checkpointrecord.java

用java语言简单实现数据库的初步功能

package simpledb.tx.recovery;

import simpledb.log.BasicLogRecord;

/**
 * The CHECKPOINT log record.
 * @author Edward Sciore
 */
class CheckpointRecord implements LogRecord {

	/**
	 * Creates a quiescent checkpoint record.
	 */
	public CheckpointRecord() {}
	
	/**
	 * Creates a log record by reading no other values from the basic log record.
	 * @param rec the basic log record
	 */
	public CheckpointRecord(BasicLogRecord rec) {}

	/** 
	 * Writes a checkpoint record to the log.
	 * This log record contains the CHECKPOINT operator,
	 * and nothing else.
	 * @return the LSN of the last log value
	 */
	public int writeToLog() {
		Object[] rec = new Object[] {CHECKPOINT};
		return logMgr.append(rec);
	}

	public int op() {
		return LogRecord.CHECKPOINT;
	}

	public int txNumber() {
		return -1; // dummy value
	}

	public void undo(int txnum) {}

	public String toString() {
		return "<CHECKPOINT>";
	}
}