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From Jack Tang <him...@gmail.com>
Subject Re: Per-page crawling policy
Date Thu, 05 Jan 2006 15:24:46 GMT
Hi Andrzej

The idea brings vertical search into nutch and definitely it is great:)
I think nutch should add information retrieving layer into the who
architecture, and export some abstract interface, say
UrlBasedInformationRetrieve(you can implement your url grouping idea
here?), TextBasedInformationRetrieve, DomBasedInformationRetrieve. The
user can implement these in their vertical search by their own.

/Jack

On 1/5/06, Andrzej Bialecki <ab@getopt.org> wrote:
> Hi,
>
> I've been toying with the following idea, which is an extension of the
> existing URLFilter mechanism and the concept of a "crawl frontier".
>
> Let's suppose we have several initial seed urls, each with a different
> subjective quality. We would like to crawl these, and expand the
> "crawling frontier" using outlinks. However, we don't want to do it
> uniformly for every initial url, but rather propagate certain "crawling
> policy" through the expanding trees of linked pages. This "crawling
> policy" could consist of url filters, scoring methods, etc - basically
> anything configurable in Nutch could be included in this "policy".
> Perhaps it could even be the new version of non-static NutchConf ;-)
>
> Then, if a given initial url is a known high-quality source, we would
> like to apply a "favor" policy, where we e.g. add pages linked from that
> url, and in doing so we give them a higher score. Recursively, we could
> apply the same policy for the next generation pages, or perhaps only for
> pages belonging to the same domain. So, in a sense the original notion
> of high-quality would cascade down to other linked pages. The important
> aspect of this to note is that all newly discovered pages would be
> subject to the same policy - unless we have compelling reasons to switch
> the policy (from "favor" to "default" or to "distrust"), which at that
> point would essentially change the shape of the expanding frontier.
>
> If a given initial url is a known spammer, we would like to apply a
> "distrust" policy for adding pages linked from that url (e.g. adding or
> not adding, if adding then lowering their score, or applying different
> score calculation). And recursively we could apply a similar policy of
> "distrust" to any pages discovered this way. We could also change the
> policy on the way, if there are compelling reasons to do so. This means
> that we could follow some high-quality links from low-quality pages,
> without drilling down the sites which are known to be of low quality.
>
> Special care needs to be taken if the same page is discovered from pages
> with different policies, I haven't thought about this aspect yet... ;-)
>
> What would be the benefits of such approach?
>
> * the initial page + policy would both control the expanding crawling
> frontier, and it could be differently defined for different starting
> pages. I.e. in a single web database we could keep different
> "collections" or "areas of interest" with differently specified
> policies. But still we could reap the benefits of a single web db,
> namely the link information.
>
> * URLFilters could be grouped into several policies, and it would be
> easy to switch between them, or edit them.
>
> * if the crawl process realizes it ended up on a spam page, it can
> switch the page policy to "distrust", or the other way around, and stop
> crawling unwanted content. From now on the pages linked from that page
> will follow the new policy. In other words, if a crawling frontier
> reaches pages with known quality problems, it would be easy to change
> the policy on-the-fly to avoid them or pages linked from them, without
> resorting to modifications of URLFilters.
>
> Some of the above you can do even now with URLFilters, but any change
> you do now has global consequences. You may also end up with awfully
> complicated rules if you try to cover all cases in one rule set.
>
> How to implement it? Surprisingly, I think that it's very simple - just
> adding a CrawlDatum.policyId field would suffice, assuming we have a
> means to store and retrieve these policies by ID; and then instantiate
> it and call appropriate methods whenever we use today the URLFilters and
> do the score calculations.
>
> Any comments?
>
> --
> Best regards,
> Andrzej Bialecki     <><
>  ___. ___ ___ ___ _ _   __________________________________
> [__ || __|__/|__||\/|  Information Retrieval, Semantic Web
> ___|||__||  \|  ||  |  Embedded Unix, System Integration
> http://www.sigram.com  Contact: info at sigram dot com
>
>
>


--
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