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https://issues.apache.org/jira/browse/NUTCH-2144?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15141285#comment-15141285
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Thamme Gowda N commented on NUTCH-2144:
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Hi [~lewismc]
* I think relying on URL suffix based mimetype detection is a nice precision 
trade-off for the gained speed.  I did this for one of my homework and to be 
honest, i disabled HEAD based MIME type detection because it was taking lot of 
time. This patch is using basic java Regex to filter. [~chrismattmann] I am not 
sure if Tika can take an URL and guess possible mime type without making a HEAD 
call. Can you point me to an example, if there is one?
* Agreed. The same logic can be applied to permit certain urls in intra-domain. 

I am glad you liked it, Let me know what improvements needed to make this 
useful for wide audience.




> Plugin to override db.ignore.external to exempt interesting external domain 
> URLs
> --------------------------------------------------------------------------------
>
>                 Key: NUTCH-2144
>                 URL: https://issues.apache.org/jira/browse/NUTCH-2144
>             Project: Nutch
>          Issue Type: New Feature
>          Components: crawldb, fetcher
>            Reporter: Thamme Gowda N
>            Assignee: Chris A. Mattmann
>            Priority: Minor
>             Fix For: 1.12
>
>         Attachments: ignore-exempt.patch, ignore-exempt.patch
>
>
> Create a rule based urlfilter plugin that allows focused crawler 
> (db.ignore.external.links=true) to fetch static resources from external 
> domains.
> The generalized version of this: This plugin should permit interesting URLs 
> from external domains (by overriding db.ignore.external). The interesting 
> urls are decided from a combination of regex and mime-type rules.
> Concrete use case:
>   When using Nutch to crawl images from a set of domains, the crawler needs 
> to fetch all images which may be linked from CDNs and other domains. In this 
> scenario, allowing all external links and then writing hundreds of regular 
> expressions is not feasible for large number of domains.



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