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From "Uma Maheswara Rao G (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2713) HA : An alternative approach to clients handling Namenode failover.
Date Thu, 22 Dec 2011 16:01:31 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2713?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13174862#comment-13174862

Uma Maheswara Rao G commented on HDFS-2713:

Thanks a lot, Aaron for the patience in review. :-)

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.

After any DFSClient operation fails due to Namenode unavailability, the most important thing
to do is to detect when the Active Namenode becomes available again.
So the background thread is not doing any unnecessary work, it is doing the high priority
The difference in our approaches is that, the importance given to the failover till it succeeds.
In the approch I described, it is considered very important in the sense that one thread is
dedicated to find the Active Namenode and after finding only it will exit.
If the RetryDecision is FAILOVER_AND_RETRY,then only the failover is done.
If there are many issued to Namenode whose RetryDecision is FAIL, failover won't happen.

My intention is that, when one client call finds failover is required and not able to complete
the failover within the wait time, then why do I need to wait till next call comes to try
again and failover after mindealy wait? 
Even though the first call fails, this background thread will ensure to find the active proxy
instance. If next call comes now(this is user thread),  it need not wait to connect and failover
again. Immediately it can make use of that proxy instance and goahead.
I will try to integrate the logic with *ConfiguredFailverProxyProvider* and upload a temp
patch for more understanding.

> 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
> 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
> 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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