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From "Jan Lukavsky (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-5757) TableInputFormat should handle as many errors as possible
Date Wed, 09 May 2012 07:58:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-5757?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13271197#comment-13271197
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Jan Lukavsky commented on HBASE-5757:
-------------------------------------

Hi Jon,

I'm not sure, but IMO the purpose of DoNotRetryIOException is to instruct the HTable client
not to retry the request. In TableInputFormat we are working on higher level, so retrying
is OK. DNRIOEx is to distinguish exceptions that might be caused by region reassignment for
instance, and that might disappear if the request is resent (and possibly dropping the cached
region location and quering .META. again). UnknonwnScannerException on the other hand will
not 'disapper' if the *same* request is sent by HTable client. But in the InputFormat we can
restart the scanner, and so we will not send the same request, hence it can succeed.

Retrying the request just once and then giving up is to avoid infinite cycles, and mostly
it suffices to retry just once, because a typical cause of the UnknownScannerException or
LeaseException is too slow Mapper (there could be other like scanning for too sparse column,
but this will not be solved by this issue :)). There is possibility to lower scanner caching,
but this might be inefficient (eg. when the 99.99% of time the caching is just OK, and then
there exists some strange records, that take the Mapper longer to process). Lowering the caching
globally just because of these few records doesn't sound like the 'correct' solution.


                
> TableInputFormat should handle as many errors as possible
> ---------------------------------------------------------
>
>                 Key: HBASE-5757
>                 URL: https://issues.apache.org/jira/browse/HBASE-5757
>             Project: HBase
>          Issue Type: Bug
>          Components: mapred, mapreduce
>    Affects Versions: 0.90.6
>            Reporter: Jan Lukavsky
>         Attachments: HBASE-5757.patch
>
>
> Prior to HBASE-4196 there was different handling of IOExceptions thrown from scanner
in mapred and mapreduce API. The patch to HBASE-4196 unified this handling so that if exception
is caught a reconnect is attempted (without bothering the mapred client). After that, HBASE-4269
changed this behavior back, but in both mapred and mapreduce APIs. The question is, is there
any reason not to handle all errors that the input format can handle? In other words, why
not try to reissue the request after *any* IOException? I see the following disadvantages
of current approach
>  * the client may see exceptions like LeaseException and ScannerTimeoutException if he
fails to process all fetched data in timeout
>  * to avoid ScannerTimeoutException the client must raise hbase.regionserver.lease.period
>  * timeouts for tasks is aready configured in mapred.task.timeout, so this seems to me
a bit redundant, because typically one needs to update both these parameters
>  * I don't see any possibility to get rid of LeaseException (this is configured on server
side)
> I think all of these issues would be gone, if the DoNotRetryIOException would not be
rethrown. -On the other hand, handling errors in InputFormat has disadvantage, that it may
hide from the user some inefficiency. Eg. if I have very big scanner.caching, and I manage
to process only a few rows in timeout, I will end up with single row being fetched many times
(and will not be explicitly notified about this). Could we solve this problem by adding some
counter to the InputFormat?-

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