Package org.apache.nutch.crawl

Class AdaptiveFetchSchedule

  • All Implemented Interfaces:
    Configurable, FetchSchedule
    Direct Known Subclasses:
    MimeAdaptiveFetchSchedule
    public class AdaptiveFetchSchedule
extends AbstractFetchSchedule
    This class implements an adaptive re-fetch algorithm. This works as follows:
    • for pages that has changed since the last fetchTime, decrease their fetchInterval by a factor of DEC_FACTOR (default value is 0.2f).
    • for pages that haven't changed since the last fetchTime, increase their fetchInterval by a factor of INC_FACTOR (default value is 0.2f).
      If SYNC_DELTA property is true, then:
      • calculate a delta = fetchTime - modifiedTime
      • 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 fetchTime + fetchInterval - delta * SYNC_DELTA_RATE
      • if the adjusted fetch interval is bigger than the delta, then fetchInterval = delta.
    • the minimum value of fetchInterval may not be smaller than MIN_INTERVAL (default is 1 minute).
    • the maximum value of fetchInterval may not be bigger than MAX_INTERVAL (default is 365 days).

    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 main(String[]) method to test the values before applying them in a production system.

    The class also allows specifying custom min. and max. re-fetch intervals per hostname, in adaptive-host-specific-intervals.txt. If they are specified, the calculated re-fetch interval for a URL matching the hostname will not be allowed to fall outside of the corresponding range, instead of the default range.
    Author:
    Andrzej Bialecki

    • Field Detail

      • INC_RATE

        protected float INC_RATE

      • DEC_RATE

        protected float DEC_RATE

    • Constructor Detail

      • AdaptiveFetchSchedule

        public AdaptiveFetchSchedule()

    • Method Detail

      • getHostName

        public static String getHostName​(String url)
                          throws URISyntaxException
        Strip a URL, leaving only the hostname.
        Parameters:
        url - the URL for which to get the hostname
        Returns:
        hostname
        Throws:
        URISyntaxException - if the given string violates RFC 2396

      • getCustomMaxInterval

        public Float getCustomMaxInterval​(Text url)
        Returns the custom max. refetch interval for this URL, if specified for the corresponding hostname.
        Parameters:
        url - the URL to be scheduled
        Returns:
        the configured max. interval or null

      • getCustomMinInterval

        public Float getCustomMinInterval​(Text url)
        Returns the custom min. refetch interval for this URL, if specified for the corresponding hostname.
        Parameters:
        url - the URL to be scheduled
        Returns:
        the configured min. interval or null

      • setFetchSchedule

        public CrawlDatum setFetchSchedule​(Text url,
                                   CrawlDatum datum,
                                   long prevFetchTime,
                                   long prevModifiedTime,
                                   long fetchTime,
                                   long modifiedTime,
                                   int state)
        Description copied from class: AbstractFetchSchedule
        Sets the fetchInterval and fetchTime on a successfully fetched page. NOTE: this implementation resets the retry counter - extending classes should call super.setFetchSchedule() to preserve this behavior.
        Specified by:
        setFetchSchedule in interface FetchSchedule
        Overrides:
        setFetchSchedule in class AbstractFetchSchedule
        Parameters:
        url - url of the page
        datum - page description to be adjusted. NOTE: this instance, passed by reference, may be modified inside the method.
        prevFetchTime - previous value of fetch time, or 0 if not available.
        prevModifiedTime - previous value of modifiedTime, or 0 if not available.
        fetchTime - the latest time, when the page was recently re-fetched. Most FetchSchedule implementations should update the value in @see CrawlDatum to something greater than this value.
        modifiedTime - last time the content was modified. This information comes from the protocol implementations, or is set to < 0 if not available. Most FetchSchedule implementations should update the value in @see CrawlDatum to this value.
        state - if FetchSchedule.STATUS_MODIFIED, then the content is considered to be "changed" before the fetchTime, if FetchSchedule.STATUS_NOTMODIFIED then the content is known to be unchanged. This information may be obtained by comparing page signatures before and after fetching. If this is set to FetchSchedule.STATUS_UNKNOWN, then it is unknown whether the page was changed; implementations are free to follow a sensible default behavior.
        Returns:
        adjusted page information, including all original information. NOTE: this may be a different instance than @see CrawlDatum, but implementations should make sure that it contains at least all information from @see CrawlDatum}.