AdaptiveFetchSchedule.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.nutch.crawl;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.io.Text;
import org.apache.nutch.crawl.CrawlDatum;
import org.apache.nutch.util.NutchConfiguration;

/**
 * This class implements an adaptive re-fetch algorithm. This works as follows:
 * <ul>
 * <li>for pages that has changed since the last fetchTime, decrease their
 * fetchInterval by a factor of DEC_FACTOR (default value is 0.2f).</li>
 * <li>for pages that haven't changed since the last fetchTime, increase their
 * fetchInterval by a factor of INC_FACTOR (default value is 0.2f).<br>
 * If SYNC_DELTA property is true, then:
 * <ul>
 * <li>calculate a <code>delta = fetchTime - modifiedTime</code></li>
 * <li>try to synchronize with the time of change, by shifting the next fetchTime
 * by a fraction of the difference between the last modification time and the last
 * fetch time. I.e. the next fetch time will be set to
 * <code>fetchTime + fetchInterval - delta * SYNC_DELTA_RATE</code></li>
 * <li>if the adjusted fetch interval is bigger than the delta, then <code>fetchInterval = delta</code>.</li>
 * </ul>
 * </li>
 * <li>the minimum value of fetchInterval may not be smaller than MIN_INTERVAL
 * (default is 1s).</li>
 * <li>the maximum value of fetchInterval may not be bigger than MAX_INTERVAL
 * (default is 365 days).</li>
 * </ul>
 * <p>NOTE: values of DEC_FACTOR and INC_FACTOR higher than 0.4f may destabilize the algorithm,
 * so that the fetch interval either increases or decreases infinitely, with little
 * relevance to the page changes. Please use {@link #main(String[])} method to
 * test the values before applying them in a production system.</p>
 * 
 * @author Andrzej Bialecki
 */
public class AdaptiveFetchSchedule extends AbstractFetchSchedule {

  private float INC_RATE;

  private float DEC_RATE;

  private int MAX_INTERVAL;

  private int MIN_INTERVAL;
  
  private boolean SYNC_DELTA;

  private double SYNC_DELTA_RATE;
  
  public void setConf(Configuration conf) {
    super.setConf(conf);
    if (conf == null) return;
    INC_RATE = conf.getFloat("db.fetch.schedule.adaptive.inc_rate", 0.2f);
    DEC_RATE = conf.getFloat("db.fetch.schedule.adaptive.dec_rate", 0.2f);
    MIN_INTERVAL = conf.getInt("db.fetch.schedule.adaptive.min_interval", 60);
    MAX_INTERVAL = conf.getInt("db.fetch.schedule.adaptive.max_interval", SECONDS_PER_DAY * 365 ); // 1 year
    SYNC_DELTA = conf.getBoolean("db.fetch.schedule.adaptive.sync_delta", true);
    SYNC_DELTA_RATE = conf.getFloat("db.fetch.schedule.adaptive.sync_delta_rate", 0.2f);
  }

  @Override
  public CrawlDatum setFetchSchedule(Text url, CrawlDatum datum,
          long prevFetchTime, long prevModifiedTime,
          long fetchTime, long modifiedTime, int state) {
    super.setFetchSchedule(url, datum, prevFetchTime, prevModifiedTime,
        fetchTime, modifiedTime, state);
    long refTime = fetchTime;
    if (modifiedTime <= 0) modifiedTime = fetchTime;
    float interval = datum.getFetchInterval();
    switch (state) {
      case FetchSchedule.STATUS_MODIFIED:
        interval *= (1.0f - DEC_RATE);
        break;
      case FetchSchedule.STATUS_NOTMODIFIED:
        interval *= (1.0f + INC_RATE);
        break;
      case FetchSchedule.STATUS_UNKNOWN:
        break;
    }
    if (SYNC_DELTA) {
      // try to synchronize with the time of change
      long delta = (fetchTime - modifiedTime) / 1000L;
      if (delta > interval) interval = delta;
      refTime = fetchTime - Math.round(delta * SYNC_DELTA_RATE * 1000);
    }
    if (interval < MIN_INTERVAL) {
      interval = MIN_INTERVAL;
    } else if (interval > MAX_INTERVAL) {
      interval = MAX_INTERVAL;
    }
    datum.setFetchInterval(interval);
    datum.setFetchTime(refTime + Math.round(interval * 1000.0));
    datum.setModifiedTime(modifiedTime);
    return datum;
  }

  public static void main(String[] args) throws Exception {
    FetchSchedule fs = new AdaptiveFetchSchedule();
    fs.setConf(NutchConfiguration.create());
    // we start the time at 0, for simplicity
    long curTime = 0;
    long delta = 1000L * 3600L * 24L; // 2 hours
    // we trigger the update of the page every 30 days
    long update = 1000L * 3600L * 24L * 30L; // 30 days
    boolean changed = true;
    long lastModified = 0;
    int miss = 0;
    int totalMiss = 0;
    int maxMiss = 0;
    int fetchCnt = 0;
    int changeCnt = 0;
    // initial fetchInterval is 10 days
    CrawlDatum p = new CrawlDatum(1, 3600 * 24 * 30, 1.0f);
    p.setFetchTime(0);
    System.out.println(p);
    // let's move the timeline a couple of deltas
    for (int i = 0; i < 10000; i++) {
      if (lastModified + update < curTime) {
        //System.out.println("i=" + i + ", lastModified=" + lastModified + ", update=" + update + ", curTime=" + curTime);
        changed = true;
        changeCnt++;
        lastModified = curTime;
      }
      System.out.println(i + ". " + changed + "\twill fetch at " + (p.getFetchTime() / delta) + "\tinterval "
              + (p.getFetchInterval() / SECONDS_PER_DAY ) + " days" + "\t missed " + miss);
      if (p.getFetchTime() <= curTime) {
        fetchCnt++;
        fs.setFetchSchedule(new Text("http://www.example.com"), p,
                p.getFetchTime(), p.getModifiedTime(), curTime, lastModified,
                changed ? FetchSchedule.STATUS_MODIFIED : FetchSchedule.STATUS_NOTMODIFIED);
        System.out.println("\tfetched & adjusted: " + "\twill fetch at " + (p.getFetchTime() / delta) + "\tinterval "
                + (p.getFetchInterval() / SECONDS_PER_DAY ) + " days");
        if (!changed) miss++;
        if (miss > maxMiss) maxMiss = miss;
        changed = false;
        totalMiss += miss;
        miss = 0;
      }
      if (changed) miss++;
      curTime += delta;
    }
    System.out.println("Total missed: " + totalMiss + ", max miss: " + maxMiss);
    System.out.println("Page changed " + changeCnt + " times, fetched " + fetchCnt + " times.");
  }
}