The following steps show how to calculate and plot a Histogram:

As you can see from the figure, most of the access to NASA is at night, whereas there noontime. Also, two peaks roughly follow the tea times.

You can find the source for the recipe from src/chapter6/WeblogTimeOfDayHistogramCreator.java. As explained in the first recipe of this chapter, we will use regular expressions to parse the log file and extract the access time from the log files.

The following code segment shows the mapper function:

public void map(Object key, Text value, 
  Context context) throws IOException, InterruptedException
{
  Matcher matcher = httplogPattern.matcher(value.toString());
  if (matcher.matches())
  {
    String timeAsStr = matcher.group(2);
    Date time = dateFormatter.parse(timeAsStr);
    Calendar calendar = GregorianCalendar.getInstance();
    calendar.setTime(time);
    int hours = calendar.get(Calendar.HOUR_OF_DAY);
    context.write(new IntWritable(hours), one); 
  }
}

Map task receives each line in the log file as a different key-value pair. It parses the lines using regular expressions and extracts the access time for each web page access. Then, the mapper function extracts the hour of the day from the access time and emits the hour of the day and one as output of the mapper function.

Then, Hadoop collects all key-value pairs, sorts them, and then invokes the reducer once for each key. Each reducer walks through the values and calculates the count of page accesses for each hour.

public void reduce(IntWritable key,
  Iterable<IntWritable> values,
  Context context) throws IOException, InterruptedException
  {
    int sum = 0;
    for (IntWritableval : values) 
    {
      sum += val.get();
    }
    context.write(key, new IntWritable(sum));
  }

The main() method of the job looks similar to the WordCount example as described in the earlier recipe.