Question

Is it possible to have multiple inputs with multiple different mappers in Hadoop MapReduce? Each mapper class work on a different set of inputs, but they would all emit key-value pairs consumed by the same reducer. Note that I’m not talking about chaining mappers here, I’m talking about running different mappers in parallel, not sequentially.

Answer 1

This is called a join.

You want to use the mappers and reducers in the mapred.* packages (older, but still supported). The newer packages (mapreduce.*) only allow for one mapper input. With the mapred packages, you use the MultipleInputs class to define the join:

MultipleInputs.addInputPath(jobConf, 
                     new Path(countsSource),       
                     SequenceFileInputFormat.class, 
                     CountMapper.class);
MultipleInputs.addInputPath(jobConf, 
                     new Path(dictionarySource), 
                     SomeOtherInputFormat.class, 
                     TranslateMapper.class);

jobConf.setJarByClass(ReportJob.class);
jobConf.setReducerClass(WriteTextReducer.class);

jobConf.setMapOutputKeyClass(Text.class);
jobConf.setMapOutputValueClass(WordInfo.class);

jobConf.setOutputKeyClass(Text.class);
jobConf.setOutputValueClass(Text.class);