[jira] [Created] (HADOOP-8047) CachedDNSToSwitchMapping caches negative results forever

["Steve Loughran (Created) (JIRA)" <ji...@apache.org>]

CachedDNSToSwitchMapping caches negative results forever
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                 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 fails)
# 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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