Updating a task's status message can be done using the setStatus() method of the job's Context class.

context.setMessage("user custom message");

The source code for this chapter provides an example of using a custom task status message to display the number of rows being processed per second by the task.

public static class StatusMap extends Mapper<LongWritable, Text, LongWritable, Text> {

      private int rowCount = 0;
      private long startTime = 0;

      public void map(LongWritable key, Text value, Context context) throws IOException, InterruptedException{

         //Display rows per second every 100,000 rows
         rowCount++;
         if(startTime == 0 || rowCount % 100000 == 0)
         {
            if(startTime > 0)
{
               long estimatedTime = System.nanoTime() - startTime;
               context.setStatus("Processing: " + (double)rowCount / ((double)estimatedTime/1000000000.0) + " rows/second");
               rowCount = 0;
}

            startTime = System.nanoTime();
}

         context.write(key, value);
}
}

Two private class variables are declared: rowCount for keeping track of the number of rows that are processed and startTime for keeping track of the time when processing started. Once the map function has processed 100,000 lines, the task status is updated with the number of rows per second that are being processed.

context.setStatus("Processing: " + (double)rowCount / ((double)estimatedTime/1000000000.0) + " rows/second"); 

After the message has been updated, the rowCount and startTime variables are reset and the process starts over again. The status is stored locally in the memory of the current process. It is then sent to the Task Tracker. The next time the Task Tracker pings, the Job Tracker is also sent the updated status message. Once the Job Tracker receives the status message, this information is made available to the UI.