SPUG: Suggestions for web crawler framework, toolkit, or reusable pieces?

C.J. Adams-Collier cjac at colliertech.org
Thu Apr 22 14:12:24 PDT 2010

I like wget ;)

On Thu, 2010-04-22 at 14:08 -0700, Michael R. Wolf wrote:
> I let myself get sucker punched!  I wrote my own web crawler based on
> WWW::Mechanize because my preliminary research indicated that crawlers
> were simple.
> [Aside: If you're tempted to do that, let me save you some time.
> Don't.  They are *conceptually* simple (GET page, push links on queue,
> iterate), but there are many levels of devils lurking in the details.]
> Having finished phase 1 (way behind schedule and way over budget), I'm
> looking for a better web crawler solution for phase 2.
> Suggestions?
> Thanks, Michael
> P.S.  Even if you you told me *personally* at the previous SPUG
> meeting, please post your suggestion here so that others can learn via
> the email list and via search engines.  Thanks.
> P.P.S.  [Stop reading here unless you're interested in the
> nitty-gritty details of how I let myself get sucker punched.]
> OK... since you're interested, here's a short description of my long
> "journey", in the hope that it will help someone else.
> I'm now at the end of phase 1, looking to start a new phase.  I can
> see that my proof-of-concept crawler worked, but I can also see that
> it's a bad business and technical decision to continue investing in
> what got me here.
> I *significantly* underestimated the complexity of a production web
> crawler, and the development time it would take to
> create/test/debut/maintain it.
> In my defense, I did a bit of research, and all the papers I could
> find said that crawling was simple:
>   * initialize a queue with some seed URL's
>   * GET the next page from the queue
>   * extract the links
>   * add links to a queue
>   * [process the page for other information]
>   * loop until the queue is empty
> So, being a bit familiar with WWW::Mechanize, I build a simple crawler
> around it.  Then added more features as I bumped against them, then
> added more features, then added new features...
> Mech is a great module (thanks, Andy)!  It's well documented, well
> designed, and does what it says it does.
> But... I eventually realized that Mech is an automated browser, and an
> automated browser is *not* the same thing as a crawler.  For *simple*
> cases, that distinction is not critical, but for the kind of
> application I eventually need, the distinction is a deal-breaker.
> I even asked Andy if anyone had taken Mech to the next level to create
> a crawler.  He said that he gets that question a lot, but doesn't know
> of a project that's created a crawler framework.
> Here's a brief requirement list for what a crawler needs to do
> (i.e. code that I wrote, or need to write) beyond what Mech does:
>   * be polite (respect robots.txt, throttle page retrieveal rate to
>   prevent overloading my machine and server machine)
>    * have a revisit policy to refresh links (based on cost, expected
>    benefit, anticipated expiration, actual expiration...)
>    * cache results to prevent expensive re-access for unexpired content
>    * prevent circular (i.e. infinate) crawls
>    * avoid useless content (for many definitions of useless)
>    * recognize uncanonical duplicates of canonical URL
>    * keep crawler on same site (or virtualized duplicate servers)
>    * monitor/admminister long (multi-day) processes (stop, start,
>    pause, continue, recover, monitor, search logs, debug...)
> It's a big job to create a whole crawling environment and all the
> production support eco-system around it.  It's not as big as Google,
> since I'll only be looking at a few hundred sites, but it's *much*
> bigger problem than I want to create with WWW::Mechanize.
> -- 
> Michael R. Wolf
>      All mammals learn by playing!
>          MichaelRWolf at att.net

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