Hadoop中基於文件的數據格式（1）SequenceFile

1 概述

1 SequenceFile是Hadoop爲例存儲二進制的<k,v>而設計的一種平面文件。
2 SequenceFile的key和value是writable或者writable子類。
3 SequenceFile的存儲不按照key排序，內部類Writer提供了append方法。
4 SequenceFile作爲一個容器，可以將小文件打包到SequenceFile，高效對小文件進行存儲和處理。

2 壓縮類型

根據CompressionType的不同，有如下壓縮類型
NONE：不壓縮。每個記錄有key長度、value長度、key、value組成，長度字段分別爲4字節。
RECORD: 記錄壓縮。結構與NONE非常類似，用定義在頭部的編碼器壓縮value，key不壓縮。
BLOCK:塊壓縮。一次壓縮多條記錄，當記錄字節數達到一個閾值則天際到塊，io.seqfile.compress.blocksize控制。格式爲：記錄數，鍵長度，鍵，值長度，值。

分別對應Writer：

Writer : Uncompressed records
RecordCompressWriter : Record-compressed files, only compress values
BlockCompressWriter : Block-compressed files, both keys & values are collected in 'blocks' separately and compressed. The size of the 'block' is configurable

3 特點

優點

1 支持基於record和block的壓縮

2 支持splittable，能夠爲Mapreduce作爲輸入分片

3 修改簡單，只需要按照業務邏輯修改，不要考慮具體存儲格式

缺點

合併後的文件不易查看。hadoop fs -cat看不到，hadoop fs -text可以看到。

4 代碼

import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IOUtils;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.SequenceFile;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.util.ReflectionUtils;

public class SequenceTest {

	public static final String Output_path = "hdfs://192.x.x.x:9000/a.txt";
	private static final String[] DATA = { "a", "b", "c", };

	@SuppressWarnings("deprecation")
	public static void write(String pathStr) throws IOException {
		Configuration conf = new Configuration();
		Path path = new Path(pathStr);
		FileSystem fs = path.getFileSystem(conf);
		
		SequenceFile.Writer writer = SequenceFile.createWriter(fs, conf, path,
				Text.class, IntWritable.class);
		Text key = new Text();
		IntWritable value = new IntWritable();
		for (int i = 0; i < DATA.length; i++) {
			key.set(DATA[i]);
			value.set(i);
			System.out.printf("[%s]\t%s\t%s\n", writer.getLength(), key, value);
			writer.append(key, value);
		}
		IOUtils.closeStream(writer);
	}

	@SuppressWarnings("deprecation")
	public static void read(String pathStr) throws IOException {
		Configuration conf = new Configuration();
		Path path = new Path(pathStr);
		FileSystem fs = path.getFileSystem(conf);
		SequenceFile.Reader reader = new SequenceFile.Reader(fs, new Path(
				pathStr), conf);
		
		Writable key = (Writable) ReflectionUtils.newInstance(
				reader.getKeyClass(), conf);
		Writable value = (Writable) ReflectionUtils.newInstance(
				reader.getValueClass(), conf);

		while (reader.next(key, value)) {
			System.out.printf("%s\t%s\n", key, value);
		}
		IOUtils.closeStream(reader);
	}

	public static void main(String[] args) throws IOException {
		write(Output_path);
		read(Output_path);
	}
}