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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-1176) Contribution: FixedLengthInputFormat and FixedLengthRecordReader
Date Thu, 19 Nov 2009 19:37:39 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1176?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12780225#action_12780225

Todd Lipcon commented on MAPREDUCE-1176:


Just had a chance to look at your patch in more detail now that the initial formatting issues
have been straightened out:

- Please provide a static setter for the mapreduce.input.fixedlengthinputformat.record.length
configuration property, rather than having users have to specify this property manually. An
example of this is in o.a.h.mapreduce.lib.input.NLineInputFormat.setNumLinesPerSplit. Also
update the javadoc and tests to refer to this new method

- The following code makes me nervous:
+       long splitSize = 
+         ((long)(Math.floor((double)defaultSize / 
+                 (double)recordLength))) * recordLength;
Why can't you just keep defaultSize and recordLength as longs? Division of longs will give
you floor-like behavior and you won't have to worry about floating point inaccuracies.

- In isSplitable, you catch the exception generated by getRecordLength and turn off splitting.
If there is no record length specified doesn't that mean the input format won't work at all?
- FixedLengthRecordReader: "This record reader does not support compressed files." Is this
true? Or does it just not support *splitting* compressed files? I see that you've explicitly
disallowed it, but I don't understand this decision.
- Throughout, you've still got 4-space indentation in the method bodies. Indentation should
be by 2.
- In FixedLengthRecordReader, you hard code a 64KB buffer. Why's this? You should let the
filesystem use its default.
- In your read loop, you're not accounting for the case of read returning 0 or -1, which I
believe can happen at EOF, right? Consider using o.a.h.io.IOUtils.readFully() to replace this

As a general note, I'm not sure I agree with the design here. Rather than forcing the split
to lie on record boundaries, I think it would be simpler to simply let FileInputFormat compute
its own splits, and then when you first open the record reader, skip forward to the next record
boundary and begin reading from there. Then for the last record of the file, "over read" your
split into the beginning of the next one. This is the strategy that other input formats take,
and should be compatible with the splittable compression codecs (see TextInputFormat for example).

I don't want to harp too much on the compression thing, in my experience the sorts of datasets
that have these fixed-length records are very highly compressible - lots and lots of numeric

> Contribution: FixedLengthInputFormat and FixedLengthRecordReader
> ----------------------------------------------------------------
>                 Key: MAPREDUCE-1176
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1176
>             Project: Hadoop Map/Reduce
>          Issue Type: New Feature
>    Affects Versions: 0.20.1, 0.20.2
>         Environment: Any
>            Reporter: BitsOfInfo
>            Priority: Minor
>         Attachments: MAPREDUCE-1176-v1.patch, MAPREDUCE-1176-v2.patch
> Hello,
> I would like to contribute the following two classes for incorporation into the mapreduce.lib.input
package. These two classes can be used when you need to read data from files containing fixed
length (fixed width) records. Such files have no CR/LF (or any combination thereof), no delimiters
etc, but each record is a fixed length, and extra data is padded with spaces. The data is
one gigantic line within a file.
> Provided are two classes first is the FixedLengthInputFormat and its corresponding FixedLengthRecordReader.
When creating a job that specifies this input format, the job must have the "mapreduce.input.fixedlengthinputformat.record.length"
property set as follows
> myJobConf.setInt("mapreduce.input.fixedlengthinputformat.record.length",[myFixedRecordLength]);
> OR
> myJobConf.setInt(FixedLengthInputFormat.FIXED_RECORD_LENGTH, [myFixedRecordLength]);
> This input format overrides computeSplitSize() in order to ensure that InputSplits do
not contain any partial records since with fixed records there is no way to determine where
a record begins if that were to occur. Each InputSplit passed to the FixedLengthRecordReader
will start at the beginning of a record, and the last byte in the InputSplit will be the last
byte of a record. The override of computeSplitSize() delegates to FileInputFormat's compute
method, and then adjusts the returned split size by doing the following: (Math.floor(fileInputFormatsComputedSplitSize
/ fixedRecordLength) * fixedRecordLength)
> This suite of fixed length input format classes, does not support compressed files. 

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