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MapReduce编程实战之“高级特性”

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也可以运行大型数据集间的“”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easy的多。

演示样例一：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 = Integer.MIN_VALUE;

			while (values.hasNext()) {

				maxValue = Math.max(maxValue, values.next().get());

			}

			output.collect(key, new IntWritable(maxValue));

		}

	}

	@Override

	public int run(String[] args) throws Exception {

		args = new String[] { "/test/input/t", "/test/output/t" }; // 给定输入输出路径

		JobConf conf = JobBuilder.parseInputAndOutput(this, getConf(), args);

		if (conf == null) {

			return -1;

		}

		conf.setOutputKeyClass(Text.class);

		conf.setOutputValueClass(IntWritable.class);

		conf.setMapperClass(MaxTemeratureMapperWithCounters.class);

		conf.setCombinerClass(MaxTemperatureReduceWithCounters.class);

		conf.setReducerClass(MaxTemperatureReduceWithCounters.class);

		JobClient.runJob(conf);

		return 0;

	}

	public static void main(String[] args) throws Exception {

		int exitCode = ToolRunner.run(new MaxTemperatureWithCounters(), args);

		System.exit(exitCode);

	}

}

演示样例二：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。

虽然应用本身可能并不须要对数据排序，但仍可能使用MapReduce的排序功能来组织数据。以下将讨论几种不同的数据集排序方法。以及怎样控制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);

	}

}

演示样例二、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可以运行大型数据集间的“”连接（join）操作，可是从头编写相关代码来运行连接比較麻烦。

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

其它章节也可能用到：）

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。忘记了非常多。学以致用是非常重要的。没用应用的学习，终于会忘记大部分，感兴趣的就须要多多温习了。

05-11 22:35
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