本文主要研究一下BitCaskLock
bitcask-java/src/main/java/com/trifork/bitcask/BitCaskLock.java
public class BitCaskLock {
public static enum Stale {
OK, NOT_STALE
}
public static enum Type {
WRITE, MERGE;
private String type_name() {
if (this == MERGE) return "merge";
if (this == WRITE) return "write";
throw new RuntimeException();
}
}
private RandomAccessFile file;
private boolean is_write_lock;
private File filename;
private BitCaskLock(RandomAccessFile file, File filename, boolean isWriteLock) {
this.file = file;
this.filename = filename;
this.is_write_lock = isWriteLock;
}
public static BitCaskLock acquire(Type type, File dirname) throws IOException {
File lock_filename = lock_filename(type, dirname);
foo: do {
try {
BitCaskLock lock = lock_acquire(lock_filename, true);
String lock_contents = Integer.toString(OS.getpid()) " \n";
lock.write_data(ByteString.copyFromUtf8(lock_contents));
return lock;
} catch (FileAlreadyExistsException e) {
delete_stale_lock(lock_filename);
continue foo;
}
} while (false);
return null;
}
public void release() throws IOException {
if (file != null) {
if (is_write_lock) {
file.close();
filename.delete();
}
}
file = null;
}
//......
}
- BitCaskLock的acquire方法，先通过lock_acquire获取BitCaskLock，如果出现FileAlreadyExistsException则执行delete_stale_lock；其release方法针对is_write_lock执行file.close()及filename.delete()
bitcask-java/src/main/java/com/trifork/bitcask/BitCaskLock.java
private static BitCaskLock lock_acquire(File lockFilename, boolean is_write_lock)
throws IOException {
if (is_write_lock) {
if (lockFilename.createNewFile() == false) {
// file already exists, so we fail!
throw new FileAlreadyExistsException(lockFilename);
}
}
RandomAccessFile f = new RandomAccessFile(lockFilename,
is_write_lock ? "rws" : "r");
return new BitCaskLock(f, lockFilename, is_write_lock);
}
- lock_acquire方法针对is_write_lock，执行lockFilename.createNewFile()，若为false则抛出FileAlreadyExistsException
bitcask-java/src/main/java/com/trifork/bitcask/BitCaskLock.java
public static Stale delete_stale_lock(Type type, File dirname) throws IOException {
return delete_stale_lock(lock_filename(type, dirname));
}
private static Stale delete_stale_lock(File lockFilename) throws IOException {
BitCaskLock l = null;
try {
l = lock_acquire(lockFilename, false);
} catch (FileNotFoundException e) {
return Stale.OK;
} catch (IOException e) {
return Stale.NOT_STALE;
}
try {
int pid = l.read_lock_data_pid();
if (OS.pid_exists(pid)) {
return Stale.NOT_STALE;
} else {
lockFilename.delete();
return Stale.OK;
}
} catch (IOException e) {
return Stale.NOT_STALE;
} finally {
l.release();
}
}
- delete_stale_lock方法先执行lock_acquire，若出现FileNotFoundException，则返回Stale.OK，若出现IOException则返回Stale.NOT_STALE；否则通过l.read_lock_data_pid()获取pid，若OS.pid_exists(pid)则返回Stale.NOT_STALE，否则执行lockFilename.delete()，返回Stale.OK；最后执行l.release()
BitCaskLock的acquire方法，先通过lock_acquire获取BitCaskLock，如果出现FileAlreadyExistsException则执行delete_stale_lock；其release方法针对is_write_lock执行file.close()及filename.delete()
doc- BitCaskLock
