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https://issues.apache.org/jira/browse/HDFS-2713?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13174419#comment-13174419
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Aaron T. Myers commented on HDFS-2713:
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bq. Yes , i agree existing implementation also supports.But the main difference
is, lets take a case with existing implementation, after a finite period of
time, if it is not able to get active node proxy instance it will throw
exception and fail. After some time if other call comes, then again it will do
failover. But with my proposed one, Background thread will continue failover
indefinitely until it finds active proxy instance. So, by the time next call
come this background thread may make ready of active node proxy.
I still don't see what benefit the background thread has. In the case you
describe, with the current implementation, the second client request (after the
failed one which had timed out retrying/failing over) would just simply
succeed, or fail over immediately and then succeed. So, the background thread
won't have saved much if any work, and instead may indefinitely be doing
(potentially unnecessary) work in the background.
What am I missing?
> 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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