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https://issues.apache.org/jira/browse/HDFS-5203?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13797405#comment-13797405
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Colin Patrick McCabe commented on HDFS-5203:
--------------------------------------------

I guess it makes sense to be able to add multiple cache directives for the same 
path.  They just get different IDs.  As you mentioned, that will make it easier 
for other programs to use our API

> Concurrent clients that add a cache directive on the same path may 
> prematurely uncache from each other.
> -------------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-5203
>                 URL: https://issues.apache.org/jira/browse/HDFS-5203
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: namenode
>    Affects Versions: HDFS-4949
>            Reporter: Chris Nauroth
>            Assignee: Chris Nauroth
>
> When a client adds a cache directive, we assign it a unique ID and return 
> that ID to the client.  If multiple clients add a cache directive for the 
> same path, then we return the same ID.  If one client then removes the cache 
> entry for that ID, then it is removed for all clients.  Then, when this 
> change becomes visible in subsequent cache reports, the datanodes may 
> {{munlock}} the block before the other clients are done with it.



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