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https://issues.apache.org/jira/browse/MAPREDUCE-2007?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Owen O'Malley resolved MAPREDUCE-2007.
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Resolution: Won't Fix
The framework can't assume that all of the values fit into memory, so it is not
possible to make the API require a List object.
If you are just counting values, you should consider replacing the value with
an integer and implement a combiner that adds the counts together. It will be
much more efficient. Look at the word count example for an example of how to do
this.
If you just need the first N values, just iterate through the values you need
and return from the reduce method. There is no need to exhaust the iterator.
> Is it possible that use ArrayList or other type instead Iterable when use
> reduce(Object, Iterable, Context)?
> --------------------------------------------------------------------------------------------------------------
>
> Key: MAPREDUCE-2007
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-2007
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Affects Versions: 0.20.2
> Reporter: Hui Wen Han
> Fix For: 0.20.2
>
>
> 1) Sometimes We only need get the elements count of the input values of
> Reducer task,
> but we have to iterate all the input values to calculate it.
> 2) Sometimes We only need get a few elements (for example top n,last n ,or
> random ) from the input values of Reducer task,
> if it can use ArrayList or other type instead Iterable when use
> reduce(Object, Iterable, Context),it 's more conveniency.
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