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From "Jing Zhao (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7858) Improve HA Namenode Failover detection on the client
Date Fri, 24 Jul 2015 21:02:06 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7858?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14641067#comment-14641067
] 

Jing Zhao commented on HDFS-7858:
---------------------------------

bq. I need this to get name of the proxy which was successful (so i can key into the targetProxies
map). CallResult catches the exception and sets it as the result.

Yeah, I did notice the exception has been captured by CallResult. But maybe we can use a future-->proxy
map here? In this way we do not need to have a wrapper class like CallResult so maybe the
code can be further simplified.

> Improve HA Namenode Failover detection on the client
> ----------------------------------------------------
>
>                 Key: HDFS-7858
>                 URL: https://issues.apache.org/jira/browse/HDFS-7858
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Arun Suresh
>            Assignee: Arun Suresh
>              Labels: BB2015-05-TBR
>         Attachments: HDFS-7858.1.patch, HDFS-7858.10.patch, HDFS-7858.2.patch, HDFS-7858.2.patch,
HDFS-7858.3.patch, HDFS-7858.4.patch, HDFS-7858.5.patch, HDFS-7858.6.patch, HDFS-7858.7.patch,
HDFS-7858.8.patch, HDFS-7858.9.patch
>
>
> In an HA deployment, Clients are configured with the hostnames of both the Active and
Standby Namenodes.Clients will first try one of the NNs (non-deterministically) and if its
a standby NN, then it will respond to the client to retry the request on the other Namenode.
> If the client happens to talks to the Standby first, and the standby is undergoing some
GC / is busy, then those clients might not get a response soon enough to try the other NN.
> Proposed Approach to solve this :
> 1) Since Zookeeper is already used as the failover controller, the clients could talk
to ZK and find out which is the active namenode before contacting it.
> 2) Long-lived DFSClients would have a ZK watch configured which fires when there is a
failover so they do not have to query ZK everytime to find out the active NN
> 2) Clients can also cache the last active NN in the user's home directory (~/.lastNN)
so that short-lived clients can try that Namenode first before querying ZK



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