[jira] [Comment Edited] (SPARK-4040) Update spark documentation for local mode and spark-streaming.

["jay vyas (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-4040?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14185848#comment-14185848 ] 

jay vyas edited comment on SPARK-4040 at 10/27/14 10:26 PM:
------------------------------------------------------------

Reopening:  After thinking some more about this, i think the docs could be clarified a little bit , just submitted a pull request to that effect :) + update the jira name.  apologies for all the churn on this.  but hope the minor doc improvement is useful .


was (Author: jayunit100):
Reopening:  After thinking some more about this, i think the docs could be clarified a little bit , just submitted a pull request to that effect :) ! 

> Update spark documentation for local mode and spark-streaming. 
> ---------------------------------------------------------------
>
>                 Key: SPARK-4040
>                 URL: https://issues.apache.org/jira/browse/SPARK-4040
>             Project: Spark
>          Issue Type: Bug
>          Components: Documentation
>            Reporter: jay vyas
>             Fix For: 1.2.0
>
>
> Please note that Im somewhat new to spark streaming's API, and am not a spark expert - so I've done the best to write up and reproduce this "bug".  If its not a bug i hope an expert will help to explain why and promptly close it.  However, it appears it could be a bug after discussing with [~rnowling] who is a spark contributor.
> CC [~rnowling] [~willbenton] 
>  
> It appears that in a DStream context, a call to   {{MappedRDD.count()}} blocks progress and prevents emission of RDDs from a stream.
> {noformat}
>     tweetStream.foreachRDD((rdd,lent)=> {
>       tweetStream.repartition(1)
>       //val count = rdd.count()  DONT DO THIS !
>       checks += 1;
>       if (checks > 20) {
>         ssc.stop()
>       }
>    }
> {noformat} 
> The above code block should inevitably halt, after 20 intervals of RDDs... However, if we *uncomment the call* to {{rdd.count()}}, it turns out that we get an *infinite stream which emits no RDDs*, and thus our program *runs forever* (ssc.stop is unreachable), because *forEach doesnt receive any more entries*.  
> I suspect this is actually because the foreach block never completes, because {{count()}} is winds up calling {{compute}}, which ultimately just reads from the stream.
> I havent put together a minimal reproducer or unit test yet, but I can work on doing so if more info is needed.
> I guess this could be seen as an application bug - but i think spark might be made smarter to throw its hands up when people execute blocking code in a stream processor. 



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