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courses:mapreduce-tutorial:step-2 [2012/01/24 08:56]
straka 		courses:mapreduce-tutorial:step-2 [2012/01/29 16:03] (current)
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 ====== MapReduce tutorial : Input and output format, testing data. ====== ====== MapReduce tutorial : Input and output format, testing data. ======
 +
 +The MapReduce framework is frequently using (key, value) pairs. These pairs can be read from a file and written to a file and there are several formats available.
 +
 +===== Input formats =====
 +  * ''TextInputFormat'' -- values are lines of UTF8 plain text files, keys are the positions of their first character in the file.
 +  * ''KeyValueTextInputFormat'' -- every line of UTF8 plain text file is split using first TAB character, forming key and value. If there is no TAB character, the value is empty.
 +  * ''SequenceFileInputFormat'' -- binary format.
 +The input format can be compressed and will be decompressed transparently by the MR framework.
 +
 +===== Output formats =====
 +  * ''TextOutputFormat'' -- (key, value) pair is printed using UTF8 on one line separated by a TAB character. If key or value is empty, no TAB character is used.
 +  * ''SequenceFileOutputFormat'' -- binary format.
 +The output format can be compressed on demand.
 +
 +===== Input data =====
 +Testing data are available in several formats and sizes:
 +  * ''/home/straka/wiki/cs-seq'' -- compressed SequenceFile of Czech Wikipedia, 85MB.
 +  * ''/home/straka/wiki/cs-seq-medium'' -- compressed SequenceFile of Czech Wikipedia, 8MB.
 +  * ''/home/straka/wiki/cs-seq-small'' -- compressed SequenceFile of Czech Wikipedia, 35kB.
 +  * ''/home/straka/wiki/cs-text'' -- uncompressed plain text files of Czech Wikipedia in the ''KeyValueTextInputFormat'', 200MB.
 +  * ''/home/straka/wiki/cs-text-medium'' -- uncompressed plain text files of Czech Wikipedia in the ''KeyValueTextInputFormat'', 16MB.
 +  * ''/home/straka/wiki/cs-text-small'' -- uncompressed plain text files of Czech Wikipedia in the ''KeyValueTextInputFormat'', 70kB.
 +  * ''/home/straka/wiki/en-seq'' -- compressed SequenceFile of English Wikipedia, 1.9GB.
 +It is recommended to use the text format in the tutorial, so that both input and output files are readable.
 +
 +----
 +
 +<html>
 +<table style="width:100%">
 +<tr>
 +<td style="text-align:left; width: 33%; "></html>[[step-1|Step 1]]: Setting the environment.<html></td>
 +<td style="text-align:center; width: 33%; "></html>[[.|Overview]]<html></td>
 +<td style="text-align:right; width: 33%; "></html>[[step-3|Step 3]]: Basic mapper.<html></td>
 +</tr>
 +</table>
 +</html>
  

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