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https://issues.apache.org/jira/browse/HDFS-1623?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13008395#comment-13008395
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Ivan Kelly commented on HDFS-1623:
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I assume that in what is envisioned, the code running on active and standby
will be identical, with the only difference being whether an active switch is
enabled.
It would be good if the code for active/standby detection was pluggable. So
that different options for failover could be provided. It wouldn't be good to
require that a zookeeper ensemble be set up just to run a namenode.
How does heartbeat deal with network partition? My understanding of it is that
it sends packets at intervals to the other node, and if they don't get through
it considers the other dead. This could create a situation where both active
and standby think that the other is dead, and both become active, leading to
divergent filesystem states on each machine.
Also, the design indicates that more than 2 NN is out of scope. Why? Surely
it's as easy to design for N namenodes as it is for 2 namenodes.
If you want manual failover, from the server perspective you need to do
nothing. Operators can have 2 namenode machines, with the namenode only running
on one, writing to shared storage. When the want to failover to the standby
they just have to ensure that the active is down and start the namenode daemon
on the standby.
I proposed a design last week for streaming updates from an active to a
standby, it may be interesting to you (ZOOKEEPER-1016). It does have some
mentions of active/standby detection, which I should remove. It occurs to me
now, that this functionallity should be separated out completely from the
WALing and should live at the level of NameNode.java.
> High Availability Framework for HDFS NN
> ---------------------------------------
>
> Key: HDFS-1623
> URL: https://issues.apache.org/jira/browse/HDFS-1623
> Project: Hadoop HDFS
> Issue Type: New Feature
> Reporter: Sanjay Radia
> Assignee: Sanjay Radia
> Attachments: Namenode HA Framework.pdf
>
>
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