About GenericOptionsParser getRemainingArgs method - mapreduce

Question

package com.ibm.dw61;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.util.GenericOptionsParser;
import com.ibm.dw61.MaxTempReducer;
import com.ibm.dw61.MaxTempMapper;
public class MaxMonthlyTemp {
public static void main(String[] args) throws Exception {
Configuration conf = new Configuration();
String[] programArgs = new GenericOptionsParser(conf, args)
.getRemainingArgs();
if (programArgs.length != 2) {
System.err.println("Usage: MaxTemp <in> <out>");
System.exit(2);
}
Job job = new Job(conf, "Monthly Max Temp");
job.setJarByClass(MaxMonthlyTemp.class);
job.setMapperClass(MaxTempMapper.class);
job.setCombinerClass(MaxTempReducer.class);
job.setReducerClass(MaxTempReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(IntWritable.class);
FileInputFormat.addInputPath(job, new Path(programArgs[0]));
FileOutputFormat.setOutputPath(job, new Path(programArgs[1]));
// Submit the job and wait for it to finish.
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}
Questions :
1) This is a map-reduce code to extract max temperature for each month. The coder is trying to get non-generic options using the getRemainingArgs method. But the next line says if the number of non-generic options is not 2, that means there is an error and the program will immediately abort. I couldn’t figure out what is the coder’s logic here. Anyone kind enough to explain?
2) In another example Wordcount, the coder didn’t perform this step of getting non-generic options. So under what circumstances do we have to perform this step and testing whether the non-generic options numbers 2?

Answer 1

as you can see in the Hadoop API documentation, purpose of the method getRemainingArgs is to extract application-specific arguments , those that are not related to Hadoop framework. in this code, you should specify two arguments, first your input and then output, as you can see in the Usage