadoop Common
Issue Type: New Feature
Components: util
Affects Versions: 0.24.0
Reporter: Steve Loughran
Priority: Minor
Getting mapping scripts right is surprisingly hard -and if its wrong in
production bad things happen. It would be good to have something simpler for
beginners -and one that is trivial to generate by a machine based on
infrastructure data.
I propose adding an alternative mapper, one driven by a java property file
# the specific topology mapper must be identified for loading
# it uses another key to identify the property file to load. This is checked
for on startup -if missing, fail.
# one property, perhaps "default-rack" identifies the default rack mapping for
any host not in the list
# every other entry lists a hostname to rack mapping
# hostname mapping is done on the first entry in the FQDN, to be less brittle
to domain resolution.
Example
{code}
default-rack=/rack1
host1=/rack1
host2=/rack1
host3=/rack2
host4=/rack2
{code}
Implementation
* add a new mapper that builds a concurrent hash map
* read in every entry in the specified property file, add it to the map
* when queried, extract the hostname (i.e. everything before any ".")
* match that in the hash table, return if found
* if not found: return the default rack
Feature creep would be to poll this file for changes at a (specified)
frequency, and pick up the changes when they occur. This would require removing
the caching topology mapper that wraps all others in the NN and RM.
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