--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/org/tmatesoft/hg/internal/Transaction.java	Wed May 15 20:10:09 2013 +0200
@@ -0,0 +1,106 @@
+/*
+ * Copyright (c) 2013 TMate Software Ltd
+ *  
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program 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 General Public License for more details.
+ *
+ * For information on how to redistribute this software under
+ * the terms of a license other than GNU General Public License
+ * contact TMate Software at support@hg4j.com
+ */
+package org.tmatesoft.hg.internal;
+
+import java.io.File;
+import java.io.IOException;
+
+import org.tmatesoft.hg.core.HgIOException;
+import org.tmatesoft.hg.core.SessionContext;
+import org.tmatesoft.hg.repo.HgInvalidStateException;
+
+/**
+ * Implementation strategies possible:<ul>
+ * <li> Get a copy, write changes to origin, keep copy as backup till #commit
+ *   <p>(-) doesn't break hard links 
+ * <li> Get a copy, write changes to a copy, on commit rename copy to origin. 
+ *   <p>(-) What if we read newly written data (won't find it);
+ *   <p>(-) complex #commit
+ *   <p>(+) simple rollback
+ * <li> Get a copy, rename origin to backup (breaks hard links), rename copy to origin, write changes 
+ *   <p>(+) Modified file is in place right away;
+ *   <p>(+) easy #commit
+ * <li> Do not copy, just record file size, truncate to that size on rollback
+ * <li> ...?
+ * </ul> 
+ * @author Artem Tikhomirov
+ * @author TMate Software Ltd.
+ */
+public abstract class Transaction {
+	/**
+	 * Record the file is going to be modified during this transaction, obtain actual
+	 * destination to write to.
+	 */
+	public abstract File prepare(File f) throws HgIOException;
+	/**
+	 * overwrites backup if exists, backup is kept after successful {@link #commit()}
+	 */
+	public abstract File prepare(File origin, File backup) throws HgIOException;
+	/**
+	 * Tell that file was successfully processed
+	 */
+	public abstract void done(File f) throws HgIOException;
+	/**
+	 * optional?
+	 */
+	public abstract void failure(File f, IOException ex);
+	/**
+	 * Complete the transaction
+	 */
+	public abstract void commit() throws HgIOException;
+	/**
+	 * Undo all the changes
+	 */
+	public abstract void rollback() throws HgIOException;
+
+	public interface Factory {
+		public Transaction create(SessionContext.Source ctxSource);
+	}
+
+	public static class NoRollback extends Transaction {
+
+		@Override
+		public File prepare(File f) throws HgIOException {
+			return f;
+		}
+
+		@Override
+		public File prepare(File origin, File backup) throws HgIOException {
+			return origin;
+		}
+
+		@Override
+		public void done(File f) throws HgIOException {
+			// no-op
+		}
+
+		@Override
+		public void failure(File f, IOException ex) {
+			// no-op
+		}
+
+		@Override
+		public void commit() throws HgIOException {
+			// no-op
+		}
+
+		@Override
+		public void rollback() throws HgIOException {
+			throw new HgInvalidStateException("This transaction doesn't support rollback");
+		}
+	}
+}