datafile.java

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/*  Sesame - Storage and Querying architecture for RDF and RDF Schema
 *  Copyright (C) 2001-2005 Aduna
 *
 *  Contact: 
 *  	Aduna
 *  	Prinses Julianaplein 14 b
 *  	3817 CS Amersfoort
 *  	The Netherlands
 *  	tel. [phone]
 *  	fax. [phone]
 *
 *  	http://aduna.biz/
 *  	http://www.openrdf.org/
 *  
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser General Public
 *  License as published by the Free Software Foundation; either
 *  version 2.1 of the License, or (at your option) any later version.
 *
 *  This library is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 *  Lesser General Public License for more details.
 *
 *  You should have received a copy of the GNU Lesser General Public
 *  License along with this library; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

package org.openrdf.sesame.sailimpl.nativerdf.datastore;

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.util.NoSuchElementException;

/**
 * Class supplying access to a data file. A data file stores data sequentially.
 * Each entry starts with the entry's length (4 bytes), followed by the data
 * itself. File offsets are used to identify entries.
 *
 * @author Arjohn Kampman
 * @version $Revision: 1.11 $
 **/
public class DataFile {
/*-------------+
| Constants    |
+-------------*/

	private static final long HEADER_LENGTH = 4L;

/*-------------+
| Variables    |
+-------------*/

	private File _file;
	private RandomAccessFile _raf;
	private FileChannel _fileChannel;

	private File _txnFile;
	private RandomAccessFile _txnRaf;
	private FileChannel _txnChannel;

	/** Flag indicating whether the current transaction is/should be an isolated one. **/
	private boolean _isolatedTransaction;

	private boolean _dataCleared;

/*-------------+
| Constructors |
+-------------*/

	public DataFile(File file)
		throws IOException
	{
		_file = file;
		
		// Make sure the file exists
		_file.createNewFile();

		// Open a read/write channel to the file
		_raf = new RandomAccessFile(file, "rw");
		_fileChannel = _raf.getChannel();

		if (_fileChannel.size() == 0L) {
			// Empty file, insert 4 dummy bytes at the start of the file
			_fileChannel.write( ByteBuffer.wrap(new byte[] {0, 0, 0, 0}) );
		}
	}

/*----------+
| Methods   |
+----------*/

	public void startTransaction(boolean isolateTransaction)
		throws IOException
	{
		_isolatedTransaction = isolateTransaction;

		if (isolateTransaction) {
			// Make sure the transaction file exists and that it is empty
			_txnFile = new File(_file.getParentFile(), "txn_" + _file.getName());
			_txnFile.createNewFile();

			_txnRaf = new RandomAccessFile(_txnFile, "rw");
			_txnChannel = _txnRaf.getChannel();
			_txnChannel.truncate(0L);

			_dataCleared = false;
		}
	}

	public void commitTransaction()
		throws IOException
	{
		if (_isolatedTransaction) {
			if (_dataCleared) {
				// Discard existing data
				_fileChannel.truncate(HEADER_LENGTH);
			}

			// Append the txn file to the data file
			_txnChannel.position(0L);
			TransferUtil.transferFrom(_txnChannel, _fileChannel.size(), _txnChannel.size(), _fileChannel);

			_txnRaf.close();
			_txnRaf = null;
			_txnChannel = null;

			// Delete the txn file
			_txnFile.delete();
			_txnFile = null;
		}
	}

	public void rollbackTransaction()
		throws IOException
	{
		if (_isolatedTransaction) {
			// Close and discard the txn channel
			_txnRaf.close();
			_txnRaf = null;
			_txnChannel = null;

			// Delete the txn file
			_txnFile.delete();
			_txnFile = null;
		}
		else {
			throw new IOException("Unisolated transactions cannot be rolled back");
		}
	}

	/**
	 * Stores the specified data and returns the byte-offset at which it has
	 * been stored.
	 **/
	public long storeData(byte[] data)
		throws IOException
	{
		if (_isolatedTransaction) {
			// Append the data to the txn file
			long offset = _txnChannel.size();
			_writeData(_txnChannel, offset, data);

			// Txn file will be appended to data file on commit,
			// add size of data file to the offset
			if (_dataCleared) {
				// data file will be truncated to 4 bytes on commit
				return HEADER_LENGTH + offset;
			}
			else {
				return _fileChannel.size() + offset;
			}
		}
		else {
			long offset = _fileChannel.size();
			_writeData(_fileChannel, offset, data);
			return offset;
		}
	}

	/**
	 * Updates (replaces) the data at the specified offset with the supplied data.
	 **/
	public void updateData(long offset, byte[] data)
		throws IOException
	{
		FileChannel channel = _fileChannel;

		if (_isolatedTransaction && (_dataCleared || offset >= _fileChannel.size())) {
			// Write the data to the txn file
			channel = _txnChannel;
			offset -= _dataCleared ? HEADER_LENGTH : _fileChannel.size();
		}

		_writeData(channel, offset, data);
	}

	private void _writeData(FileChannel channel, long offset, byte[] data)
		throws IOException
	{
		ByteBuffer buf = ByteBuffer.allocate(data.length + 4);
		buf.putInt(data.length);
		buf.put(data);
		buf.rewind();

		channel.write(buf, offset);
	}

	/**
	 * Gets the data that is stored at the specified offset.
	 *
	 * @param offset An offset in the data file.
	 * @param dirtyReads Flag indicating whether data that hasn't been committed
	 * yet should be searched too.
	 * @return The data that was found on the specified offset.
	 * @exception IOException If an I/O error occurred.
	 **/
	public byte[] getData(long offset, boolean dirtyReads)
		throws IOException
	{
		FileChannel channel = _fileChannel;

		if (dirtyReads && _isolatedTransaction && (_dataCleared || offset >= _fileChannel.size())) {
			// Check txn file
			channel = _txnChannel;
			offset -= _dataCleared ? HEADER_LENGTH : _fileChannel.size();
		}

		// FIXME: maybe get more data in one go is more efficient?
		ByteBuffer buf = ByteBuffer.allocate(4);
		channel.read(buf, offset);
		int dataLength = buf.getInt(0);

		byte[] data = new byte[dataLength];
		buf = ByteBuffer.wrap(data);
		channel.read(buf, offset + 4L);

		return data;
	}

	public void clear()
		throws IOException
	{
		if (_isolatedTransaction) {
			_txnChannel.truncate(0L);

			// Existing data is deleted on commit
			_dataCleared = true;
		}
		else {
			_fileChannel.truncate(HEADER_LENGTH);
		}
	}

	public void close()
		throws IOException
	{
		if (_txnChannel != null) {
			rollbackTransaction();
		}

		_raf.close();
	}

	public DataIterator iterator() {
		return new DataIterator();
	}

	public class DataIterator {

		private long _position = HEADER_LENGTH;

		public boolean hasNext()
			throws IOException
		{
			return _position < _fileChannel.size();
		}

		public byte[] next()
			throws IOException
		{
			if (!hasNext()) {
				throw new NoSuchElementException();
			}

			byte[] data = getData(_position, false);
			_position += (4 + data.length);
			return data;
		}
	}
}