heron/api/src/java/com/twitter/heron/api/windowing/evictors/TimeEvictionPolicy.java

//  Copyright 2017 Twitter. All rights reserved.
//
//  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.twitter.heron.api.windowing.evictors;

import java.util.logging.Logger;

import com.twitter.heron.api.windowing.Event;
import com.twitter.heron.api.windowing.EvictionContext;
import com.twitter.heron.api.windowing.EvictionPolicy;

/**
 * Eviction policy that evicts events based on time duration.
 */
public class TimeEvictionPolicy<T> implements EvictionPolicy<T> {
  private static final Logger LOG = Logger.getLogger(TimeEvictionPolicy.class.getName());

  private final long windowLength;
  protected volatile EvictionContext evictionContext;
  private long delta;

  /**
   * Constructs a TimeEvictionPolicy that evicts events older
   * than the given window length in millis
   *
   * @param windowLength the duration in milliseconds
   */
  public TimeEvictionPolicy(long windowLength) {
    this.windowLength = windowLength;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public EvictionPolicy.Action evict(Event<T> event) {
    long now =
        evictionContext == null ? System.currentTimeMillis() : evictionContext.getReferenceTime();
    long diff = now - event.getTimestamp();
    if (diff >= (windowLength + delta)) {
      return Action.EXPIRE;
    } else if (diff < 0) { // do not process events beyond current ts
      return Action.KEEP;
    }
    return Action.PROCESS;
  }

  @Override
  public void track(Event<T> event) {
    // NOOP
  }

  @Override
  public void setContext(EvictionContext context) {
    EvictionContext prevContext = evictionContext;
    evictionContext = context;
    // compute window length adjustment (delta) to account for time drift
    if (context.getSlidingInterval() != null) {
      if (prevContext == null) {
        delta = Integer.MAX_VALUE; // consider all events for the initial window
      } else {
        delta = context.getReferenceTime() - prevContext.getReferenceTime()
            - context.getSlidingInterval();
        if (Math.abs(delta) > 100) {
          LOG.warning(String.format("Possible clock drift or long running computation in window; "
              + "Previous eviction time: %s, current eviction time: %s", prevContext
              .getReferenceTime(), context.getReferenceTime()));
        }
      }
    }
  }

  @Override
  public EvictionContext getContext() {
    return evictionContext;
  }

  @Override
  public void reset() {
    // NOOP
  }

  @Override
  public String toString() {
    return "TimeEvictionPolicy{" + "windowLength=" + windowLength + ", evictionContext="
        + evictionContext + '}';
  }
}