Alternative Generator which can generate several segments in one parse of the
crawlDB
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Key: NUTCH-762
URL: https://issues.apache.org/jira/browse/NUTCH-762
Project: Nutch
Issue Type: New Feature
Components: generator
Affects Versions: 1.0.0
Reporter: Julien Nioche
When using Nutch on a large scale (e.g. billions of URLs), the operations
related to the crawlDB (generate - update) tend to take the biggest part of the
time. One solution is to limit such operations to a minimum by generating
several fetchlists in one parse of the crawlDB then update the Db only once on
several segments. The existing Generator allows several successive runs by
generating a copy of the crawlDB and marking the URLs to be fetched. In
practice this approach does not work well as we need to read the whole crawlDB
as many time as we generate a segment.
The patch attached contains an implementation of a MultiGenerator which can
generate several fetchlists by reading the crawlDB only once. The
MultiGenerator differs from the Generator in other aspects:
* can filter the URLs by score
* normalisation is optional
* IP resolution is done ONLY on the entries which have been selected for
fetching (during the partitioning). Running the IP resolution on the whole
crawlDb is too slow to be usable on a large scale
* can max the number of URLs per host or domain (but not by IP)
* can choose to partition by host, domain or IP
Typically the same unit (e.g. domain) would be used for maxing the URLs and for
partitioning; however as we can't count the max number of URLs by IP another
unit must be chosen while partitioning by IP.
We found that using a filter on the score can dramatically improve the
performance as this reduces the amount of data being sent to the reducers.
The MultiGenerator is called via : nutch org.apache.nutch.crawl.MultiGenerator
...
with the following options :
MultiGenerator <crawldb> <segments_dir> [-force] [-topN N] [-numFetchers
numFetchers] [-adddays numDays] [-noFilter] [-noNorm] [-maxNumSegments num]
where most parameters are similar to the default Generator - apart from :
-noNorm (explicit)
-topN : max number of URLs per segment
-maxNumSegments : the actual number of segments generated could be less than
the max value select e.g. not enough URLs are available for fetching and fit in
less segments
Please give it a try and less me know what you think of it
Julien Nioche
http://www.digitalpebble.com
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