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Hadoop: The Definitive Guide by Tom White

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Chapter 7. MapReduce Types and Formats

MapReduce has a simple model of data processing: inputs and outputs for the map and reduce functions are key-value pairs. This chapter looks at the MapReduce model in detail, and, in particular, how data in various formats, from simple text to structured binary objects, can be used with this model.

MapReduce Types

The map and reduce functions in Hadoop MapReduce have the following general form:

map: (K1, V1) → list(K2, V2)
reduce: (K2, list(V2)) → list(K3, V3)

In general, the map input key and value types (K1 and V1) are different from the map output types (K2 and V2). However, the reduce input must have the same types as the map output, although the reduce output types may be different again (K3 and V3). The Java interfaces mirror this form:

public interface Mapper<K1, V1, K2, V2> extends JobConfigurable, Closeable {
  
  void map(K1 key, V1 value, OutputCollector<K2, V2> output, Reporter reporter)
    throws IOException;
}

public interface Reducer<K2, V2, K3, V3> extends JobConfigurable, Closeable {
  
  void reduce(K2 key, Iterator<V2> values,
    OutputCollector<K3, V3> output, Reporter reporter) throws IOException;
}    

Recall that the OutputCollector is purely for emitting key-value pairs (and is hence parameterized with their types), while the Reporter is for updating counters and status. (In the new MapReduce API in release 0.20.0 and later, these two functions are combined in a single context object.)

If a combine function is used then it is the same form as ...

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