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https://issues.apache.org/jira/browse/HADOOP-13742?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15669568#comment-15669568
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Vinayakumar B commented on HADOOP-13742:
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bq. First of all, the use of "Vs" is probably not the best choice. "Vs" is for 
two comparable things.
I think "userToConnectionsMap" might be better.
bq. I am wondering whether introducing a separate metrics class is better. 
(Something similar to RpcDetailedMetrics) For typical metrics, a counter is 
simply incremented and the diff shows the rate. If users are only interested in 
instantaneous counts and do not care at all about cumulative counts, the 
approach in the patch might be okay. But I am thinking cumulative values are 
useful too.
That's a good idea and may be required to for detailed analysis about each 
user's usage. In simple terms there could be separate instances of 
RpcDetailedMetrics for each user, Just an Idea. I feel this could be done in a 
follow-up jira and detailed discussions could be made.

bq. If we are to proceed with the proposed approach, I suggest simply 
synchronize on the lock object inside incrUserConnections() and 
decrUserConnections() and lose the private methods. Then no need to have 
AtomicInteger. The ideal way will be range-locking the map, but I don't know 
whether there is any known good implementation of it. In any case, the number 
of rpc reader threads is typically small, so this level of 
synchronization/monitor lock should be okay.
In this jira only simple count of connections can be exposed as a metric, as 
title says.
 [~brahmareddy], please update the patch with simple synchronization, as 
suggested by [~kihwal].

> Expose "NumOpenConnectionsPerUser" as a metric
> ----------------------------------------------
>
>                 Key: HADOOP-13742
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13742
>             Project: Hadoop Common
>          Issue Type: Improvement
>            Reporter: Brahma Reddy Battula
>            Assignee: Brahma Reddy Battula
>         Attachments: HADOOP-13742-002.patch, HADOOP-13742-003.patch, 
> HADOOP-13742-004.patch, HADOOP-13742.patch
>
>
> To track user level connections( How many connections for each user) in busy 
> cluster where so many connections to server.



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