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https://issues.apache.org/jira/browse/HADOOP-8292?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Alejandro Abdelnur updated HADOOP-8292:
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    Attachment: HADOOP-8292.patch

new patch addressing Todd's comments.

the new patch removes the 'hot path' by using a daemon thread that does the 
check/reload and swapping the Map using an AtomicRef. It does a 2 seconds lag 
check as you suggested. Regarding the clearCache(), the underlaying impl is a 
ConcurrentMap, so that would not be an issue (still, now the clearcache is 
called after swapping the new Map in the daemon thread).

Regarding your last question on how it may impact HDFS, I don't know I'll dig.
                
> TableMapping does not refresh when topology is updated
> ------------------------------------------------------
>
>                 Key: HADOOP-8292
>                 URL: https://issues.apache.org/jira/browse/HADOOP-8292
>             Project: Hadoop Common
>          Issue Type: Bug
>    Affects Versions: 2.0.0
>            Reporter: Philip Zeyliger
>            Assignee: Alejandro Abdelnur
>         Attachments: HADOOP-8292.patch, HADOOP-8292.patch
>
>
> HADOOP-7030 introduced TableMapping, an implementation of DNSToSwitchMapping 
> which uses a file to map from IPs/hosts to their racks.  It's intended to 
> replace ScriptBasedMapping for cases where the latter was just a complicated 
> way of looking up the rack in a file.
> Though there was discussion of it on the JIRA, the TableMapping 
> implementation is not 'refreshable'.  i.e., if you want to add a host to your 
> cluster, and that host wasn't in the topology file to begin with, it will 
> never be added.
> TableMapping should refresh, either based on a command that can be executed, 
> or, perhaps, if the file on disk changes.
> I'll also point out that TableMapping extends CachedDNSToSwitchMapping, but, 
> since it does no refreshing, I don't see what the caching gets you: I think 
> the cache ends up being a second copy of the underlying map, always.

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