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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.cassandra.utils;
import java.lang.reflect.Constructor;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.function.LongSupplier;
import com.google.common.annotations.VisibleForTesting;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.apache.cassandra.concurrent.ScheduledExecutors;
import org.apache.cassandra.config.Config;
import static java.util.concurrent.TimeUnit.MILLISECONDS;
/**
* Wrapper around time related functions that are either implemented by using the default JVM calls
* or by using a custom implementation for testing purposes.
*
* See {@link #preciseTime} for how to use a custom implementation.
*
* Please note that {@link java.time.Clock} wasn't used, as it would not be possible to provide an
* implementation for {@link #now()} with the exact same properties of {@link System#nanoTime()}.
*/
public interface MonotonicClock
{
/**
* Static singleton object that will be instantiated by default with a system clock
* implementation. Set cassandra.clock system property to a FQCN to use a
* different implementation instead.
*/
public static final MonotonicClock preciseTime = Defaults.precise();
public static final MonotonicClock approxTime = Defaults.approx(preciseTime);
/**
* @see System#nanoTime()
*
* Provides a monotonic time that can be compared with any other such value produced by the same clock
* since the application started only; these times cannot be persisted or serialized to other nodes.
*
* Nanosecond precision.
*/
public long now();
/**
* @return nanoseconds of potential error
*/
public long error();
public MonotonicClockTranslation translate();
public boolean isAfter(long instant);
public boolean isAfter(long now, long instant);
static class Defaults
{
private static final Logger logger = LoggerFactory.getLogger(MonotonicClock.class);
private static MonotonicClock precise()
{
String sclock = System.getProperty("cassandra.clock");
if (sclock == null)
sclock = System.getProperty("cassandra.monotonic_clock.precise");
if (sclock != null)
{
try
{
logger.debug("Using custom clock implementation: {}", sclock);
return (MonotonicClock) Class.forName(sclock).newInstance();
}
catch (Exception e)
{
logger.error(e.getMessage(), e);
}
}
return new SystemClock();
}
private static MonotonicClock approx(MonotonicClock precise)
{
String sclock = System.getProperty("cassandra.monotonic_clock.approx");
if (sclock != null)
{
try
{
logger.debug("Using custom clock implementation: {}", sclock);
Class clazz = (Class) Class.forName(sclock);
if (SystemClock.class.equals(clazz) && SystemClock.class.equals(precise.getClass()))
return precise;
try
{
Constructor withPrecise = clazz.getConstructor(MonotonicClock.class);
return withPrecise.newInstance(precise);
}
catch (NoSuchMethodException nme)
{
}
return clazz.newInstance();
}
catch (Exception e)
{
logger.error(e.getMessage(), e);
}
}
return new SampledClock(precise);
}
}
static abstract class AbstractEpochSamplingClock implements MonotonicClock
{
private static final Logger logger = LoggerFactory.getLogger(AbstractEpochSamplingClock.class);
private static final String UPDATE_INTERVAL_PROPERTY = Config.PROPERTY_PREFIX + "NANOTIMETOMILLIS_TIMESTAMP_UPDATE_INTERVAL";
private static final long UPDATE_INTERVAL_MS = Long.getLong(UPDATE_INTERVAL_PROPERTY, 10000);
@VisibleForTesting
static class AlmostSameTime implements MonotonicClockTranslation
{
final long millisSinceEpoch;
final long monotonicNanos;
final long error; // maximum error of millis measurement (in nanos)
@VisibleForTesting
AlmostSameTime(long millisSinceEpoch, long monotonicNanos, long errorNanos)
{
this.millisSinceEpoch = millisSinceEpoch;
this.monotonicNanos = monotonicNanos;
this.error = errorNanos;
}
public long fromMillisSinceEpoch(long currentTimeMillis)
{
return monotonicNanos + MILLISECONDS.toNanos(currentTimeMillis - millisSinceEpoch);
}
public long toMillisSinceEpoch(long nanoTime)
{
return millisSinceEpoch + TimeUnit.NANOSECONDS.toMillis(nanoTime - monotonicNanos);
}
public long error()
{
return error;
}
}
