storm-client/src/jvm/org/apache/storm/windowing/CountEvictionPolicy.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;

import java.util.concurrent.atomic.AtomicLong;

/**
 * An eviction policy that tracks event counts and can evict based on a threshold count.
 *
 * @param <T> the type of event tracked by this policy.
 */
public class CountEvictionPolicy<T> implements EvictionPolicy<T, Long> {
    protected final int threshold;
    protected final AtomicLong currentCount;
    private EvictionContext context;

    public CountEvictionPolicy(int count) {
        this.threshold = count;
        this.currentCount = new AtomicLong();
    }

    @Override
    public Action evict(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) {
        if (!event.isWatermark()) {
            currentCount.incrementAndGet();
        }
    }

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

    @Override
    public void setContext(EvictionContext context) {
        this.context = context;
    }

    @Override
    public String toString() {
        return "CountEvictionPolicy{"
                + "threshold=" + threshold
                + ", currentCount=" + currentCount
                + '}';
    }

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

    @Override
    public Long getState() {
        return currentCount.get();
    }

    @Override
    public void restoreState(Long state) {
        currentCount.set(state);
    }
}