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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 Wed, 21 Dec 2011 19:59:30 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2713?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13174366#comment-13174366
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Uma Maheswara Rao G commented on HDFS-2713:
-------------------------------------------

Thanks a lot, Aaron for taking a look.
{quote}
    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.
{quote}
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.

This is the main difference i wanted to explain.

{quote}
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?
{quote}
here two ways, one way is to enhance the existing ConfiguredFailOverProxyProvider implementation
to incorporate the this proposal. (separating the failover logic into separate background
thread)
Other way is to keep the new proposal as separete FailOverProxyProvider implementation.

Which one is preferable for you?

                
> 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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