Fetching a clean copy of a changing web page

John Nagle nagle at animats.com
Mon Jul 16 19:14:42 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.
> -Miles

      Yes, they're updating it non-atomically.

      I'm now reading it twice and comparing, which works.
Actually, it's read up to 5 times, until the same contents
appear twice in a row.  Two tries usually work, but if the
server is updating, it may require more.

      Ugly, and doubles the load on the server, but necessary to
get a consistent copy of the data.

					John Nagle

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