Map Reduce 101

Emitter.java

/**
 * Collects results of map phase, persists it to temporary storage
 * results with same key (globally) can be accessed together
 */ 
interface Emitter<M> {
  void emit(String key, M result);
}

Mapper.java

import java.util.Iterator;
/**
 * Runs locally on each file
 */
interface Mapper<M> {
  void map(Iterator<String> lines, Emmiter<M> output);
}

Reducer.java

import java.util.Iterator;
/**
 * Runs on <some> host, one reducer for each group (group is defined by key at map phase), group is assembled from 
 * results from all hosts
 */
interface Reducer<R> {
  R reduce(Iterator<String> lines, Emmiter<M> output);
}