[jira] [Commented] (HDFS-6371) In HA setup, the standby NN should redirect WebHDFS write requests to the active NN

["Haohui Mai (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-6371?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13995293#comment-13995293 ] 

Haohui Mai commented on HDFS-6371:
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In HA design the client always takes responsibility of retries instead of the server. In the use case mentioned in HDFS-6334, the client and the sever might have different views on HA, thus it looks to me that retrying on the client-side might be the only viable solution.

> 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: Bug
>          Components: 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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