storm-core/src/jvm/org/apache/storm/windowing/WatermarkTimeEvictionPolicy.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.apache.storm.windowing;

/**
 * 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 int lag;

    /**
     * Constructs a WatermarkTimeEvictionPolicy that evicts events older
     * than the given window length in millis.
     *
     * @param windowLength the window length in milliseconds
     */
    public WatermarkTimeEvictionPolicy(int windowLength) {
        this(windowLength, Integer.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(int windowLength, int lag) {
        super(windowLength);
        referenceTime = 0L;
        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) {
        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();
    }
}