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From "Doug Cook (JIRA)" <j...@apache.org>
Subject [jira] Updated: (NUTCH-409) Add "short circuit" notion to filters to speedup mixed site/subsite crawling
Date Sun, 26 Nov 2006 00:20:03 GMT
     [ http://issues.apache.org/jira/browse/NUTCH-409?page=all ]

Doug Cook updated NUTCH-409:
----------------------------

    Attachment: shortcircuit.patch

> Add "short circuit" notion to filters to speedup mixed site/subsite crawling
> ----------------------------------------------------------------------------
>
>                 Key: NUTCH-409
>                 URL: http://issues.apache.org/jira/browse/NUTCH-409
>             Project: Nutch
>          Issue Type: Improvement
>          Components: fetcher
>    Affects Versions: 0.8
>            Reporter: Doug Cook
>            Priority: Minor
>         Attachments: shortcircuit.patch
>
>
> In the case where one is crawling a mixture of sites and sub-sites, the prefix matcher
can match the sites quite quickly, but either the regex or automaton filters are considerably
slower matching the sub-sites. In the current model of AND-ing all the filters together, the
pattern-matching filter will be run on every site that matches the prefix matcher -- even
if that entire site is to be crawled and there are no sub-site patterns. If only a small portion
of the sites actually need sub-site pattern matching, this is much slower than it should be.
> I propose (and attach) a simple modification allowing considerable speedup for this usage
pattern. I define the notion of a "short circuit" match that means "accept this URL and don't
run any of the remaining filters in the filter chain." 
> Though with this change, any filter plugin can in theory return a short-circuit match,
I have only implemented the short-circuit match for the PrefixURLFilter. The configuration
file format is backwards-compatible; shortcircuit matches just have SHORTCIRCUIT: in front
of them.
> One minor "gotcha":
> * Because the shortcircuit matches will avoid running any later filters, all of the site-independent
filters need to be BEFORE the PrefixURLFilter in the chain.
> I get my best performance using the following filter chain:
> 1) The SuffixURLFilter  to throw away anything with unwanted extensions
> 2) The RegexURLFilter to do site-independent cleanup (ad removal, skipping mailto:, bulletin-board
pages, etc.)
> 3) The PrefixURLFilter, with SHORTCIRCUIT: in front of every site name EXCEPT the sites
needing subsite matching
> 4) The AutomatonURLFilter to match those sites needing subsite pattern matching.
> I have tens of thousands of sites and an order of magnitude fewer subsites, so skipping
step #4 90% of the time speeds things up considerably (my reduce time on a round of crawling
is down from some 26 hours to less than 10).
> There are only two drawbacks to the implementation, and I think they're pretty minor:
> 1) Because I pass a special token (_PASS_) in the place of the URL to implement the short
circuit, if for some reason someone wanted to crawl a URL named "_PASS_", there would be problems.
I find this highly unlikely, since that's an invalid URL.
> 2) The correct behavior of steps #3 and #4 above depends upon coordination of the config
files between the prefix and automaton filters, making an opportunity for user screwup. I
thought about creating a "new kind of filter" which essentially combined prefix & automaton's
behaviors, took one config file, and internally handled the short-circuiting. But I think
the approach I took is simpler, cleaner, more flexible, and avoids creating yet another kind
of filter. Coordinating the config files is pretty easy (I generate them programmatically).
> As this is my first contribution to Nutch I'm sure that there are things I've missed,
whether in coding style or desired patch format. I welcome any feedback, suggestions, etc.
> Doug

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