logrecord.java

用java语言简单实现数据库的初步功能

package simpledb.tx.recovery;

import simpledb.log.LogMgr;
import simpledb.server.SimpleDB;

/**
 * The interface implemented by each type of log record.
 * @author Edward Sciore
 */
public interface LogRecord {
	/**
	 * The six different types of log record
	 */
	static final int CHECKPOINT = 0, START = 1,
					 COMMIT = 2, ROLLBACK  = 3,
					 SETINT = 4, SETSTRING = 5;

	static final LogMgr logMgr = SimpleDB.logMgr();

	/**
	 * Writes the record to the log and returns its LSN.
	 * @return the LSN of the record in the log
	 */
	int writeToLog();
	
	/**
	 * Returns the log record's type. 
	 * @return the log record's type
	 */
	int op();
	
	/**
	 * Returns the transaction id stored with
	 * the log record.
	 * @return the log record's transaction id
	 */
	int txNumber();
	
	/**
	 * Undoes the operation encoded by this log record.
	 * The only log record types for which this method
	 * does anything interesting are SETINT and SETSTRING.
	 * @param txnum the id of the transaction that is performing the undo.
	 */
	void undo(int txnum);
}