org.apache.nutch.crawl

Class AbstractFetchSchedule

java.lang.Object

org.apache.hadoop.conf.Configured

org.apache.nutch.crawl.AbstractFetchSchedule

All Implemented Interfaces:

org.apache.hadoop.conf.Configurable, FetchSchedule

Direct Known Subclasses:

AdaptiveFetchSchedule, DefaultFetchSchedule

public abstract class AbstractFetchSchedule
extends org.apache.hadoop.conf.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(org.apache.hadoop.conf.Configuration conf)

Method Summary

Methods

Modifier and Type	Method and Description
long	calculateLastFetchTime(CrawlDatum datum) This method return the last fetch time of the CrawlDatum
CrawlDatum	forceRefetch(org.apache.hadoop.io.Text url, CrawlDatum datum, boolean asap) This method resets fetchTime, fetchInterval, modifiedTime, retriesSinceFetch and page signature, so that it forces refetching.
CrawlDatum	initializeSchedule(org.apache.hadoop.io.Text url, CrawlDatum datum) Initialize fetch schedule related data.
void	setConf(org.apache.hadoop.conf.Configuration conf)
CrawlDatum	setFetchSchedule(org.apache.hadoop.io.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(org.apache.hadoop.io.Text url, CrawlDatum datum, long prevFetchTime, long prevModifiedTime, long fetchTime) This method specifies how to schedule refetching of pages marked as GONE.
CrawlDatum	setPageRetrySchedule(org.apache.hadoop.io.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(org.apache.hadoop.io.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(org.apache.hadoop.conf.Configuration conf)

Method Detail

setConf

public void setConf(org.apache.hadoop.conf.Configuration conf)

Specified by:

setConf in interface org.apache.hadoop.conf.Configurable

Overrides:

setConf in class org.apache.hadoop.conf.Configured

initializeSchedule

public CrawlDatum initializeSchedule(org.apache.hadoop.io.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(org.apache.hadoop.io.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(org.apache.hadoop.io.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(org.apache.hadoop.io.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(org.apache.hadoop.io.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(org.apache.hadoop.io.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.