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courses:mapreduce-tutorial:step-29 [2012/01/30 00:44] straka |
courses:mapreduce-tutorial:step-29 [2012/02/05 18:57] straka |
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- | ====== MapReduce Tutorial : Custom | + | ====== MapReduce Tutorial : Custom |
- | Every custom format reading keys of type '' | + | ====== Custom sorting comparator ====== |
- | ===== FileAsPathInputFormat ===== | + | The keys are sorted before processed by a reducer, using a |
+ | [[http:// | ||
- | We start by creating '' | + | <code java> |
+ | public static class IntPair implements WritableComparable< | ||
+ | private int first = 0; | ||
+ | private int second = 0; | ||
- | When implementing new input format, we must | + | public void set(int left, int right) { first = left; second = right; } |
- | | + | |
- | | + | |
- | Our '' | + | |
- | <code java> | + | |
- | public class FileAsPathInputFormat extends FileInputFormat< | + | |
- | // Helper class, which does the actual work -- produce the (path, offset-length) input pair. | + | |
- | public static class FileAsPathRecordReader extends RecordReader< | + | |
- | private Path file; | + | |
- | long start, length; | + | |
- | private Text key, value; | + | |
- | + | ||
- | | + | |
- | | + | |
- | 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(" | + | |
- | + | ||
- | 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 void write(DataOutput out) throws IOException { |
- | // Use the helper class as a RecordReader in out file format. | + | out.writeInt(first); |
- | public RecordReader< | + | |
- | | + | } |
- | } | + | |
- | + | | |
- | | + | |
- | protected boolean isSplittable(JobContext context, Path filename) { | + | |
- | | + | |
- | return | + | |
} | } | ||
} | } | ||
</ | </ | ||
- | ===== WholeFileInputFormat ===== | + | If we would like in a Hadoop job to sort the '' |
- | + | ||
- | Next we create | + | |
<code java> | <code java> | ||
- | public class WholeFileInputFormat extends FileInputFormat<Text, BytesWritable> { | + | public |
- | | + | |
- | public static class WholeFileRecordReader extends RecordReader<Text, BytesWritable> { | + | public static class FirstOnlyComparator implements RawComparator<IntPair> { |
- | | + | |
- | | + | |
- | private Text key; | + | int first2 |
- | private BytesWritable value; | + | |
- | DataInputStream in; | + | |
- | + | ||
- | public void initialize(InputSplit genericSplit, TaskAttemptContext context) throws IOException | + | |
- | | + | |
- | file = split.getPath(); | + | |
- | | + | |
- | key = null; | + | |
- | value = null; | + | |
- | + | ||
- | FileSystem fs = file.getFileSystem(context.getConfiguration()); | + | |
- | | + | |
- | + | ||
- | CompressionCodecFactory compressionCodecs | + | |
- | CompressionCodec codec = compressionCodecs.getCodec(file); | + | |
- | if (codec != null) | + | |
- | in = new DataInputStream(codec.createInputStream(in)); | + | |
} | } | ||
+ | public int compare(IntPair x, IntPair y) { | ||
+ | return x.getFirst() < y.getFirst() ? -1 : x.getFirst() == y.getFirst() ? 0 : 1; | ||
+ | } | ||
+ | } | ||
+ | } | ||
- | public boolean nextKeyValue() throws IOException { | + | ... |
- | if (key != null) return false; | + | |
- | byte[] data = new byte[length]; | + | job.setSortComparatorClass(IntPair.FirstOnlyComparator.class); |
- | in.readFully(data); | + | </ |
+ | Notice we used helper function '' | ||
- | key = new Text(file.toString()); | + | ====== Grouping comparator ====== |
- | value = new BytesWritable(data); | + | |
- | return true; | + | 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. |
- | } | + | |
- | public Text getCurrentKey() { return | + | That is possible only to some degree. The (key, value) |
- | public BytesWritable getCurrentValue() { return | + | |
- | public float getProgress() { return | + | |
- | 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< | + | |
- | return new WholeFileRecordReader(); | + | |
- | } | + | |
- | | + | < |
- | | + | <table style=" |
- | return false; | + | < |
- | } | + | <td style=" |
- | } | + | <td style=" |
- | </code> | + | <td style=" |
+ | </tr> | ||
+ | </ | ||
+ | </html> | ||
- | ===== Exercise: ParagraphTextInputFormat ===== |