-Re: A couple of Questions on InputFormat
Harsh J 2013-09-24, 03:13
Yes, that is right.
On Mon, Sep 23, 2013 at 9:04 PM, Steve Lewis <[EMAIL PROTECTED]> wrote:
> 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:
>> (I'm assuming 1.0~ MR here)
>> On Sun, Sep 22, 2013 at 1:00 AM, Steve Lewis <[EMAIL PROTECTED]>
>> > 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
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