Thursday, December 11, 2008

Crawling AJAX in practice. Part 1

Some theory

Traditionally, a web spider system is tasked with connecting to a server, pulling down the HTML document, scanning the document for anchor links to other HTTP URLs and repeating the same process on all of the discovered URLs. Each URL represents a different state of the traditional web site. In an AJAX application, much of the page content isn't contained in the HTML document, but is dynamically inserted by Javascript during page load. Furthermore, anchor links can trigger Javascript events instead of pointing to other documents. The state of the application is defined by the series of Javascript events that were triggered after page load. The result is that the traditional spider is only able to see a small fraction of the site's content and is unable to index any of the application's state information.

Some findings

I've googled around for a few days and have found various information about crawling tools. There are more, but some are forgotten to mention or haven't been tried. Here is a quick summary of tools for getting page source.

1) Right click, View page source. Well, a simple way to get page source, but you fall laughing when you see the dynamic page source. I was playing around with -> quotes -> instruments.
2) Perl::Mechanize. It's a useful tool, i was happy using it, but the result is sadly though - static. It fills forms and follows links, but the final page source is static. It digs text, digs .css underneath, but avoids JavaScript. You can get a page source browsing only static version of webpage (which has). This tools is useful for sabotage, for example you create disposable email accounts, vote online for a car of the year, read the passkey from you temp email, vote and repeat the loop again. No user interface needed, while testing use browser and
compare it to Mechanize agent->content.
3) Various Firefox extensions. Crap, the target was to build a GUI-less mechanism, these implementations though require user interaction. Extensions are based on tracking AJAX requests, recording user actions (like macros in Office). iMacros, ChickenFoot, Selenium. Here are the names. iMacros are error sensitive, for example first visit and getting a cookie differs from the subsequent visits. Mechanize for example is error proof for such actions. I haven't found use of other extensions, because most of them doesn't work on Minefield, nor RedHat's Firefox one point zero something.
4) Ruby + Watir. AJAX crawling after all! But there is a huge "but" - presenting IE. There are also Watir implementations on FF and Safari, haven't tested them. Ruby is also error proof, and i have achieved some progress, but the result is STATIC. Spent half a day looking for a way to save page content to file, spent one day more for looking how to get a full page source, but didn't manage that. Please inform me if i'm wrong, but anyways - skype has an emotion for coding on windows: (puke).
5) Yes you are right, saved the best for dessert. XULRunner + Crowbar. This stuff works and rocks and has an implementation, more about it in Part 2. Here is a quote: "...a server-side headless mozilla-based browser". It even sounds promising. It runs as a daemon, you can ask it, push it, get contents, and get AJAXed source. Its a browser based execution environment with a scraping tool on it. After you get your desired page source, its kinda trash: no newlines, bunch of HTML tags. Then there are few solutions - pass it to Lynx or process manually with a custom C code parser. At last we have the numbers, not a JavaScript function names.


The purpose of this task is not to exactly crawl from one page to other, its only numbers that are pushed (AJAX'ed) via JavaScript that matters.

Part 2

No comments: