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From "Steve Loughran (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HADOOP-8047) CachedDNSToSwitchMapping caches negative results forever
Date Thu, 09 Feb 2012 13:09:59 GMT
CachedDNSToSwitchMapping caches negative results forever

                 Key: HADOOP-8047
                 URL: https://issues.apache.org/jira/browse/HADOOP-8047
             Project: Hadoop Common
          Issue Type: Bug
          Components: util
    Affects Versions: 1.0.0, 0.23.0, 0.24.0
            Reporter: Steve Loughran
            Priority: Trivial

This is very minor, just worth filing in JIRA unless someone wants to rethink topology caching
for a dynamic world.

# The CachedDNSToSwitchMapping caches the results from all relayed DNS queries.
# The DNS script mapper returns the default rack for all unknown entries (or when the script
# The Cache stores this in its map and never re-resolves it.

As a result, if a node is added to a live cluster that the existing script cannot resolve,
then it won't get assigned to a rack unless the script is updated before the rack map is resolved.

This isn't usually that important, it just means "update your scripts before adding new racks".
Perhaps there should be a page on that activity, "runbook and checklist for adding new servers
and racks".

Where it would matter if anyone started playing with dynamic topologies, but in that situation
the cached mapping itself would become the liability, as it assumes that servers never switch
switches in a live system: the topology is static for existing nodes. 

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