heron/api/src/java/com/twitter/heron/api/windowing/evictors/WatermarkCountEvictionPolicy.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.concurrent.atomic.AtomicLong;

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

/**
 * An eviction policy that tracks count based on watermark ts and
 * evicts events up to the watermark based on a threshold count.
 *
 * @param <T> the type of event tracked by this policy.
 */
public class WatermarkCountEvictionPolicy<T> implements EvictionPolicy<T> {
  protected final int threshold;
  protected final AtomicLong currentCount;
  private EvictionContext context;

  private volatile long processed;

  public WatermarkCountEvictionPolicy(int count) {
    threshold = count;
    currentCount = new AtomicLong();
  }

  public EvictionPolicy.Action evict(Event<T> event) {
    if (getContext() == 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;
    }

    Action action;
    if (event.getTimestamp() <= getContext().getReferenceTime() && processed < currentCount.get()) {
      action = doEvict(event);
      if (action == Action.PROCESS) {
        ++processed;
      }
    } else {
      action = Action.KEEP;
    }
    return action;
  }

  private Action doEvict(Event<T> event) {
        /*
         * atomically decrement the count if its greater than threshold and
         * return if the event should be evicted
         */
    while (true) {
      long curVal = currentCount.get();
      if (curVal > threshold) {
        if (currentCount.compareAndSet(curVal, curVal - 1)) {
          return Action.EXPIRE;
        }
      } else {
        break;
      }
    }
    return Action.PROCESS;
  }

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

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

  @Override
  public void setContext(EvictionContext context) {
    this.context = context;
    if (context.getCurrentCount() != null) {
      currentCount.set(context.getCurrentCount());
    } else {
      currentCount.set(processed + context.getSlidingCount());
    }
    processed = 0;
  }

  @Override
  public void reset() {
    processed = 0;
  }

  @Override
  public String toString() {
    return "WatermarkCountEvictionPolicy{" + "} " + super.toString();
  }
}