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From "Kiyan Ahmadizadeh (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CRUNCH-58) Implement PObject in Crunch/Scrunch
Date Thu, 06 Sep 2012 19:47:07 GMT

    [ https://issues.apache.org/jira/browse/CRUNCH-58?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13449983#comment-13449983
] 

Kiyan Ahmadizadeh commented on CRUNCH-58:
-----------------------------------------

Discussion of implementing PObjects started on the CRUNCH-57 ticket.  Josh gave this suggestion
for an implementation: 

--
Kiyan, do you have an opinion on how you want to go about this one? Do you want to take on
defining PObject (which in my mind, could just be a simple wrapper that materialized a PCollection
and then implemented some abstract function that did a computation on the materialized Iterable)
and incorporate it here?
--

Josh, I think PObject should be a wrapper around PCollection, but the underlying PCollection
should contain only one element (or be treated as such).  In other words, it should wrap the
result of a distributed computation that reduced/combined a source PCollection into a target
PCollection of 1 element.  Then PObject could have a getValue method that materialized the
underlying PCollection and returned the singleton element found within.  I'm not sure if we
want to strongly enforce that the underlying PCollection for a PObject contains one element
by throwing an exception, or if we simply ignore any element but the first in the underlying
PCollection. 

Your suggestion for "some abstract function that did a computation on the materialized Iterable"
doesn't make sense to me, since in my mind a PObject should only care about the first element
in its underlying PCollection.  Could you clarify?  

                
> Implement PObject in Crunch/Scrunch
> -----------------------------------
>
>                 Key: CRUNCH-58
>                 URL: https://issues.apache.org/jira/browse/CRUNCH-58
>             Project: Crunch
>          Issue Type: New Feature
>    Affects Versions: 0.3.0
>            Reporter: Kiyan Ahmadizadeh
>            Assignee: Kiyan Ahmadizadeh
>
> FlumeJava has the concept of a PObject<T>, a container for a singleton of type
T.  It is meant represent the result of a distributed computation that yields a singleton
value (for example max, min, and length methods on PCollection<T>).  Generally speaking,
the result of any computation that combines/reduces a PCollection into a singleton value could
be represented by a PObject.  
> Like PCollection, a PObject defers distributed computation until its value is actually
used.  

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