/**
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
*/
package com.bigdata.util;
import java.util.UUID;
/**
* An instance of this class is thrown if we observe that the timestamps
* generated by two or more services violate a requirement for synchronized
* clocks.
*
* @author <a href="mailto:thompsonbry@users.sourceforge.net">Bryan Thompson</a>
*/
public class ClocksNotSynchronizedException extends RuntimeException {
/**
*
*/
private static final long serialVersionUID = 1L;
public ClocksNotSynchronizedException() {
super();
}
public ClocksNotSynchronizedException(String msg) {
super(msg);
}
/**
* Assert that <code>t1</code> LT <code>t2</code>, where <code>t1</code> and
* <code>t2</code> are timestamps obtain such that this relation will be
* <code>true</code> if the clocks on the nodes are synchronized.
* <p>
* Note: Clock synchronization errors can arise across nodes if the nodes
* are not using a common network time source.
* <p>
* Note: Synchronization errors can arise on a single node if the clock is
* changed on that node - specifically if the clock is move backwards to
* before the most recent commit timestamp. For example, if the timezone is
* changed.
*
* @param serviceId1
* The service that reported the timestamp <code>t1</code>.
* @param serviceId2
* The service that reported the timestamp <code>t2</code>.
* @param t1
* A timestamp from one service.
* @param t2
* A timestamp from the another service.
* @param maxSkew
* The maximum allowed clock skew (typically on the order of
* seconds).
*
* @throws ClocksNotSynchronizedException
*/
static public void assertBefore(final UUID serviceId1,
final UUID serviceId2, final long t1, final long t2,
final long maxSkew) throws ClocksNotSynchronizedException {
if (t1 < t2) {
/*
* Strictly LT.
*
* Note: There can be large latencies between t1 and t2. If t1 is
* taken on the leader before t2 is taken on the follower, then a
* full GC on either node before t2 is taken will result in a large
* latency between t1 and t2. Our concern here is to identify skew
* that violates the BEFORE semantics, not latency in which time
* moves forward.
*/
return;
}
final long delta = Math.abs(t1 - t2);
if (delta <= maxSkew)
return;
// if (t2 >= min && t2 <= max) {
// // The 2nd clock is within the allowed window.
// return;
// }
throw new ClocksNotSynchronizedException("service1=" + serviceId1
+ ", serviceId2=" + serviceId2 + ", skew=" + delta
+ "ms exceeds maximumSkew=" + maxSkew + "ms.");
}
}