com.datastax.driver.core.Clock Maven / Gradle / Ivy

/*
 * Copyright DataStax, Inc.
 *
 * Licensed 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 com.datastax.driver.core;

import static java.util.concurrent.TimeUnit.MILLISECONDS;
import static java.util.concurrent.TimeUnit.NANOSECONDS;
import static java.util.concurrent.TimeUnit.SECONDS;

import java.util.concurrent.atomic.AtomicReference;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * A small abstraction around system clock that aims to provide microsecond precision with the best
 * accuracy possible.
 */
interface Clock {

  /**
   * Returns the current time in microseconds.
   *
   * @return the difference, measured in microseconds, between the current time and and the Epoch
   *     (that is, midnight, January 1, 1970 UTC).
   */
  long currentTimeMicros();
}

/**
 * Factory that returns the best Clock implementation depending on what native libraries are
 * available in the system. If LibC is available through JNR, and if the system property {@code
 * com.datastax.driver.USE_NATIVE_CLOCK} is set to {@code true} (which is the default value), then
 * {@link NativeClock} is returned, otherwise {@link SystemClock} is returned.
 */
class ClockFactory {

  private static final Logger LOGGER = LoggerFactory.getLogger(ClockFactory.class);

  private static final String USE_NATIVE_CLOCK_SYSTEM_PROPERTY =
      "com.datastax.driver.USE_NATIVE_CLOCK";

  static Clock newInstance() {
    if (SystemProperties.getBoolean(USE_NATIVE_CLOCK_SYSTEM_PROPERTY, true)
        && Native.isGettimeofdayAvailable()) {
      LOGGER.info("Using native clock to generate timestamps.");
      return new NativeClock();
    } else {
      LOGGER.info("Using java.lang.System clock to generate timestamps.");
      return new SystemClock();
    }
  }
}

/**
 * Default implementation of a clock that delegates its calls to the system clock.
 *
 * @see System#currentTimeMillis()
 */
class SystemClock implements Clock {

  @Override
  public long currentTimeMicros() {
    return System.currentTimeMillis() * 1000;
  }
}

/**
 * Provides the current time with microseconds precision with some reasonable accuracy through the
 * use of {@link Native#currentTimeMicros()}.
 *
 * Because calling JNR methods is slightly expensive, we only call it once per second and add the
 * number of nanoseconds since the last call to get the current time, which is good enough an
 * accuracy for our purpose (see CASSANDRA-6106).
 *
 * This reduces the cost of the call to {@link NativeClock#currentTimeMicros()} to levels
 * comparable to those of a call to {@link System#nanoTime()}.
 */
class NativeClock implements Clock {

  private static final long ONE_SECOND_NS = NANOSECONDS.convert(1, SECONDS);
  private static final long ONE_MILLISECOND_NS = NANOSECONDS.convert(1, MILLISECONDS);

  /**
   * Records a time in micros along with the System.nanoTime() value at the time the time is
   * fetched.
   */
  private static class FetchedTime {

    private final long timeInMicros;
    private final long nanoTimeAtCheck;

    private FetchedTime(long timeInMicros, long nanoTimeAtCheck) {
      this.timeInMicros = timeInMicros;
      this.nanoTimeAtCheck = nanoTimeAtCheck;
    }
  }

  private final AtomicReference lastFetchedTime =
      new AtomicReference(fetchTimeMicros());

  @Override
  public long currentTimeMicros() {
    FetchedTime spec = lastFetchedTime.get();
    long curNano = System.nanoTime();
    if (curNano > spec.nanoTimeAtCheck + ONE_SECOND_NS) {
      lastFetchedTime.compareAndSet(spec, spec = fetchTimeMicros());
    }
    return spec.timeInMicros + ((curNano - spec.nanoTimeAtCheck) / 1000);
  }

  private static FetchedTime fetchTimeMicros() {
    // To compensate for the fact that the Native.currentTimeMicros call could take
    // some time, instead of picking the nano time before the call or after the
    // call, we take the average of both.
    long start = System.nanoTime();
    long micros = Native.currentTimeMicros();
    long end = System.nanoTime();
    // If it turns out the call took us more than 1 millisecond (can happen while
    // the JVM warms up, unlikely otherwise, but no reasons to take risks), fall back
    // to System.currentTimeMillis() temporarily
    if ((end - start) > ONE_MILLISECOND_NS)
      return new FetchedTime(System.currentTimeMillis() * 1000, System.nanoTime());
    return new FetchedTime(micros, (end + start) / 2);
  }
}