/**
Copyright (C) SYSTAP, LLC DBA Blazegraph 2006-2016. All rights reserved.
Contact:
SYSTAP, LLC DBA Blazegraph
2501 Calvert ST NW #106
Washington, DC 20008
licenses@blazegraph.com
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.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
/*
* Created on Oct 26, 2006
*/
package com.bigdata.util;
/**
* Timestamp factory class with no more than nanosecond resolution - values
* produced by this class MUST NOT be persisted.
* <p>
* Note: There are several problems with {@link System#nanoTime()} to date and
* the values MUST NOT be persisted. The underlying problem is that the epoch
* MAY (and in practice does) change from VM instance to VM instance, often when
* the machine is rebooted. For this reason, nano time can appear to "go
* backward" rendering it unsuitable for placing timestamps on commit records.
* This means that we do not have access to time-based method with more than
* millisecond resolution of creating "distinctions" for transaction
* identifiers.
* <p>
* Note: Nano time could be made to work in a robust nano time service as long
* as the base time for the service is adjusted on service start or rollover to
* never go backward.
*
* @author <a href="mailto:thompsonbry@users.sourceforge.net">Bryan Thompson</a>
* @version $Id$
*/
public class NanosecondTimestampFactory {
static long lastNanoTime = System.nanoTime();
/**
* Generates a timestamp with nanosecond precision that is guarenteed to be
* distinct from the last timestamp generated by this method within the same
* VM instance.
*
* @return A timestamp with nanosecond precision that MUST NOT be persisted.
*/
public static long nextNanoTime() {
final int limit = 1000;
int i = 0;
long nanoTime;
do {
nanoTime = System.nanoTime();
if( i++ >= limit ) throw new AssertionError();
} while( nanoTime == lastNanoTime );
if(nanoTime<lastNanoTime) {
throw new AssertionError("Nano time goes backward: lastNanoTime="
+ lastNanoTime + ", nanoTime=" + nanoTime);
}
lastNanoTime = nanoTime;
return nanoTime;
}
}