[jira] [Commented] (HDFS-7858) Improve HA Namenode Failover detection on the client

["Arpit Agarwal (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-7858?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14626950#comment-14626950 ] 

Arpit Agarwal commented on HDFS-7858:
-------------------------------------

Hi [~asuresh], were you thinking of posting an updated patch. The overall approach looks good.

One comment from a quick look - RequestHedgingProxyProvider sends all requests to both NNs. Should it skip the standby for subsequent requests?

> 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-7858.2.patch, HDFS-7858.2.patch, HDFS-7858.3.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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