- 从JDK提供的GZIP工具来窥视GZIP格式
- java.util.zip.GZIPOutputStream
- java.util.zip.DeflaterOutputStream
- Flume中 HDFSCompressedDataStream的一个玩笑
- 战场转至Hadoop GzipCodec源码
- 回到问题，如何处理
从JDK提供的GZIP工具来窥视GZIP格式
java.util.zip.GZIPOutputStream
class GZIPOutputStream extends DeflaterOutputStream {
public GZIPOutputStream(OutputStream out, int size, boolean syncFlush) {
super(out, new Deflater(Deflater.DEFAULT_COMPRESSION, true), size, syncFlush);
writeHeader();
}
/**
* Writes array of bytes to the compressed output stream. This method
* will block until all the bytes are written.
* @param buf the data to be written
* @param off the start offset of the data
* @param len the length of the data
* @exception IOException If an I/O error has occurred.
*/
public synchronized void write(byte[] buf, int off, int len) {
super.write(buf, off, len);
crc.update(buf, off, len);
}
public void finish() throws IOException {
if (!def.finished()) {
def.finish();
while (!def.finished()) {
int len = def.deflate(buf, 0, buf.length);
if (def.finished() && len <= buf.length - TRAILER_SIZE) {
// last deflater buffer. Fit trailer at the end
writeTrailer(buf, len);
len = len + TRAILER_SIZE;
out.write(buf, 0, len);
return;
}
if (len > 0)
out.write(buf, 0, len);
}
// if we can't fit the trailer at the end of the last
// deflater buffer, we write it separately
byte[] trailer = new byte[TRAILER_SIZE];
writeTrailer(trailer, 0);
out.write(trailer);
}
}
/*
* Writes GZIP member header.
*/
private void writeHeader() throws IOException {
out.write(new byte[] {
(byte) GZIP_MAGIC, // Magic number (short)
(byte)(GZIP_MAGIC >> 8), // Magic number (short)
Deflater.DEFLATED, // Compression method (CM)
0, // Flags (FLG)
0, // Modification time MTIME (int)
0, // Modification time MTIME (int)
0, // Modification time MTIME (int)
0, // Modification time MTIME (int)
0, // Extra flags (XFLG)
0 // Operating system (OS)
});
}
/*
* Writes GZIP member trailer to a byte array, starting at a given
* offset.
*/
private void writeTrailer(byte[] buf, int offset) throws IOException {
writeInt((int)crc.getValue(), buf, offset); // CRC-32 of uncompr. data
writeInt(def.getTotalIn(), buf, offset + 4); // Number of uncompr. bytes
}
}
从上述源码可知一下信息：
- GZIPOutputStream 继承至 DeflaterOutputStream
- constructor：创建DeflaterOutputStream，并write header
- write：交给DeflaterOutputStream进行write，并更新crc
- finish：将数据全部处理完成之后，write tailer
总的来说：GZIPOutputStream在DeflaterOutputStream外层包了一层Header & Tailer
java.util.zip.DeflaterOutputStream
public class DeflaterOutputStream extends FilterOutputStream {
/**
* Writes an array of bytes to the compressed output stream. This
* method will block until all the bytes are written.
* @param b the data to be written
* @param off the start offset of the data
* @param len the length of the data
* @exception IOException if an I/O error has occurred
*/
public void write(byte[] b, int off, int len) throws IOException {
if (def.finished()) {
throw new IOException("write beyond end of stream");
}
if ((off | len | (off + len) | (b.length - (off + len))) < 0) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return;
}
if (!def.finished()) {
// 有兴趣的可以单独学习一下Deflater的用法
def.setInput(b, off, len);
while (!def.needsInput()) {
deflate();
}
}
}
/**
* Writes next block of compressed data to the output stream.
* @throws IOException if an I/O error has occurred
*/
protected void deflate() throws IOException {
int len = def.deflate(buf, 0, buf.length);
if (len > 0) {
out.write(buf, 0, len);
}
}
}
NOTE：从write方法中可以看出，每次的write都会出发def的操作。所以为了得到一定程度的压缩效果，最好不要传入小byte数组，不然压缩效果不明显。
部分总结
- Gzip与Deflater的关系：Gzip是一种格式，Deflater是一种压缩算法
- Gzip的格式：Gzip使用Deflater算法进行数据压缩，并添加Header和Tailer
Flume中 HDFSCompressedDataStream的一个玩笑
@Override
public void sync() throws IOException {
// We must use finish() and resetState() here -- flush() is apparently not
// supported by the compressed output streams (it's a no-op).
