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.nutch.storage.WebPage;

/**
 * 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 1 minute).</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 void setFetchSchedule(String url, WebPage page, long prevFetchTime,
      long prevModifiedTime, long fetchTime, long modifiedTime, int state) {
    super.setFetchSchedule(url, page, prevFetchTime, prevModifiedTime,
        fetchTime, modifiedTime, state);
    long refTime = fetchTime;
    if (modifiedTime <= 0)
      modifiedTime = fetchTime;
    int interval = page.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
      // TODO: different from normal class (is delta in seconds)?
      int delta = (int) ((fetchTime - modifiedTime) / 1000L);
      if (delta > interval)
        interval = delta;
      refTime = fetchTime - Math.round(delta * SYNC_DELTA_RATE);
    }
    if (interval < MIN_INTERVAL)
      interval = MIN_INTERVAL;
    if (interval > MAX_INTERVAL)
      interval = MAX_INTERVAL;

    page.setFetchInterval(interval);
    page.setFetchTime(refTime + interval * 1000L);
    page.setModifiedTime(modifiedTime);
    page.setPrevModifiedTime(prevModifiedTime);
  }

}