idfile.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;

/**
 * Class supplying access to an ID file.
 *
 * @author Arjohn Kampman
 * @version $Revision: 1.11 $
 **/
public class IDFile {

/*-------------+
| Constants    |
+-------------*/

	private static final long HEADER_LENGTH = 8L;
	private static final long ITEM_SIZE = 8L;

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

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

	private File _txnFile;
	private RandomAccessFile _txnRaf;
	private FileChannel _txnChannel;
	private ByteBuffer _txnBuffer = ByteBuffer.allocate(8);

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

	private boolean _dataCleared;

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

	public IDFile(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 8 dummy bytes at the start of the file
			_fileChannel.write( ByteBuffer.wrap(new byte[] {0, 0, 0, 0, 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());
			if (!_txnFile.exists()) {
				_txnFile.createNewFile();
			}

			_txnRaf = new RandomAccessFile(_txnFile, "rw");
			_txnChannel = _txnRaf.getChannel();
			if (_txnChannel.size() > 0L) {
				_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");
		}
	}

	/**
	 * Gets a new and unused ID that can be assign to new data.
	 **/
	public int getNewID()
		throws IOException
	{
		if (_isolatedTransaction) {
			// Txn file will be appended to data file on commit,
			// combine both file size to calculate the next ID
			if (_dataCleared) {
				return (int)((_txnChannel.size() + HEADER_LENGTH) / ITEM_SIZE);
			}
			else {
				return (int)((_txnChannel.size() + _fileChannel.size()) / ITEM_SIZE);
			}
		}
		else {
			return (int)(_fileChannel.size() / ITEM_SIZE);
		}
	}

	public int getMaxID()
		throws IOException
	{
		return (int)(_fileChannel.size() / ITEM_SIZE) - 1;
	}

	/**
	 * Stores the offset of a new data entry with the specified ID.
	 **/
	public void storeOffset(int id, long offset)
		throws IOException
	{
		// Create and fill ByteBuffer
		_txnBuffer.clear();
		_txnBuffer.putLong(0, offset);

		// Insert the offset at 8*id
		long txnOffset = ITEM_SIZE*id;

		FileChannel channel = _fileChannel;

		if (_isolatedTransaction) {
			channel = _txnChannel;
			txnOffset -= _dataCleared ? HEADER_LENGTH : _fileChannel.size();
		}

		channel.write(_txnBuffer, txnOffset);
	}

	/**
	 * Gets the offset of the data entry with the specified ID.
	 **/
	public long getOffset(int id, boolean dirtyReads)
		throws IOException
	{
		long offset = ITEM_SIZE * id;

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

		ByteBuffer buf = ByteBuffer.allocate(8);
		channel.read(buf, offset);
		return buf.getLong(0);
	}

	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();
	}
}