// Also, since resetState() writes headers, avoid calling it without an
// additional write/append operation.
// Note: There are bugs in Hadoop & JDK w/ pure-java gzip; see HADOOP-8522.
serializer.flush();
if (!isFinished) {
cmpOut.finish(); // 触发finish逻辑
isFinished = true;
}
fsOut.flush();
hflushOrSync(this.fsOut);
}
何时会调用sync
- 当write中的batchCounter(计数器) 等于batchSize时
- 当HDFSEventSink处理完一次事务之后
可以发现有这么一段神奇的注释
Note: There are bugs in Hadoop & JDK w/ pure-java gzip; see HADOOP-8522.
摘录至：https://issues.apache.org/jira/browse/HADOOP-8522
* ResetableGzipOutputStream creates invalid gzip files when finish() and resetState() are used *
Description
ResetableGzipOutputStream creates invalid gzip files when finish() and resetState() are used. The issue is that finish() flushes the compressor buffer and writes the gzip CRC32 + data length trailer. After that, resetState() does not repeat the gzip header, but simply starts writing more deflate-compressed data. The resultant files are not readable by the Linux "gunzip" tool. ResetableGzipOutputStream should write valid multi-member gzip files.
转换一下描述（针对ResetableGzipOutputStream）：
- 开始时，写入Header信息
- 随后，写入压缩数据
- finish时，写入Tailer信息
- 之后再调用resetState，再写入压缩数据，重复以往
这样的逻辑下生成的gz文件，gunzip是只能解析最前面的一块数据的。以上就是 pure-java gzip 的bug。
那么，很显然，按照以上的逻辑，Flume HDFS Sink不能使用Gzip进行数据压缩。但是也很神奇，Flume官网就是说支持（捂脸.png）。
战场转至Hadoop GzipCodec源码
public class GzipCodec extends DefaultCodec {
public CompressionOutputStream createOutputStream(OutputStream out) {
return (CompressionOutputStream)(ZlibFactory.isNativeZlibLoaded(this.conf) ?
new CompressorStream(out, this.createCompressor(), this.conf.getInt("io.file.buffer.size", 4096))
: new GzipCodec.GzipOutputStream(out));
}
public CompressionOutputStream createOutputStream(OutputStream out, Compressor compressor) {
return (CompressionOutputStream)(compressor != null ?
new CompressorStream(out, compressor, this.conf.getInt("io.file.buffer.size", 4096))
: this.createOutputStream(out));
}
}
显然，可以发现，GzipCodec中提供了两种Gzip实现：
- GzipOutputStream，正是上面提到的“玩笑”
- CompressorStream，这个则需要依赖于ZlibFactory.isNativeZlibLoaded(this.conf)的结果
那么NativeZlib是个什么鬼呢？跟踪一下，发现：
public class ZlibCompressor implements Compressor {
static {
if (NativeCodeLoader.isNativeCodeLoaded()) {
try {
// Initialize the native library
initIDs();
nativeZlibLoaded = true;
} catch (Throwable t) {
// Ignore failure to load/initialize native-zlib
}
}
}
}
结合一些其他资料，可以得出这样的结论：
Hadoop会load native包，并在GzipCodec使用时 根据 zlib包是否loaded 来决定 是否使用GzipOutputStream（这个Bug）。这样说，可能大家不是很清楚，但是 相信大家一定都见过这样一个WARN
常见的WARN：Unable to load native-hadoop library for your platform
这个WARN就是Hadoop没有找到native包 于是就无法成功load时的信息
public final class NativeCodeLoader {
static {
try {
System.loadLibrary("hadoop");
LOG.debug("Loaded the native-hadoop library");
nativeCodeLoaded = true;
} catch (Throwable t) {
// Ignore failure to load
LOG.debug("Failed to load native-hadoop with error: " + t);
LOG.debug("java.library.path=" + System.getProperty("java.library.path"));
}
if (!nativeCodeLoaded) {
LOG.warn("Unable to load native-hadoop library for your platform... using builtin-java classes where applicable");
}
}
}
这里大家请自行查阅System.loadLibrary()，然后就会发现这么个东西 – java.library.path
由于内容太多，这里就不继续往下了。
回到问题，如何处理
通过上面的说明 已经一定程度上描述了问题，接下来就是如何处理这个问题：
其实，最为本质的方式 就是调整环境让Hadoop load 到 native包，这样就能让GzipCodec不使用那个BugClass了
但是还有一种绕道的处理方式，就是通过控制Flume参数，让Sink时每次finish都产生一个完整的File。