Working of RecordReader in Hadoop

Question

Can anyone explain how the RecordReader actually works? How are the methods nextkeyvalue(), getCurrentkey() and getprogress() work after the program starts executing?

Answer 1

(new API): The default Mapper class has a run method which looks like this:

public void run(Context context) throws IOException, InterruptedException {
    setup(context);
    while (context.nextKeyValue()) {
        map(context.getCurrentKey(), context.getCurrentValue(), context);
    }
    cleanup(context);
}

The Context.nextKeyValue(), Context.getCurrentKey() and Context.getCurrentValue() methods are wrappers for the RecordReader methods. See the source file src/mapred/org/apache/hadoop/mapreduce/MapContext.java.

So this loop executes and calls your Mapper implementation's map(K, V, Context) method.

Specifically, what else would you like to know?