Hadoop文件系统支持释疑之S3

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一、引言
Hadoop 版本提供了对多种文件系统的支持，但是这些文件系统是以何种方式实现的，其实现原理是什么以前并没有深究过。今天正好有人咨询我这个问题：Hadoop 对 S3 的支持原理是什么？特此总结一下。Hadoop 支持的文件系统包括：

文件系统 URI 前缀 hadoop 的具体实现类
Local file fs.LocalFileSystem
HDFS hdfs hdfs.DistributedFileSystem
HFTP hftp hdfs.HftpFileSystem
HSFTP hsftp hdfs.HsftpFileSystem
HAR har fs.HarFileSystem
KFS kfs fs.kfs.KosmosFileSystem
FTP ftp fs.ftp.FTPFileSystem
S3 (native) s3n fs.s3native.NativeS3FileSystem
S3 (blockbased) s3 fs.s3.S3FileSystem

二、争议观点
1.Hadoop 对 S3 文件系统的支持是通过自己实现 S3 文件系统来做的吗？
2.Hadoop 对 S3 文件系统的支持是通过 S3 文件系统接口，实现的对 S3 文件系统的整合？

三、源码解析

package org.apache.hadoop.fs.s3;

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.Closeable;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URI;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.TreeSet;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.fs.s3.INode.FileType;
import org.jets3t.service.S3Service;
import org.jets3t.service.S3ServiceException;
import org.jets3t.service.impl.rest.httpclient.RestS3Service;
import org.jets3t.service.model.S3Bucket;
import org.jets3t.service.model.S3Object;
import org.jets3t.service.security.AWSCredentials;

class Jets3tFileSystemStore implements FileSystemStore {

private static final String FILE_SYSTEM_NAME = “fs”;
private static final String FILE_SYSTEM_VALUE = “Hadoop”;

private static final String FILE_SYSTEM_TYPE_NAME = “fs-type”;
private static final String FILE_SYSTEM_TYPE_VALUE = “block”;

private static final String FILE_SYSTEM_VERSION_NAME = “fs-version”;
private static final String FILE_SYSTEM_VERSION_VALUE = “1”;

private static final Map<String, String> METADATA =
new HashMap<String, String>();

static {
METADATA.put(FILE_SYSTEM_NAME, FILE_SYSTEM_VALUE);
METADATA.put(FILE_SYSTEM_TYPE_NAME, FILE_SYSTEM_TYPE_VALUE);
METADATA.put(FILE_SYSTEM_VERSION_NAME, FILE_SYSTEM_VERSION_VALUE);
}

private static final String PATH_DELIMITER = Path.SEPARATOR;
private static final String BLOCK_PREFIX = “block_”;

private Configuration conf;

private S3Service s3Service;

private S3Bucket bucket;

private int bufferSize;

public void initialize(URI uri, Configuration conf) throws IOException {

this.conf = conf;

S3Credentials s3Credentials = new S3Credentials();
s3Credentials.initialize(uri, conf);
try {
AWSCredentials awsCredentials =
new AWSCredentials(s3Credentials.getAccessKey(),
s3Credentials.getSecretAccessKey());
this.s3Service = new RestS3Service(awsCredentials);
} catch (S3ServiceException e) {
if (e.getCause() instanceof IOException) {
throw (IOException) e.getCause();
}
throw new S3Exception(e);
}
bucket = new S3Bucket(uri.getHost());

this.bufferSize = conf.getInt(“io.file.buffer.size”, 4096);
}

public String getVersion() throws IOException {
return FILE_SYSTEM_VERSION_VALUE;
}

private void delete(String key) throws IOException {
try {
s3Service.deleteObject(bucket, key);
} catch (S3ServiceException e) {
if (e.getCause() instanceof IOException) {
throw (IOException) e.getCause();
}
throw new S3Exception(e);
}
}

public void deleteINode(Path path) throws IOException {
delete(pathToKey(path));
}

public void deleteBlock(Block block) throws IOException {
delete(blockToKey(block));
}

public boolean inodeExists(Path path) throws IOException {
InputStream in = get(pathToKey(path), true);
if (in == null) {
return false;
}
in.close();
return true;
}

public boolean blockExists(long blockId) throws IOException {
InputStream in = get(blockToKey(blockId), false);
if (in == null) {
return false;
}
in.close();
return true;
}

private InputStream get(String key, boolean checkMetadata)
throws IOException {

try {
S3Object object = s3Service.getObject(bucket, key);
if (checkMetadata) {
checkMetadata(object);
}
return object.getDataInputStream();
} catch (S3ServiceException e) {
if (“NoSuchKey”.equals(e.getS3ErrorCode())) {
return null;
}
if (e.getCause() instanceof IOException) {
throw (IOException) e.getCause();
}
throw new S3Exception(e);
}
}

private InputStream get(String key, long byteRangeStart) throws IOException {
try {
S3Object object = s3Service.getObject(bucket, key, null, null, null,
null, byteRangeStart, null);
return object.getDataInputStream();
} catch (S3ServiceException e) {
if (“NoSuchKey”.equals(e.getS3ErrorCode())) {
return null;
}
if (e.getCause() instanceof IOException) {
throw (IOException) e.getCause();
}
throw new S3Exception(e);
}
}

private void checkMetadata(S3Object object) throws S3FileSystemException,
S3ServiceException {

