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MapReduce, mail # user - A couple of Questions on InputFormat


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Steve Lewis 2013-09-21, 19:30
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Harsh J 2013-09-23, 11:30
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Re: A couple of Questions on InputFormat
Steve Lewis 2013-09-23, 15:34
Thank you for your thorough answer
The last question is essentially this - while I can write a custom input
format to handle things like hyphens I
could do almost the same thing in the mapper by saving any hyphenated words
from the last line (ignoring hyphenated words that
cross a split boundary) as long as  LineRecordReader guarantees that each
line in the split is sent to the same mapper in the order read.
This seems to be the case - right?
On Mon, Sep 23, 2013 at 4:30 AM, Harsh J <[EMAIL PROTECTED]> wrote:

> Hi,
>
> (I'm assuming 1.0~ MR here)
>
> On Sun, Sep 22, 2013 at 1:00 AM, Steve Lewis <[EMAIL PROTECTED]>
> wrote:
> > Classes implementing InputFormat implement
> >  public List<InputSplit> getSplits(JobContext job) which a List if
> > InputSplits. for FileInputFormat the Splits have Path.start and End
> >
> > 1) When is this method called and on which JVM on Which Machine and is it
> > called only once?
>
> Called only at a client, i.e. your "hadoop jar" JVM. Called only once.
>
> > 2) Do the number of Map task correspond to the number of splits returned
> by
> > getSplits?
>
> Yes, number of split objects == number of mappers.
>
> > 3) InputFormat implements a method
> >  RecordReader<K,V> createRecordReader(InputSplit split,TaskAttemptContext
> > context ). Is this  executed within the JVM of the Mapper on the slave
> > machine and does the RecordReader run within that JVM
>
> RecordReaders are not created on the client side JVM. RecordReaders
> are created on the Map task JVMs, and run inside it.
>
> > 4) The default RecordReaders read a file from the start position to the
> end
> > position emitting values in the order read. With such a reader, assume
> it is
> > reading lines of text, is it reasonable to assume that the values the
> mapper
> > received are in the same order they were found in a file? Would it, for
> > example, be possible for WordCount to see a word that was hyphen-
> > ated at the end of one line and append the first word of the next line it
> > sees (ignoring the case where the word is at the end of a split)
>
> If you speak of the LineRecordReader, each map() will simply read a
> line, i.e. until \n. It is not language-aware to understand meaning of
> hyphens, etc..
>
> You can implement a custom reader to do this however - there should be
> no problems so long as your logic covers the case of not having any
> duplicate reads across multiple maps.
>
> --
> Harsh J
>

--
Steven M. Lewis PhD
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Skype lordjoe_com
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Harsh J 2013-09-24, 03:13