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https://issues.apache.org/jira/browse/HDFS-12943?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16504261#comment-16504261
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Chao Sun commented on HDFS-12943:
---------------------------------
[~xiaochen], on the second question, current the transition from SBN to
Observer is done via a haadmin command: {{haadmin -transitionToObserver}}, and
vise versa you can transition Observer to SBN via {{haadmin
-transitionToStandby}}. There is no automatic transition between the two, and
no transition is allowed between Observer and ANN.
In terms of failover, details are yet to be discussed. Ideally we'd like to
allow Observer to participate in the failover too but it is yet to be resolved.
I did some preliminary work on that which you can find in the comments of
HDFS-12975. The failover handling is tracked by HDFS-13182.
> Consistent Reads from Standby Node
> ----------------------------------
>
> Key: HDFS-12943
> URL: https://issues.apache.org/jira/browse/HDFS-12943
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: hdfs
> Reporter: Konstantin Shvachko
> Priority: Major
> Attachments: ConsistentReadsFromStandbyNode.pdf,
> ConsistentReadsFromStandbyNode.pdf
>
>
> StandbyNode in HDFS is a replica of the active NameNode. The states of the
> NameNodes are coordinated via the journal. It is natural to consider
> StandbyNode as a read-only replica. As with any replicated distributed system
> the problem of stale reads should be resolved. Our main goal is to provide
> reads from standby in a consistent way in order to enable a wide range of
> existing applications running on top of HDFS.
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