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https://issues.apache.org/jira/browse/HDFS-12943?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16687438#comment-16687438
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Chao Sun commented on HDFS-12943:
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[~xiangheng]: we are still working on the support for state transition between
standby/observer in the auto failover environment. You can watch HDFS-14067,
HDFS-13182 and HDFS-14059 for more detailed information.
At the moment, one workaround is to not launch ZK failover controller on the
host where the observer is at. Let me know if this works for you.
> 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,
> TestPlan-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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