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Date: Tue, 13 May 2014 13:39:16 +0000 (UTC)
From: "Daryn Sharp (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Message-ID: <JIRA.12713635.1399901558659.337360.1399988356169@arcas>
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Subject: [jira] [Commented] (HDFS-6371) In HA setup, the standby NN should
 redirect WebHDFS write requests to the active NN
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    [ https://issues.apache.org/jira/browse/HDFS-6371?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13996394#comment-13996394 ] 

Daryn Sharp commented on HDFS-6371:
-----------------------------------

Agreed on client side retries.  With IP-based failover, a standby NN that attempts to redirect to the other NN will create a redirect loop to itself until the IP failover to the active occurs.

> In HA setup, the standby NN should redirect WebHDFS write requests to the active NN
> -----------------------------------------------------------------------------------
>
>                 Key: HDFS-6371
>                 URL: https://issues.apache.org/jira/browse/HDFS-6371
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode, webhdfs
>            Reporter: Tsz Wo Nicholas Sze
>            Assignee: Tsz Wo Nicholas Sze
>
> The current WebHDFS implementation in namenode does not check its HA state -- it does the same thing no matter it is active or standby.
> Suppose a http client talk to the standby NN via WebHDFS.  For the read operations, there is no problem.  For the write operations, if the operation requires http redirect (e.g. creating a file), it will work since the standby NN will also redirect the client to a DN.  When the client connect to the DN, the DN will fulfill the request with the active NN.  However, for the write operations not requiring http redirect (e.g. mkdir), the operation will fail with StandbyException since it will be executed with the standby NN.
> There are two solutions:
> # The http client could catch StandbyException and then retries with the other NN in this case.
> # The standby NN redirects the request to the active NN.
> The second solution seems better since the client does not need to know both active NN and standby NN.
> Note that WebHdfsFileSystem is already able to handle HA failover.  The JIRA is for other http clients.


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