Zookeeper(四))持久化日志文件
持久化用途
存储两种文件
- snapshot：内存快照
- log：事务日志，类似MySQL的binlog，存储数据节点的操作日志
问题
- 序列化的本质其实就是将原数据重新写入
- roll中的BufferedOutputStream.flush和commit中的FileChannel.force()都是强制刷新：有什么区别
基本术语
- FileTxnSnapLog，封装了TxnLog和SnapShot。 是操作数据文件和快照文件的对外API
- TxnLog，接口类型，读取事务性日志的接口。
- FileTxnLog，实现TxnLog接口，添加了访问该事务性日志的API。
- Snapshot，接口类型，持久层快照接口。
- FileSnap，实现Snapshot接口，负责存储、序列化、反序列化、访问快照。
- Util，工具类，提供持久化所需的API。
实现
TxnLog
public interface TxnLog {
/**
* Setter for ServerStats to monitor fsync threshold exceed
* @param serverStats used to update fsyncThresholdExceedCount
*/
void setServerStats(ServerStats serverStats);
// 滚动日志，从当前日志滚到下一个日志，不是回滚
void rollLog() throws IOException;
// 添加一个请求至事务性日志
boolean append(TxnHeader hdr, Record r) throws IOException;
// 读取事务性日志
TxnIterator read(long zxid) throws IOException;
// 事务性操作的最新zxid
long getLastLoggedZxid() throws IOException;
// 清空zxid以后的日志
boolean truncate(long zxid) throws IOException;
// 获取数据库的id
long getDbId() throws IOException;
// 提交事务并进行确认
void commit() throws IOException;
long getTxnLogSyncElapsedTime();
// 关闭事务性日志
void close() throws IOException;
}
FileTxnLog
实现TxnLog接口，提供操作事务日志的api
public class FileTxnLog implements TxnLog {
//最新的日志zxid
long lastZxidSeen;
//日志文件
volatile BufferedOutputStream logStream = null;
volatile OutputArchive oa;
//日志存储文件
File logFileWrite = null;
private FilePadding filePadding = new FilePadding();
private LinkedList<FileOutputStream> streamsToFlush =
new LinkedList<FileOutputStream>();
//重置日志文件
public synchronized void rollLog() throws IOException {
if (logStream != null) {
this.logStream.flush();
this.logStream = null;
oa = null;
}
}
//添加事务日志 hdr：事务标题 txn：事务本身
public synchronized boolean append(TxnHeader hdr, Record txn)
throws IOException {
if (hdr == null) {
return false;
}
//判断并更新最新的zxid
if (hdr.getZxid() <= lastZxidSeen) {
LOG.warn("Current zxid " + hdr.getZxid()
+ " is <= " + lastZxidSeen + " for " + hdr.getType());
} else {
lastZxidSeen = hdr.getZxid();
}
//构建事务日志文件
if (logStream==null) {
if(LOG.isInfoEnabled()){
LOG.info("Creating new log file: " + Util.makeLogName(hdr.getZxid()));
}
//1. 生成新的log文件
logFileWrite = new File(logDir, Util.makeLogName(hdr.getZxid()));
//2. 生成log文件输出流
fos = new FileOutputStream(logFileWrite);
//为写入给定的输出流而创建缓冲输出流
logStream=new BufferedOutputStream(fos);
//获取二进制序列化类 TODO
//BinaryOutputArchive内部维护一个DataOutput 根据值传递特性
//oa序列化写入时其实就是写入log文件
oa = BinaryOutputArchive.getArchive(logStream);
//3. 用TXNLOG_MAGIC VERSION dbId来生成事务日志文件头
FileHeader fhdr = new FileHeader(TXNLOG_MAGIC,VERSION, dbId);
//4. 将事务日志文件头序列化到文件上
fhdr.serialize(oa, "fileheader");
//确保文件扩展之前 魔数已被写入
logStream.flush();
filePadding.setCurrentSize(fos.getChannel().position());
streamsToFlush.add(fos);
}
//5. 剩余空间不足时 填充文件
filePadding.padFile(fos.getChannel());
//6. 将事务头部和本身序列化为字节数组
byte[] buf = Util.marshallTxnEntry(hdr, txn);
if (buf == null || buf.length == 0) {
throw new IOException("Faulty serialization for header " + "and txn");
}
//生成验证算法 校验数据流
Checksum crc = makeChecksumAlgorithm();
crc.update(buf, 0, buf.length);
oa.writeLong(crc.getValue(), "txnEntryCRC");
//6. 将当前序列化的事务记录写入到oa
Util.writeTxnBytes(oa, buf);
return true;
}
//从给定的zxid开始读取日志文件
public TxnIterator read(long zxid, boolean fastForward) throws IOException {
return new FileTxnIterator(logDir, zxid, fastForward);
}
//提交日志 保存到磁盘
public synchronized void commit() throws IOException {
//刷到磁盘
if (logStream != null) {
logStream.flush();
}
//强刷到磁盘
for (FileOutputStream log : streamsToFlush) {
log.flush();
if (forceSync) {
long startSyncNS = System.nanoTime();
FileChannel channel = log.getChannel();
//会强制将所有未写入磁盘的数据都强制写入磁盘 比如还在缓冲区中的数据
channel.force(false);
syncElapsedMS = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startSyncNS);
if (syncElapsedMS > fsyncWarningThresholdMS) {
if(serverStats != null) {
serverStats.incrementFsyncThresholdExceedCount();
}
LOG.warn("fsync-ing the write ahead log in "
+ Thread.currentThread().getName()
+ " took " + syncElapsedMS
+ "ms which will adversely effect operation latency. "
+ "File size is " + channel.size() + " bytes. "
+ "See the ZooKeeper troubleshooting guide");
}
ServerMetrics.FSYNC_TIME.add(syncElapsedMS);
}
}
//移除流并关闭
while (streamsToFlush.size() > 1) {
streamsToFlush.removeFirst().close();
}
//当日志文件大小超过限制 强刷到磁盘并重置
if(txnLogSizeLimit > 0) {
long logSize = getCurrentLogSize();
if (logSize > txnLogSizeLimit) {
LOG.debug("Log size limit reached: {}", logSize);
rollLog();
}
}
}
}
FileTxnIterator：用于读取事务日志
public static class FileTxnIterator implements TxnLog.TxnIterator {
public FileTxnIterator(File logDir, long zxid, boolean fastForward)
throws IOException {
this.logDir = logDir;
this.zxid = zxid;
init();
if (fastForward && hdr != null) {
while (hdr.getZxid() < zxid) {
if (!next())
break;
}
}
}
void init() throws IOException {
storedFiles = new ArrayList<File>();
//找出大于等于snapshot中最大的zxid的logfile及后续logfile并升序
List<File> files = Util.sortDataDir(FileTxnLog.getLogFiles(logDir.listFiles(), 0), LOG_FILE_PREFIX, false);
for (File f: files) {
if (Util.getZxidFromName(f.getName(), LOG_FILE_PREFIX) >= zxid) {
storedFiles.add(f);
}
// add the last logfile that is less than the zxid
else if (Util.getZxidFromName(f.getName(), LOG_FILE_PREFIX) < zxid) {
storedFiles.add(f);
break;
}
}
goToNextLog();
next();
}
}