[jira] [Created] (NUTCH-1564) AdaptiveFetchSchedule: sync_delta forces immediat refetch for documents not modified

["Sebastian Nagel (JIRA)" <ji...@apache.org>]

Sebastian Nagel created NUTCH-1564:
--------------------------------------

             Summary: AdaptiveFetchSchedule: sync_delta forces immediat refetch for documents not modified
                 Key: NUTCH-1564
                 URL: https://issues.apache.org/jira/browse/NUTCH-1564
             Project: Nutch
          Issue Type: Bug
          Components: crawldb
    Affects Versions: 2.1, 1.6
            Reporter: Sebastian Nagel
            Priority: Critical


In a continuous crawl with adaptive fetch scheduling documents not modified for a longer time are may be fetched in every cycle.

A continous crawl is run daily with a 3 cycles and the following scheduling intervals (freshness matters):
{code}
db.fetch.schedule.class = org.apache.nutch.crawl.AdaptiveFetchSchedule
db.fetch.schedule.adaptive.sync_delta   = true (default)
db.fetch.schedule.adaptive.sync_delta_rate = 0.3 (default)
db.fetch.interval.default               = 172800 (2 days)
db.fetch.schedule.adaptive.min_interval =  86400 (1 day)
db.fetch.schedule.adaptive.max_interval = 604800 (7 days)
db.fetch.interval.max                   = 604800 (7 days)
{code}

At Apr 18 a URL is generated and fetched (from segment dump):
{code}
Crawl Generate::
Status: 2 (db_fetched)
Fetch time: Mon Apr 15 19:43:22 CEST 2013
Modified time: Tue Mar 19 01:07:42 CET 2013
Retries since fetch: 0
Retry interval: 604800 seconds (7 days)

Crawl Fetch::
Status: 33 (fetch_success)
Fetch time: Thu Apr 18 01:23:51 CEST 2013
Modified time: Tue Mar 19 01:07:42 CET 2013
Retries since fetch: 0
Retry interval: 604800 seconds (7 days)
{code}

Running CrawlDb update results in a next fetch time in the past (which forces an immediate refetch in the next cycle):
{code}
Status: 6 (db_notmodified)
Fetch time: Tue Apr 16 01:37:00 CEST 2013
Modified time: Tue Mar 19 01:07:42 CET 2013
Retries since fetch: 0
Retry interval: 604800 seconds (7 days)
{code}

This behavior is caused by the sync_delta calculation in AdaptiveFetchSchedule:
{code}
  if (SYNC_DELTA) {
    // try to synchronize with the time of change
    long delta = (fetchTime - modifiedTime) / 1000L;
    if (delta > interval) interval = delta;
    refTime = fetchTime - Math.round(delta * SYNC_DELTA_RATE * 1000);
  }
  if (interval < MIN_INTERVAL) {
    interval = MIN_INTERVAL;
  } else if (interval > MAX_INTERVAL) {
    interval = MAX_INTERVAL;
  }
...
datum.setFetchTime(refTime + Math.round(interval * 1000.0));
{code}
{{delta}} is 30 days (Apr 18 - Mar 19). {{refTime}} is then 9 days in the past ({{delta}} * 0.3). After adding {{interval}} (adjusted to {{MAX_INTERVAL}} = 7 days) to {{refTime}} the next fetch "should" take place 2 days in the past (Apr 16).

According to the [javadoc|http://nutch.apache.org/apidocs-2.1/org/apache/nutch/crawl/AdaptiveFetchSchedule.html] (if understood right), there are to aims of the sync_delta if we know that a document hasn't been modified for long:
* increase the fetch interval immediately (not step by step)
* because we expect the document to be changed within the adaptive interval (but it hasn't), we shift the "reference time", i.e. we expect a change soon.

These two aims are somehow in contradiction. In any case, the next fetch time should be always within the range of (currentFetchTime + MIN_INTERVAL) and (currentFetchTime + MAX_INTERVAL) and never in the past.

This problem has been noted by [~pascaldimassimo] in [1|http://pascaldimassimo.com/2010/06/11/how-to-re-crawl-with-nutch/] and [2|http://lucene.472066.n3.nabble.com/Adaptive-sync-with-the-time-of-page-change-td870842.html#a897234].


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