本篇介绍MapReduce的一些高级特性，如计数器、数据集的排序和连接。计数器是一种收集作业统计信息的有效手段，排序是MapReduce的核心技术，MapReduce也能够执行大型数据集间的“”连接（join）操作。

计数器

计数器是一种收集作业统计信息的有效手段，用于质量控制或应用级统计。计数器还可用于辅助诊断系统故障。对于大型分布式系统来说，获取计数器比分析日志文件容易的多。

示例一：气温缺失及不规则数据计数器

import java.io.IOException;
import java.util.Iterator;

import org.apache.hadoop.conf.Configured;
import org.apache.hadoop.io.*;
import org.apache.hadoop.mapred.JobClient;
import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.mapred.MapReduceBase;
import org.apache.hadoop.mapred.Mapper;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reducer;
import org.apache.hadoop.mapred.Reporter;
import org.apache.hadoop.util.Tool;
import org.apache.hadoop.util.ToolRunner;

//统计最高气温的作业，也统计气温值缺少的记录，不规范的记录
public class MaxTemperatureWithCounters extends Configured implements Tool {

	enum Temperature {
		MiSSING, MALFORMED
	}

	static class MaxTemeratureMapperWithCounters extends MapReduceBase implements
			Mapper<LongWritable, Text, Text, IntWritable> {

		private NcdcRecordParser parser = new NcdcRecordParser();

		@Override
		public void map(LongWritable key, Text value,
				OutputCollector<Text, IntWritable> output, Reporter reporter)
				throws IOException {
			parser.parse(value);
			if (parser.isValidTemperature()) {
				int airTemperature = parser.getAirTemperature();
				output.collect(new Text(parser.getYear()), new IntWritable(
						airTemperature));
			} else if (parser.isMa1formedTemperature()) {
				reporter.incrCounter(Temperature.MALFORMED, 1);
			} else if (parser.IsMissingTemperature()) {
				reporter.incrCounter(Temperature.MALFORMED, 1);
			}

		}

	}

	static class MaxTemperatureReduceWithCounters extends MapReduceBase implements
			Reducer<Text, IntWritable, Text, IntWritable> {
		public void reduce(Text key, Iterator<IntWritable> values,
				OutputCollector<Text, IntWritable> output, Reporter reporter)
				throws IOException {
			int maxValue = http://www.mamicode.com/Integer.MIN_VALUE;>
示例二：统计气温信息缺失记录所占比例

import org.apache.hadoop.conf.*;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.util.*;
//统计气温缺失记录所占比例

public class MissingTemperatureFields extends Configured implements Tool {

	@Override
	public int run(String[] args) throws Exception {
		String jobID = args[0];
		JobClient jobClient = new JobClient(new JobConf(getConf()));
		RunningJob job = jobClient.getJob(JobID.forName(jobID));
		if (job == null) {
			System.err.printf("No job with ID %s found.\n", jobID);
			return -1;
		}
		if (!job.isComplete()) {
			System.err.printf("Job %s is not complete.\n", jobID);
			return -1;
		}
		Counters counters = job.getCounters();
		long missing = counters
				.getCounter(MaxTemperatureWithCounters.Temperature.MiSSING);
		long total = counters.findCounter(
				"org.apache.hadoop.mapred.Task$Counter", "MAP_INPUT_RECORDS")
				.getCounter();
		System.out.printf("Records with missing temperature fields:%.2f%%\n",
				100.0 * missing / total);
		return 0;
	}

	public static void main(String[] args) throws Exception {
		int exitCode = ToolRunner.run(new MissingTemperatureFields(), args);
		System.exit(exitCode);
	}
}

hadoop jar xx.jar MissingTemperatureFields job_1400072670556_0001

排序

排序是MapReduce的核心技术。尽管应用本身可能并不需要对数据排序，但仍可能使用MapReduce的排序功能来组织数据。下面将讨论几种不同的数据集排序方法，以及如何控制MapReduce的排序。

实例一、数据准备：将天气数据转成顺序文件格式

import java.io.IOException;

import org.apache.hadoop.conf.*;
import org.apache.hadoop.io.*;
import org.apache.hadoop.io.SequenceFile.CompressionType;
import org.apache.hadoop.io.compress.GzipCodec;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.util.*;

public class SortDataPreprocessor extends Configured implements Tool {
	static class CleanerMapper extends MapReduceBase implements
			Mapper<LongWritable, Text, IntWritable, Text> {

		private NcdcRecordParser parser = new NcdcRecordParser();

		@Override
		public void map(LongWritable key, Text value,
				OutputCollector<IntWritable, Text> output, Reporter reporter)
				throws IOException {
			parser.parse(value);
			if (parser.isValidTemperature()) {
				output.collect(new IntWritable(parser.getAirTemperature()),
						value);
			}
		}
	}

