org.apache.nutch.crawl

## Class AbstractFetchSchedule

- java.lang.Object
- - org.apache.hadoop.conf.Configured
  - - org.apache.nutch.crawl.AbstractFetchSchedule

- All Implemented Interfaces:
    Configurable, FetchSchedule
  Direct Known Subclasses:
    AdaptiveFetchSchedule, DefaultFetchSchedule

      public abstract class AbstractFetchSchedule
      extends Configured
      implements FetchSchedule

  This class provides common methods for implementations of `FetchSchedule`.
  Author:
    Andrzej Bialecki

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected int` | `defaultInterval`
    `protected int` | `maxInterval`

    - ### Fields inherited from interface org.apache.nutch.crawl.FetchSchedule

      `SECONDS_PER_DAY, STATUS_MODIFIED, STATUS_NOTMODIFIED, STATUS_UNKNOWN`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `AbstractFetchSchedule()`
    `AbstractFetchSchedule(Configuration conf)`

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `long` | `calculateLastFetchTime(CrawlDatum datum)` This method return the last fetch time of the CrawlDatum
    `CrawlDatum` | `forceRefetch(Text url, CrawlDatum datum, boolean asap)` This method resets fetchTime, fetchInterval, modifiedTime, retriesSinceFetch and page signature, so that it forces refetching.
    `CrawlDatum` | `initializeSchedule(Text url, CrawlDatum datum)` Initialize fetch schedule related data.
    `void` | `setConf(Configuration conf)`
    `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 class org.apache.hadoop.conf.Configured

      `getConf`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

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

      `getConf`

- - ### Field Detail

    - #### defaultInterval

          protected int defaultInterval

    - #### maxInterval

          protected int maxInterval

  - ### Constructor Detail

    - #### AbstractFetchSchedule

          public AbstractFetchSchedule()

    - #### AbstractFetchSchedule

          public AbstractFetchSchedule(Configuration conf)

  - ### Method Detail

    - #### setConf

          public void setConf(Configuration conf)

      Specified by:
        `setConf` in interface `Configurable`
      Overrides:
        `setConf` in class `Configured`

    - #### initializeSchedule

          public CrawlDatum initializeSchedule(Text url,
                                               CrawlDatum datum)

      Initialize fetch schedule related data. Implementations should at least set the `fetchTime` and `fetchInterval`. The default implementation sets the `fetchTime` to now, using the default `fetchInterval`.
      Specified by:
        `initializeSchedule` in interface `FetchSchedule`
      Parameters:
        `url` - URL of the page.
        `datum` - datum instance to be initialized (modified in place).
      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

          public 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. NOTE: this implementation resets the retry counter - extending classes should call super.setFetchSchedule() to preserve this behavior.
      Specified by:
        `setFetchSchedule` in interface `FetchSchedule`
      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}.

    - #### setPageGoneSchedule

          public 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% but the value may never exceed `maxInterval`.
      Specified by:
        `setPageGoneSchedule` in interface `FetchSchedule`
      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

          public 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.
      Specified by:
        `setPageRetrySchedule` in interface `FetchSchedule`
      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

          public long calculateLastFetchTime(CrawlDatum datum)

      This method return the last fetch time of the CrawlDatum
      Specified by:
        `calculateLastFetchTime` in interface `FetchSchedule`
      Returns:
        the date as a long.

    - #### shouldFetch

          public 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.
      Specified by:
        `shouldFetch` in interface `FetchSchedule`
      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

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

      This method resets fetchTime, fetchInterval, modifiedTime, retriesSinceFetch and page signature, so that it forces refetching.
      Specified by:
        `forceRefetch` in interface `FetchSchedule`
      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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