org.apache.nutch.crawl

## Interface FetchSchedule

- All Superinterfaces:
    Configurable
  All Known Implementing Classes:
    AbstractFetchSchedule, AdaptiveFetchSchedule, DefaultFetchSchedule, MimeAdaptiveFetchSchedule

      public interface FetchSchedule
      extends Configurable

  This interface defines the contract for implementations that manipulate fetch times and re-fetch intervals.
  Author:
    Andrzej Bialecki

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static int` | `SECONDS_PER_DAY`
    `static int` | `STATUS_MODIFIED` Page is known to have been modified since our last visit.
    `static int` | `STATUS_NOTMODIFIED` Page is known to remain unmodified since our last visit.
    `static int` | `STATUS_UNKNOWN` It is unknown whether page was changed since our last visit.

  - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method and Description
    --- | ---
    `long` | `calculateLastFetchTime(CrawlDatum datum)` Calculates last fetch time of the given CrawlDatum.
    `CrawlDatum` | `forceRefetch(Text url, CrawlDatum datum, boolean asap)` This method resets fetchTime, fetchInterval, modifiedTime and page signature, so that it forces refetching.
    `CrawlDatum` | `initializeSchedule(Text url, CrawlDatum datum)` Initialize fetch schedule related data.
    `CrawlDatum` | `setFetchSchedule(Text url, CrawlDatum datum, long prevFetchTime, long prevModifiedTime, long fetchTime, long modifiedTime, int state)` Sets the `fetchInterval` and `fetchTime` on a successfully fetched page.
    `CrawlDatum` | `setPageGoneSchedule(Text url, CrawlDatum datum, long prevFetchTime, long prevModifiedTime, long fetchTime)` This method specifies how to schedule refetching of pages marked as GONE.
    `CrawlDatum` | `setPageRetrySchedule(Text url, CrawlDatum datum, long prevFetchTime, long prevModifiedTime, long fetchTime)` This method adjusts the fetch schedule if fetching needs to be re-tried due to transient errors.
    `boolean` | `shouldFetch(Text url, CrawlDatum datum, long curTime)` This method provides information whether the page is suitable for selection in the current fetchlist.

    - ### Methods inherited from interface org.apache.hadoop.conf.Configurable

      `getConf, setConf`

- - ### Field Detail

    - #### STATUS_UNKNOWN

          static final int STATUS_UNKNOWN

      It is unknown whether page was changed since our last visit.
      See Also:
        Constant Field Values

    - #### STATUS_MODIFIED

          static final int STATUS_MODIFIED

      Page is known to have been modified since our last visit.
      See Also:
        Constant Field Values

    - #### STATUS_NOTMODIFIED

          static final int STATUS_NOTMODIFIED

      Page is known to remain unmodified since our last visit.
      See Also:
        Constant Field Values

    - #### SECONDS_PER_DAY

          static final int SECONDS_PER_DAY

      See Also:
        Constant Field Values

  - ### Method Detail

    - #### initializeSchedule

          CrawlDatum initializeSchedule(Text url,
                                        CrawlDatum datum)

      Initialize fetch schedule related data. Implementations should at least set the `fetchTime` and `fetchInterval`. The default implementation set the `fetchTime` to now, using the default `fetchInterval`.
      Parameters:
        `url` - URL of the page.
        `datum` - datum instance to be initialized.
      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.

    - #### setFetchSchedule

          CrawlDatum setFetchSchedule(Text url,
                                      CrawlDatum datum,
                                      long prevFetchTime,
                                      long prevModifiedTime,
                                      long fetchTime,
                                      long modifiedTime,
                                      int state)

      Sets the `fetchInterval` and `fetchTime` on a successfully fetched page. Implementations may use supplied arguments to support different re-fetching schedules.
      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 `STATUS_MODIFIED`, then the content is considered to be "changed" before the `fetchTime`, if `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 `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}.

    - #### setPageGoneSchedule

          CrawlDatum setPageGoneSchedule(Text url,
                                         CrawlDatum datum,
                                         long prevFetchTime,
                                         long prevModifiedTime,
                                         long fetchTime)

      This method specifies how to schedule refetching of pages marked as GONE. Default implementation increases fetchInterval by 50%, and if it exceeds the `maxInterval` it calls `forceRefetch(Text, CrawlDatum, boolean)`.
      Parameters:
        `url` - URL of the page
        `datum` - datum instance to be adjusted.
      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.

    - #### setPageRetrySchedule

          CrawlDatum setPageRetrySchedule(Text url,
                                          CrawlDatum datum,
                                          long prevFetchTime,
                                          long prevModifiedTime,
                                          long fetchTime)

      This method adjusts the fetch schedule if fetching needs to be re-tried due to transient errors. The default implementation sets the next fetch time 1 day in the future and increases the retry counter.
      Parameters:
        `url` - URL of the page.
        `datum` - page information.
        `prevFetchTime` - previous fetch time.
        `prevModifiedTime` - previous modified time.
        `fetchTime` - current fetch time.
      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.

    - #### calculateLastFetchTime

          long calculateLastFetchTime(CrawlDatum datum)

      Calculates last fetch time of the given CrawlDatum.
      Returns:
        the date as a long.

    - #### shouldFetch

          boolean shouldFetch(Text url,
                              CrawlDatum datum,
                              long curTime)

      This method provides information whether the page is suitable for selection in the current fetchlist. NOTE: a true return value does not guarantee that the page will be fetched, it just allows it to be included in the further selection process based on scores. The default implementation checks `fetchTime`, if it is higher than the curTime it returns false, and true otherwise. It will also check that fetchTime is not too remote (more than `maxInterval`), in which case it lowers the interval and returns true.
      Parameters:
        `url` - URL of the page.
        `datum` - datum instance.
        `curTime` - reference time (usually set to the time when the fetchlist generation process was started).
      Returns:
        true, if the page should be considered for inclusion in the current fetchlist, otherwise false.

    - #### forceRefetch

          CrawlDatum forceRefetch(Text url,
                                  CrawlDatum datum,
                                  boolean asap)

      This method resets fetchTime, fetchInterval, modifiedTime and page signature, so that it forces refetching.
      Parameters:
        `url` - URL of the page.
        `datum` - datum instance.
        `asap` - if true, force refetch as soon as possible - this sets the fetchTime to now. If false, force refetch whenever the next fetch time is set.
      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.

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