heron/api/src/java/com/twitter/heron/api/windowing/EvictionPolicy.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;

/**
 * Eviction policy tracks events and decides whether
 * an event should be evicted from the window or not.
 *
 * @param <T> the type of event that is tracked.
 */
public interface EvictionPolicy<T> {
  /**
   * The action to be taken when {@link EvictionPolicy#evict(Event)} is invoked.
   */
  enum Action {
    /**
     * expire the event and remove it from the queue.
     */
    EXPIRE, /**
     * process the event in the current window of events.
     */
    PROCESS, /**
     * don't include in the current window but keep the event
     * in the queue for evaluating as a part of future windows.
     */
    KEEP, /**
     * stop processing the queue, there cannot be anymore events
     * satisfying the eviction policy.
     */
    STOP
  }

  /**
   * Decides if an event should be expired from the window, processed in the current
   * window or kept for later processing.
   *
   * @param event the input event
   * @return the {@link EvictionPolicy.Action} to be taken based on the input event
   */
  Action evict(Event<T> event);

  /**
   * Tracks the event to later decide whether
   * {@link EvictionPolicy#evict(Event)} should evict it or not.
   *
   * @param event the input event to be tracked
   */
  void track(Event<T> event);

  /**
   * Sets a context in the eviction policy that can be used while evicting the events.
   * E.g. For TimeEvictionPolicy, this could be used to set the reference timestamp.
   *
   * @param context the eviction context
   */
  void setContext(EvictionContext context);

  /**
   * Returns the current context that is part of this eviction policy.
   *
   * @return the eviction context
   */
  EvictionContext getContext();

  /**
   * Resets the eviction policy.
   */
  void reset();
}