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https://issues.apache.org/jira/browse/HADOOP-8047?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13204554#comment-13204554
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Daryn Sharp commented on HADOOP-8047:
-------------------------------------
This is actually a general problem if the cache is in a persistent daemon like
the namenode. Transient DNS problems can cause false entries in the negative
cache.
> 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: 0.23.0, 0.24.0, 1.0.0
> 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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