io.seldon.memcache.DogpileHandler.java Source code

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Here is the source code for io.seldon.memcache.DogpileHandler.java

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/*
 * Seldon -- open source prediction engine
 * =======================================
 *
 * Copyright 2011-2015 Seldon Technologies Ltd and Rummble Ltd (http://www.seldon.io/)
 *
 * ********************************************************************************************
 *
 * 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 io.seldon.memcache;

import org.apache.log4j.Logger;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

import java.util.concurrent.ConcurrentHashMap;

@Component
public class DogpileHandler {
    private static Logger logger = Logger.getLogger(DogpileHandler.class.getName());

    private static final float DEF_EXPIRE_FACTOR = 0.75f;
    public static final String DOGPILE_EXPIRE_PROP = "io.seldon.dogpile.expire.factor";
    public static final String DOGPILE_ACTIVE = "io.seldon.dogpile.active";

    private static DogpileHandler peer;

    @Autowired
    public DogpileHandler(@Value("${io.seldon.dogpile.expire.factor:0.75}") String expireStr,
            @Value("${io.seldon.dogpile.active:true}") String activeStr) {
        expireFactor = Float.parseFloat(expireStr);
        if ("true".equals(activeStr))
            active = true;
        logger.info("Creating dogpile cache handler with active: " + active + " and expire factor " + expireFactor);
        peer = this;
    }

    public static DogpileHandler get() {
        return peer;
    }

    ConcurrentHashMap<String, Long> expireMap = new ConcurrentHashMap<>();
    ConcurrentHashMap<String, Boolean> updatingMap = new ConcurrentHashMap<>();
    float expireFactor;
    boolean active = false;

    private DogpileHandler(float expireFactor, boolean active) {
        this.expireFactor = expireFactor;
        this.active = active;
    }

    private boolean setUpdating(String key) {
        if (updatingMap.putIfAbsent(key, true) == null) {
            // if there was not updating flag then return true to tell caller an update is needed
            //
            // There is a possibility of a race condition of 1 (or more) thread checks expire then gets stopped and another updates
            // then first (or more) thread(s) comes back in and set updateingMap successfully. However this should only happen under extreme load
            // and will cause a few extra updates
            logger.info("Returning true for memcache key " + key);
            return true;
        } else
            return false; // as another thread is updating the value
    }

    public <T> T retrieveUpdateIfRequired(String key, T cachedItem, UpdateRetriever<T> retriever, int expireSecs)
            throws Exception {

        if (updateIsRequired(key, cachedItem, expireSecs)) {
            try {
                return retriever.retrieve();
            } finally {
                updated(key, expireSecs);
            }
        } else {
            return null;
        }
    }

    public <T> boolean updateIsRequired(String key, T cachedItem, int expireSecs) {
        boolean continueWithUpdate = false;
        if (key == null || !active)
            return false;
        if (cachedItem == null) {
            continueWithUpdate = setUpdating(key);
        } else {
            Long expire = expireMap.get(key);
            if (expire == null || (expire != null && System.currentTimeMillis() > expire)) {
                continueWithUpdate = setUpdating(key);
            }
        }
        return continueWithUpdate;
    }

    public void updated(String key, int expireSecs) {
        if (!active)
            return;
        logger.info("Updating key " + key);
        int expireDeltaMilisecs = Math.round((expireSecs * 1000) * expireFactor);
        expireMap.put(key, System.currentTimeMillis() + expireDeltaMilisecs);
        updatingMap.remove(key);
    }

    public void clear(String key) {
        expireMap.remove(key);
        updatingMap.remove(key);
    }
}