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https://issues.apache.org/jira/browse/HADOOP-449?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12566819#action_12566819
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Chris Douglas commented on HADOOP-449:
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bq. I think people will understand that nesting FilterInputFormat cannot be
done with the current API
True. The case I had in mind wasn't so much nesting as it was simultaneous
instantiation. For example, the classes in mapred.join wouldn't hesitate to
accept multiple FilterInputFormats for multiple datasources, but- as you point
out- the API is such that one would quickly realize that chained filters aren't
possible without some effort. I was hoping that these classes could be
integrated into the aforementioned framework and I'm confident they can be.
On that note, the MD5PercentFilter guards access to the MessageDigest within
the instance, but multiple instances could corrupt it. It would probably be
better if it were not static.
Also, since a filter may discard the vast majority of the input, is it
necessary to update the reporter to avoid a timeout? A call to next may churn
through data for some time, and I'm uncertain whether one can expect the base
InputFormat to keep the task alive. I'd expect it to be fine for the majority
of cases, but if you felt like being paranoid it's not insane.
bq. If you see a better solution to pass the Writables to the tasks, I will be
very glad to adopt it. Or should we add setWritable() getWritable() to the
Configuration?
I don't, sorry. :) I remember the JIRA you mention, the rejection of
get/setWritable, and the reasoning probably remains sound. Other than the
solutions you propose, the only other way I can think of would be to have an
auxiliary InputFormat/input dir that slurps a set of keys (no splits!) into an
in-memory Set and assume that the OOM exceptions are a strong hint to the user.
Gross.
Still, you could probably still restrict these to Text, as long as the user is
aware of SequenceFileAsTextInputFormat and related options. Automatically
converting to String could produce some weird results if one isn't aware of how
the filter is effected. Forcing someone to figure out how to get their
WritableComparables to Text is ample warning, I think.
> Generalize the SequenceFileInputFilter to apply to any InputFormat
> ------------------------------------------------------------------
>
> Key: HADOOP-449
> URL: https://issues.apache.org/jira/browse/HADOOP-449
> Project: Hadoop Core
> Issue Type: Improvement
> Components: mapred
> Affects Versions: 0.17.0
> Reporter: Owen O'Malley
> Assignee: Enis Soztutar
> Fix For: 0.17.0
>
> Attachments: filterinputformat_v1.patch
>
>
> I'd like to generalize the SequenceFileInputFormat that was introduced in
> HADOOP-412 so that it can be applied to any InputFormat. To do this, I
> propose:
> interface WritableFilter {
> boolean accept(Writable item);
> }
> class FilterInputFormat implements InputFormat {
> ...
> }
> FilterInputFormat would look in the JobConf for:
> mapred.input.filter.source = the underlying input format
> mapred.input.filter.filters = a list of class names that implement
> WritableFilter
> The FilterInputFormat will work like the current SequenceFilter, but use an
> internal RecordReader rather than the SequenceFile. This will require adding
> a next(key) and getCurrentValue(value) to the RecordReader interface, but
> that will be addressed in a different issue.
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