String name = (String) object.getMetadata(FILE_SYSTEM_NAME);
if (!FILE_SYSTEM_VALUE.equals(name)) {
throw new S3FileSystemException(“Not a Hadoop S3 file.”);
}
String type = (String) object.getMetadata(FILE_SYSTEM_TYPE_NAME);
if (!FILE_SYSTEM_TYPE_VALUE.equals(type)) {
throw new S3FileSystemException(“Not a block file.”);
}
String dataVersion = (String) object.getMetadata(FILE_SYSTEM_VERSION_NAME);
if (!FILE_SYSTEM_VERSION_VALUE.equals(dataVersion)) {
throw new VersionMismatchException(FILE_SYSTEM_VERSION_VALUE,
dataVersion);
}
}

public INode retrieveINode(Path path) throws IOException {
return INode.deserialize(get(pathToKey(path), true));
}

public File retrieveBlock(Block block, long byteRangeStart)
throws IOException {
File fileBlock = null;
InputStream in = null;
OutputStream out = null;
try {
fileBlock = newBackupFile();
in = get(blockToKey(block), byteRangeStart);
out = new BufferedOutputStream(new FileOutputStream(fileBlock));
byte[] buf = new byte[bufferSize];
int numRead;
while ((numRead = in.read(buf)) >= 0) {
out.write(buf, 0, numRead);
}
return fileBlock;
} catch (IOException e) {
// close output stream to file then delete file
closeQuietly(out);
out = null; // to prevent a second close
if (fileBlock != null) {
fileBlock.delete();
}
throw e;
} finally {
closeQuietly(out);
closeQuietly(in);
}
}

private File newBackupFile() throws IOException {
File dir = new File(conf.get(“fs.s3.buffer.dir”));
if (!dir.exists() && !dir.mkdirs()) {
throw new IOException(“Cannot create S3 buffer directory: ” + dir);
}
File result = File.createTempFile(“input-“, “.tmp”, dir);
result.deleteOnExit();
return result;
}

public Set<Path> listSubPaths(Path path) throws IOException {
try {
String prefix = pathToKey(path);
if (!prefix.endsWith(PATH_DELIMITER)) {
prefix += PATH_DELIMITER;
}
S3Object[] objects = s3Service.listObjects(bucket, prefix, PATH_DELIMITER);
Set<Path> prefixes = new TreeSet<Path>();
for (int i = 0; i < objects.length; i++) {
prefixes.add(keyToPath(objects[i].getKey()));
}
prefixes.remove(path);
return prefixes;
} catch (S3ServiceException e) {
if (e.getCause() instanceof IOException) {
throw (IOException) e.getCause();
}
throw new S3Exception(e);
}
}

public Set<Path> listDeepSubPaths(Path path) throws IOException {
try {
String prefix = pathToKey(path);
if (!prefix.endsWith(PATH_DELIMITER)) {
prefix += PATH_DELIMITER;
}
S3Object[] objects = s3Service.listObjects(bucket, prefix, null);
Set<Path> prefixes = new TreeSet<Path>();
for (int i = 0; i < objects.length; i++) {
prefixes.add(keyToPath(objects[i].getKey()));
}
prefixes.remove(path);
return prefixes;
} catch (S3ServiceException e) {
if (e.getCause() instanceof IOException) {
throw (IOException) e.getCause();
}
throw new S3Exception(e);
}
}

private void put(String key, InputStream in, long length, boolean storeMetadata)
throws IOException {

try {
S3Object object = new S3Object(key);
object.setDataInputStream(in);
object.setContentType(“binary/octet-stream”);
object.setContentLength(length);
if (storeMetadata) {
object.addAllMetadata(METADATA);
}
s3Service.putObject(bucket, object);
} catch (S3ServiceException e) {
if (e.getCause() instanceof IOException) {
throw (IOException) e.getCause();
}
throw new S3Exception(e);
}
}

public void storeINode(Path path, INode inode) throws IOException {
put(pathToKey(path), inode.serialize(), inode.getSerializedLength(), true);
}

public void storeBlock(Block block, File file) throws IOException {
BufferedInputStream in = null;
try {
in = new BufferedInputStream(new FileInputStream(file));
put(blockToKey(block), in, block.getLength(), false);
} finally {
closeQuietly(in);
}
}

private void closeQuietly(Closeable closeable) {
if (closeable != null) {
try {
closeable.close();
} catch (IOException e) {
// ignore
}
}
}

private String pathToKey(Path path) {
if (!path.isAbsolute()) {
throw new IllegalArgumentException(“Path must be absolute: ” + path);
}
return path.toUri().getPath();
}

private Path keyToPath(String key) {
return new Path(key);
}

private String blockToKey(long blockId) {
return BLOCK_PREFIX + blockId;
}

private String blockToKey(Block block) {
return blockToKey(block.getId());
}

public void purge() throws IOException {
try {
S3Object[] objects = s3Service.listObjects(bucket);
for (int i = 0; i < objects.length; i++) {
s3Service.deleteObject(bucket, objects[i].getKey());
}
} catch (S3ServiceException e) {
if (e.getCause() instanceof IOException) {
throw (IOException) e.getCause();
}
throw new S3Exception(e);
}
}

public void dump() throws IOException {
StringBuilder sb = new StringBuilder(“S3 Filesystem, “);
sb.append(bucket.getName()).append(“\n”);
try {
S3Object[] objects = s3Service.listObjects(bucket, PATH_DELIMITER, null);
for (int i = 0; i < objects.length; i++) {
Path path = keyToPath(objects[i].getKey());
sb.append(path).append(“\n”);
INode m = retrieveINode(path);
sb.append(“\t”).append(m.getFileType()).append(“\n”);
if (m.getFileType() == FileType.DIRECTORY) {
continue;
}
for (int j = 0; j < m.getBlocks().length; j++) {
sb.append(“\t”).append(m.getBlocks()[j]).append(“\n”);
}
}
} catch (S3ServiceException e) {
if (e.getCause() instanceof IOException) {
throw (IOException) e.getCause();
}
throw new S3Exception(e);
}
System.out.println(sb);
}