Fetching a clean copy of a changing web page

John Nagle nagle at animats.com
Tue Jul 17 02:47:49 CEST 2007


Miles wrote:
> On Jul 16, 1:00 am, John Nagle <na... at animats.com> wrote:
> 
>>    I'm reading the PhishTank XML file of active phishing sites,
>>at "http://data.phishtank.com/data/online-valid/"  This changes
>>frequently, and it's big (about 10MB right now) and on a busy server.
>>So once in a while I get a bogus copy of the file because the file
>>was rewritten while being sent by the server.
>>
>>    Any good way to deal with this, short of reading it twice
>>and comparing?
>>
>>                                John Nagle
> 
> 
> Sounds like that's the host's problem--they should be using atomic
> writes, which is usally done be renaming the new file on top of the
> old one.  How "bogus" are the bad files?  If it's just incomplete,
> then since it's XML, it'll be missing the "</output>" and you should
> get a parse error if you're using a suitable strict parser.  If it's
> mixed old data and new data, but still manages to be well-formed XML,
> then yes, you'll probably have to read it twice.

    The files don't change much from update to update; typically they
contain about 10,000 entries, and about 5-10 change every hour.  So
the odds of getting a seemingly valid XML file with incorrect data
are reasonably good.

				John Nagle



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