SPUG: Suggestions for web crawler framework, toolkit, or reusable pieces?
Michael R. Wolf
MichaelRWolf at att.net
Thu Apr 22 14:08:29 PDT 2010
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
--
Michael R. Wolf
All mammals learn by playing!
MichaelRWolf at att.net
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