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Date: Thu, 28 Jun 2007 11:59:04 -0700 (PDT)
From: "Doug Cutting (JIRA)" <jira@apache.org>
To: hadoop-dev@lucene.apache.org
Subject: [jira] Commented: (HADOOP-475) The value iterator to reduce
 function should be clonable
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    [ https://issues.apache.org/jira/browse/HADOOP-475?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12508905 ] 

Doug Cutting commented on HADOOP-475:
-------------------------------------

I think the original intention was to just make the iterator cloneable, so that a reducer could iterate through the values more than once.  It might first, e.g., count the total number of values, then, having this count, filter them or somesuch.  This should work even when the values would not all fit in memory.  Whether this feature is still interesting to folks I cannot say...

> The value iterator to reduce function should be clonable
> --------------------------------------------------------
>
>                 Key: HADOOP-475
>                 URL: https://issues.apache.org/jira/browse/HADOOP-475
>             Project: Hadoop
>          Issue Type: New Feature
>          Components: mapred
>            Reporter: Runping Qi
>            Assignee: Owen O'Malley
>
> In the current framework, when the user implements the reduce method of Reducer class, 
> the user can only iterate through the value iterator once. 
> This makes it hard for the user to perform join-like operations with in the reduce method. 
> To address problem, one approach is to make the input value iterator clonable. Then the user can iterate the values in different ways.
> If the iterator can be reset, then the user can perform nested iterations over the data, thus 
> carry out join-likeoperations.
> The user code in reduce method would be something like:
>                   iterator1 = values.clone();
>                   iterator2 = values.clone();
>                  while (iterator1.hasNext()) {
>                       val1 = iterator1.next();
>                       iterator2.reset();
>                       while (iterator2.hasNext()) {
>                            val2 = iterator.next();
>                            do something vased on val1 and val2
>                            .......................
>                       }
>                  }
> One possible optimization is that if the values are sorted based on a secondary key, 
> the reset function can take a secondary key as an argument and reset the iterator to the begining
> position of the secondary key. It will be very helpful if there is a utility that returns a list of iterators,
> one per secondary key value, from the given iterator:
>                           TreeMap getIteratorsBasedOnSecondaryKey(iterator);
> Each entry in the returned map object is a pair of <secondary key, iterator for the values with the same secondary key>.
>   

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