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--- courses:mapreduce-tutorial:step-29 [2012/01/28 20:17]
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+++ courses:mapreduce-tutorial:step-29 [2012/01/29 17:40]
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-====== MapReduce Tutorial : Running multiple Hadoop jobs ======
+====== MapReduce Tutorial : Custom input formats ======
+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.
+===== FileAsPathInputFormat =====
+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.
+<code java>
+public static 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 =====
+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.
+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]].
+<code java>
+public class WholeFileInputFormat extends FileInputFormat<Text, BytesWritable> {
+  // Helper class, which does the actual work -- reads the (path, content) input pair.
+  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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