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courses:mapreduce-tutorial:step-29 [2012/01/29 17:44]
straka 		courses:mapreduce-tutorial:step-29 [2012/02/05 18:23]
straka
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-====== MapReduce Tutorial : Custom input formats ======+====== MapReduce Tutorial : Custom sorting and grouping comparators. ======
  
-Every custom format reading keys of type ''K'' and values of type ''V'' must subclass [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/InputFormat.html|InputFormat<K, V>]]. Usually it is easier to subclass [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/lib/input/FileInputFormat.html|FileInputFormat<K, V>]] -- the file listing and splitting is then solved by the ''FileInputFormat'' itself.+====== Sorting comparator ======
  
-===== FileAsPathInputFormat =====+The keys are sorted before processed by a reducer, using a 
 +[[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/io/RawComparator.html|Raw comparator]].
  
-We start by creating ''FileAsPathInputFormat'', which reads any file, splits it and for each split return exactly one input pair (file_path, start-length) with types (''Text'', ''Text''), where ''file_path'' is path to the file and ''start-length'' is a string containing two dash-separated numbers: start offset of the split and length of the split.+====== Grouping comparator ======
  
-When implementing new input format, we must +In a reduce, it is guaranteed that keys are processed in ascending order. Sometimes it would be useful if the //values associated with one key// could also be processed in ascending order.
-  * decide whether the input files are splittable. Usually uncompressed are splittable and compressed are not splittable, with the exception of ''SequenceFile'', which is always splittable. +
-  * implement [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/RecordReader.html|RecordReader<K, V>]]. The ''RecordReader'' is the one doing the real work -- it is given a file split and it reads (key, value) pairs of types (K, V), until there are any.+
  
-Our ''FileAsPathInputFormat'' is simple -- we allow splitting of uncompressed file and the ''RecordReader'' reads exactly one input pair. +----
-<code java> +
-public class FileAsPathInputFormat extends FileInputFormat<Text, Text> { +
-  public static class FileAsPathRecordReader extends RecordReader<Text, Text> { +
-    private Path file; +
-    long start, length; +
-    private Text key, value; +
-     +
-    public void initialize(InputSplit genericSplit, TaskAttemptContext context) throws IOException { +
-      FileSplit split = (FileSplit) genericSplit; +
-      file = split.getPath(); +
-      start = split.getStart(); +
-      length = split.getLength(); +
-      key = null;    +
-      value = null;  +
-    }                +
-    public boolean nextKeyValue() throws IOException { +
-      if (key != null) return false; +
-                     +
-      key = new Text(file.toString()); +
-      value = new Text(String.format("%d-%d", start, length)); +
-                     +
-      return true;   +
-    }                +
-                     +
-    public Text getCurrentKey() { return key; } +
-    public Text getCurrentValue() { return value; } +
-    public float getProgress() { return (key == null) ? 0 : 1; } +
-    public synchronized void close() throws IOException {} +
-  } +
-       +
-  public RecordReader<Text, Text> createRecordReader(InputSplit split, TaskAttemptContext context) { +
-    return new FileAsPathRecordReader(); +
-  }    +
-       +
-  protected boolean isSplittable(JobContext context, Path filename) { +
-    CompressionCodec codec = new CompressionCodecFactory(context.getConfiguration()).getCodec(filename); +
-    return codec == null; +
-  } +
-} +
-</code>+
  
-===== WholeFileInputFormat ===== +<html> 
- +<table style="width:100%"> 
-We start by creating ''WholeFileInputFormat'', which reads any file and return exactly one input pair (input_path, file_content) with types (''Text'', ''BytesWritable''). The format does not allow file splitting -- each file will be processed by exactly one mapper. +<tr> 
- +<td style="text-align:left; width: 33%; "></html>[[step-28|Step 28]]: Custom data types.<html></td> 
-The main functionality lays in ''WholeFileRecordReader'', a subclass of [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/RecordReader.html|RecordReader<Text, BytesWritable]]. +<td style="text-align:center; width: 33%; "></html>[[.|Overview]]<html></td> 
- +<td style="text-align:right; width: 33%; "></html>[[step-30|Step 30]]: Custom input formats.<html></td> 
-<code java> +</tr> 
-public class WholeFileInputFormat extends FileInputFormat<Text, BytesWritable> { +</table> 
-  // Helper class, which does the actual work -- reads the (path, content) input pair. +</html>
-  public static class WholeFileRecordReader extends RecordReader<Text, BytesWritable> { +
-    private Path file; +
-    int length; +
-    private boolean value_read; +
-    private Text key; +
-    private BytesWritable value; +
-    DataInputStream in; +
- +
-    public void initialize(InputSplit genericSplit, TaskAttemptContext context) throws IOException { +
-      FileSplit split = (FileSplit) genericSplit; +
-      file = split.getPath(); +
-      length = (int) split.getLength(); +
-      key = null; +
-      value = null; +
-      value_read = false; +
- +
-      FileSystem fs = file.getFileSystem(context.getConfiguration()); +
-      in = fs.open(split.getPath()); +
- +
-      CompressionCodecFactory compressionCodecs = new CompressionCodecFactory(context.getConfiguration()); +
-      CompressionCodec codec = compressionCodecs.getCodec(file); +
-      if (codec != null) +
-        in = new DataInputStream(codec.createInputStream(in)); +
-    } +
- +
-    public boolean nextKeyValue() throws IOException { +
-      if (value_read) return false; +
- +
-      byte[] data = new byte[length]; +
-      in.readFully(data); +
- +
-      key = new Text(file.toString()); +
-      value = new BytesWritable(data); +
-      value_read = true; +
- +
-      return true; +
-    } +
- +
-    public Text getCurrentKey() { return key; } +
-    public BytesWritable getCurrentValue() { return value; } +
-    public float getProgress() { return value_read ? 0 : 1; } +
-    public synchronized void close() throws IOException { if (in != null) { in.close(); in = null; } } +
-  } +
- +
-  // Use the helper class as a RecordReader in out file format. +
-  public RecordReader<Text, BytesWritable> createRecordReader(InputSplit split, TaskAttemptContext context) { +
-    return new WholeFileRecordReader(); +
-  } +
- +
-  // Do not allow splitting. +
-  protected boolean isSplittable(JobContext context, Path filename) { +
-    return false; +
-  } +
-} +
- +
-</code>+
  

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