final LongSupplier millisSinceEpoch;
private volatile AlmostSameTime almostSameTime = new AlmostSameTime(0L, 0L, Long.MAX_VALUE);
private Future almostSameTimeUpdater;
private static double failedAlmostSameTimeUpdateModifier = 1.0;
AbstractEpochSamplingClock(LongSupplier millisSinceEpoch)
{
this.millisSinceEpoch = millisSinceEpoch;
resumeEpochSampling();
}
public MonotonicClockTranslation translate()
{
return almostSameTime;
}
public synchronized void pauseEpochSampling()
{
if (almostSameTimeUpdater == null)
return;
almostSameTimeUpdater.cancel(true);
try { almostSameTimeUpdater.get(); } catch (Throwable t) { }
almostSameTimeUpdater = null;
}
public synchronized void resumeEpochSampling()
{
if (almostSameTimeUpdater != null)
throw new IllegalStateException("Already running");
updateAlmostSameTime();
logger.info("Scheduling approximate time conversion task with an interval of {} milliseconds", UPDATE_INTERVAL_MS);
almostSameTimeUpdater = ScheduledExecutors.scheduledFastTasks.scheduleWithFixedDelay(this::updateAlmostSameTime, UPDATE_INTERVAL_MS, UPDATE_INTERVAL_MS, MILLISECONDS);
}
private void updateAlmostSameTime()
{
final int tries = 3;
long[] samples = new long[2 * tries + 1];
samples[0] = System.nanoTime();
for (int i = 1 ; i < samples.length ; i += 2)
{
samples[i] = millisSinceEpoch.getAsLong();
samples[i + 1] = now();
}
int best = 1;
// take sample with minimum delta between calls
for (int i = 3 ; i < samples.length - 1 ; i += 2)
{
if ((samples[i+1] - samples[i-1]) < (samples[best+1]-samples[best-1]))
best = i;
}
long millis = samples[best];
long nanos = (samples[best+1] / 2) + (samples[best-1] / 2);
long error = (samples[best+1] / 2) - (samples[best-1] / 2);
AlmostSameTime prev = almostSameTime;
AlmostSameTime next = new AlmostSameTime(millis, nanos, error);
if (next.error > prev.error && next.error > prev.error * failedAlmostSameTimeUpdateModifier)
{
failedAlmostSameTimeUpdateModifier *= 1.1;
return;
}
failedAlmostSameTimeUpdateModifier = 1.0;
almostSameTime = next;
}
}
public static class SystemClock extends AbstractEpochSamplingClock
{
private SystemClock()
{
super(System::currentTimeMillis);
}
@Override
public long now()
{
return System.nanoTime();
}
@Override
public long error()
{
return 1;
}
@Override
public boolean isAfter(long instant)
{
return now() > instant;
}
@Override
public boolean isAfter(long now, long instant)
{
return now > instant;
}
}
public static class SampledClock implements MonotonicClock
{
private static final Logger logger = LoggerFactory.getLogger(SampledClock.class);
private static final int UPDATE_INTERVAL_MS = Math.max(1, Integer.parseInt(System.getProperty(Config.PROPERTY_PREFIX + "approximate_time_precision_ms", "2")));
private static final long ERROR_NANOS = MILLISECONDS.toNanos(UPDATE_INTERVAL_MS);
private final MonotonicClock precise;
private volatile long almostNow;
private Future almostNowUpdater;
public SampledClock(MonotonicClock precise)
{
this.precise = precise;
resumeNowSampling();
}
@Override
public long now()
{
return almostNow;
}
@Override
public long error()
{
return ERROR_NANOS;
}
@Override
public MonotonicClockTranslation translate()
{
return precise.translate();
}
@Override
public boolean isAfter(long instant)
{
return isAfter(almostNow, instant);
}
@Override
public boolean isAfter(long now, long instant)
{
return now - ERROR_NANOS > instant;
}
public synchronized void pauseNowSampling()
{
if (almostNowUpdater == null)
return;
almostNowUpdater.cancel(true);
try { almostNowUpdater.get(); } catch (Throwable t) { }
almostNowUpdater = null;
}
public synchronized void resumeNowSampling()
{
if (almostNowUpdater != null)
throw new IllegalStateException("Already running");
almostNow = precise.now();
logger.info("Scheduling approximate time-check task with a precision of {} milliseconds", UPDATE_INTERVAL_MS);
almostNowUpdater = ScheduledExecutors.scheduledFastTasks.scheduleWithFixedDelay(() -> almostNow = precise.now(), UPDATE_INTERVAL_MS, UPDATE_INTERVAL_MS, MILLISECONDS);
}
public synchronized void refreshNow()
{
pauseNowSampling();
resumeNowSampling();
}
}
}
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