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https://issues.apache.org/jira/browse/HDFS-2713?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13174347#comment-13174347
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Aaron T. Myers commented on HDFS-2713:
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Hey Uma, there are several things I don't understand about this proposal.
bq. 1) During failover, user threads can be controlled very accurately about
the time they wait for active namenode to be available, awaiting the retry.
Beyond this, the threads will not be made to wait; DFS Client will throw an
Exception indicating that the operation has failed.
The current system already supports this. Clients will failover and retry with
some random, exponential backoff for a finite period of time, after which the
operation will fail, throwing an exception.
bq. 2) Failover happens in a seperate thread, not in the client application
threads. The thread will keep trying to find the Active Namenode until it
succeeds.
What's the point of doing this in a separate thread? Given that client
operations still block while the failover is attempted, it doesn't seem like
this difference will be tangible to the user.
3) This also means that irrespective of whether the operation's RetryAction is
RETRY_FAILOVER or FAIL, the user thread can trigger the client's failover.
This confuses me. How does this work?
In short, this proposal just seems _different_ and not necessarily _better_
than the current implementation. This implementation also seems like a more
complex design to me, so without tangible user benefits I don't see much point
in doing it.
The other thing that's not clear to me is how you'd propose to incorporate it
into HDFS. Would it be an alternative to the current implementation? Or done as
an enhancement to the current implementation?
> HA : An alternative approach to clients handling Namenode failover.
> --------------------------------------------------------------------
>
> Key: HDFS-2713
> URL: https://issues.apache.org/jira/browse/HDFS-2713
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: ha, hdfs client
> Affects Versions: HA branch (HDFS-1623)
> Reporter: Uma Maheswara Rao G
> Assignee: Uma Maheswara Rao G
>
> This is the approach for client failover which we adopted when we developed
> HA for Hadoop. I would like to propose thia approach for others to review &
> include in the HA implementation, if found useful.
> This is similar to the ConfiguredProxyProvider in the sense that the it takes
> the address of both the Namenodes as the input. The major differences I can
> see from the current implementation are
> 1) During failover, user threads can be controlled very accurately about *the
> time they wait for active namenode* to be available, awaiting the retry.
> Beyond this, the threads will not be made to wait; DFS Client will throw an
> Exception indicating that the operation has failed.
> 2) Failover happens in a seperate thread, not in the client application
> threads. The thread will keep trying to find the Active Namenode until it
> succeeds.
> 3) This also means that irrespective of whether the operation's RetryAction
> is RETRY_FAILOVER or FAIL, the user thread can trigger the client's failover.
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