	@Override
	public int run(String[] args) throws Exception {
		args = new String[] { "/test/input/t", "/test/input/seq" };
		JobConf conf = JobBuilder.parseInputAndOutput(this, getConf(), args);
		if (conf == null) {
			return -1;
		}
		conf.setMapperClass(CleanerMapper.class);
		conf.setOutputKeyClass(IntWritable.class);
		conf.setOutputValueClass(Text.class);
		conf.setNumReduceTasks(0);
		conf.setOutputFormat(SequenceFileOutputFormat.class);
		SequenceFileOutputFormat.setCompressOutput(conf, true);
		SequenceFileOutputFormat
				.setOutputCompressorClass(conf, GzipCodec.class);
		SequenceFileOutputFormat.setOutputCompressionType(conf,
				CompressionType.BLOCK);
		JobClient.runJob(conf);
		return 0;
	}

	public static void main(String[] args) throws Exception {
		int exitCode = ToolRunner.run(new SortDataPreprocessor(), args);
		System.exit(exitCode);
	}
}

示例二、部分排序

import org.apache.hadoop.conf.*;
import org.apache.hadoop.io.*;
import org.apache.hadoop.io.SequenceFile.CompressionType;
import org.apache.hadoop.io.compress.GzipCodec;
import org.apache.hadoop.mapred.*;
import org.apache.hadoop.util.*;

public class SortByTemperatureUsingHashPartitioner extends Configured implements
		Tool {

	@Override
	public int run(String[] args) throws Exception {
		args = new String[] { "/test/input/seq", "/test/output/t" };
		JobConf conf = JobBuilder.parseInputAndOutput(this, getConf(), args);
		if (conf == null) {
			return -1;
		}
		conf.setInputFormat(SequenceFileInputFormat.class);
		conf.setOutputKeyClass(IntWritable.class);
		conf.setOutputFormat(SequenceFileOutputFormat.class);
		conf.setNumReduceTasks(5);//设置5个reduce任务，输出5个文件
		SequenceFileOutputFormat.setCompressOutput(conf, true);
		SequenceFileOutputFormat
				.setOutputCompressorClass(conf, GzipCodec.class);
		SequenceFileOutputFormat.setOutputCompressionType(conf,
				CompressionType.BLOCK);
		JobClient.runJob(conf);
		return 0;
	}

	public static void main(String[] args) throws Exception {
		int exitCode = ToolRunner.run(
				new SortByTemperatureUsingHashPartitioner(), args);
		System.exit(exitCode);
	}

}

hadoop jar test.jar SortByTemperatureUsingHashPartitioner -D mapred.reduce.tasks=30
产生多个已经排好序的小文件。

连接

MapReduce能够执行大型数据集间的“”连接（join）操作，但是从头编写相关代码来执行连接比较麻烦。也可以考虑使用一个更高级的框架，如Pig、Hive或Casading等，它们都将连接操作视为整个实现的核心部分。

本章的代码用到的基础工具类

其他章节也可能用到：）

JobBuilder

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.mapred.FileInputFormat;
import org.apache.hadoop.mapred.FileOutputFormat;
import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.util.Tool;

public class JobBuilder {

	public static JobConf parseInputAndOutput(Tool tool, Configuration conf,
			String[] args) {
		if (args.length != 2) {
			printUsage(tool, "<input><output>");
			return null;
		}
		JobConf jobConf = new JobConf(conf, tool.getClass());
		FileInputFormat.addInputPath(jobConf, new Path(args[0]));
		FileOutputFormat.setOutputPath(jobConf, new Path(args[1]));
		return jobConf;
	}

	public static void printUsage(Tool tool, String extraArgsUsage) {
		System.err.printf("Usage:%s [genericOptions] %s\n\n", tool.getClass()
				.getSimpleName(), extraArgsUsage);
	}
}

NcdcRecordParser
 
import org.apache.hadoop.io.Text;

public class NcdcRecordParser {
	private static final int MISSING_TEMPERATURE = 9999;

	private String year;
	private int airTemperature;
	private String quality;

	public void parse(String record) {
		year = record.substring(15, 19);
		String airTemperatureString;
		// Remove leading plus sign as parseInt doesn‘t like them
		if (record.charAt(87) == ‘+‘) {
			airTemperatureString = record.substring(88, 92);
		} else {
			airTemperatureString = record.substring(87, 92);
		}
		airTemperature = Integer.parseInt(airTemperatureString);
		quality = record.substring(92, 93);
	}

	public void parse(Text record) {
		parse(record.toString());
	}

	public boolean isValidTemperature() {
		return airTemperature != MISSING_TEMPERATURE
				&& quality.matches("[01459]");
	}

	public boolean isMa1formedTemperature() {
		return !quality.matches("[01459]");
	}

	public boolean IsMissingTemperature() {
		return airTemperature == MISSING_TEMPERATURE;
	}

	public String getYear() {
		return year;
	}

	public int getAirTemperature() {
		return airTemperature;
	}
}

这一篇是《Hadoop权威指南》第八章的学习笔记，好久没看Hadoop，工作中也没使用，前不久学习的东西，忘记了很多。学以致用是非常重要的，没用应用的学习，最终会忘记大部分，感兴趣的就需要多多温习了。