heron/api/src/java/com/twitter/heron/api/windowing/evictors/WatermarkTimeEvictionPolicy.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 com.twitter.heron.api.windowing.Event;

/**
 * An eviction policy that evicts events based on time duration taking
 * watermark time and event lag into account.
 */
public class WatermarkTimeEvictionPolicy<T> extends TimeEvictionPolicy<T> {
  private final long lag;

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

  /**
   * Constructs a WatermarkTimeEvictionPolicy that evicts events older
   * than the given window length in millis. The lag parameter
   * can be used in the case of event based ts to break the queue
   * scan early.
   *
   * @param windowLength the window length in milliseconds
   * @param lag the max event lag in milliseconds
   */
  public WatermarkTimeEvictionPolicy(long windowLength, long lag) {
    super(windowLength);
    this.lag = lag;
  }

  /**
   * {@inheritDoc}
   * <p/>
   * Keeps events with future ts in the queue for processing in the next
   * window. If the ts difference is more than the lag, stops scanning
   * the queue for the current window.
   */
  @Override
  public Action evict(Event<T> event) {
    if (evictionContext == null) {
      //It is possible to get asked about eviction before we have a context, due to WindowManager
      // .compactWindow.
      //In this case we should hold on to all the events. When the first watermark is received,
      // the context will be set,
      //and the events will be reevaluated for eviction
      return Action.STOP;
    }

    long referenceTime = evictionContext.getReferenceTime();
    long diff = referenceTime - event.getTimestamp();
    if (diff < -lag) {
      return Action.STOP;
    } else if (diff < 0) {
      return Action.KEEP;
    } else {
      return super.evict(event);
    }
  }

  @Override
  public String toString() {
    return "WatermarkTimeEvictionPolicy{" + "lag=" + lag + "} " + super.toString();
  }

}