#nodejs

I was recently out with a friend of mine who mentioned that he was having a tough time scraping some data off a website. After a few drinks we arrived at a barter, if I could scrape the data he’d buy me some single malt scotch which seemed like a great deal for me. I assumed I’d make a couple of HTTP requests, parse some HTML, grab the data and dump it into a CSV. In the worst case I imagined having to write some custom code to login to a web app and maybe sticky some cookies. And then I got started.

As it turned out this site was running one of the most sophisticated anti-scraping/anti-robot packages I’ve ever encountered. In a regular browser session everything looked normal but after a half dozen or so programmatic HTTP requests I started running into their anti-robot software. After poking around a bit it, the blocks they were deploying were a mix of:

  • Whitelisted User Agents – Following a few requests from PHP cURL the site started blocking requests from my IP that didn’t include a “regular” user agent.
  • Requiring cookies and Javascript – I thought this was actually really clever. After a couple of requests the site started quietly loading an intermediate page that required your browser to run Javascript to set a cookie and then complete a POST request to a URL that included a nonce in order to view a page. To a regular user, this was fairly transparent since it happened so quickly but it obviously trips up a client HTTP client.
  • Soft IP rate limits – After a couple of dozen requests from my IP I started receiving “Solve this captcha” pages in order to view the target content.

Taken all together, it’s a pretty sophisticated setup for what’s effectively a niche social networking site. With the “requires Javascript” requirement I decided to explore using Electron for this project. And turns out, it’s a perfect fit. For a quick primer, Electron is an open source project from GitHub that enables developers to build cross platform desktop applications by merging nodejs and Chrome. Developers end up writing Javascript that can leverage the nodejs ecosystem while also using Chrome’s browser internals to render windows and widgets. Electron helps in this use case because it provides a full Chrome browser that’s scriptable and has access to node’s system level modules. For completeness, you could implement all of this in a Chrome extension but in my experience extensions have more complicated non-privileged to privileged communication and lack access to node so you can’t just fire off a “fs.writeFileSync” to persist your results.

With a full browser environment, we now need to tackle the IP restrictions that cause captchas to appear. At face value, like most people, I assumed solving captchas with OCR magic would be easier than getting new IPs after a couple of requests but it turns out that’s not true. There weren’t any usable “captcha solvers” on npm so I decided to pursue the IP angle. The idea would be to grab a new IP address after a few requests to avoid having to solve a captcha which would require human intervention. Following some research, I found out that it’s possible to use Tor as a SOCKS proxy from a third party application. So concretely, we can launch a Tor circuit and then push our Electron HTTP requests through Tor to get a different IP address that your normal Internet connection.

Ok, enough talk, show me some code!

I setup a test “target page” at http://code.setfive.com/scraper_demo/ which randomly shows “content you want” and a “please solve this captcha”. The github repository at https://github.com/adatta02/electron-scraper-skeleton has all the goodies, a runnable Electron application. The money file is injected.js which looks like:

To run that locally, you’ll need to do the usual “npm install” and then also run a Tor instance if you want to get a new IP address on every request. The way it’s implemented, it’ll detect the “content you want” and also alert you when there’s a captcha by playing a “ding!” sound. To launch, first start Tor and let it connect. Then you should be able to run:

Once it loads, you’ll see the test page in what looks like a Chrome window with a devtools instance. As it refreshes, you’ll notice that the IP address is displays for you keeps updating. One “gotcha” is that by default Tor will only get a new IP address each time it opens a conduit, so you’ll notice that I run “killall” after each request which closes the Tor conduit and forces it to reopen.

And that’s about it. Using Tor with the skeleton you should be able to build a scraper that presents a new IP frequently, scrapes data, and conveniently notifies you if human input is required.

As always questions and comments are welcomed!

Posted In: Javascript

Tags: , ,

One of the nice things about nodejs is that since the majority of its libraries are asynchronous it boasts strong support for concurrently performing IO heavy workloads. Even though node is single threaded the event loop is able to concurrently progress separate operations because of the asynchronous style of its libraries. A canonical example would something like fetching 10 web pages and extracting all the links from the fetched HTML. This fits into node’s computational model nicely since the most time consuming part of an HTTP request is waiting around for the network during which node can use the CPU for something else. For the sake of discussion, let’s consider this sample implementation:

Request debugging is enabled so you’ll see that node starts fetching all the URLs at the same time and then the various events fire at different times for each URL:

So we’ve demonstrated that node will concurrently “do” several things at the same time but what happens if computationally intensive code is tying up the event loop? As a concrete example, imagine doing something like compressing the results of the HTTP request. For our purposes we’ll just throw in a while(1) so it’s easier to see what’s going on:

If you run the script you’ll notice it takes much longer to finish since we’ve now introduced a while() loop that causes each URL to take at least 5 seconds to be processed:

And now back to the original problem, how can we fetch the URLs in parallel so that our script completes in around 5 seconds? It turns out it’s possible to do this with node with the child_process module. Child_process basically lets you fire up a second nodejs instance and use IPC to pass messages between the parent and it’s child. We’ll need to move a couple of things around to get this to work and the implementation ends up looking like:

What’s happening now is that we’re launching a child process for each URL we want to process, passing a message with the target URL, and then passing the links back to the parent. And then running along with a timer results in:

It isn’t exactly 5 seconds since there’s a non-trivial amount of time required to start each of the child processes but it’s around what you’d expect. So there you have it, we’ve successfully demonstrated how you can achieve parallelism with nodejs.

Posted In: Javascript

Tags: ,

Earlier this week, a buddy of mine reached out asking for a good solution to programmatically taking screenshots of a few thousand URLs. For whatever reason, this question seems to come up ever so often so I pointed him towards PhantomJS and figured he’d be on his way. Wrong. Not one to pass up free beer and the opportunity to learn something I agreed to write up the script to generate screenshots from a list of URLs.

Looking at PhantomJS, it seems relatively straightforward but it’s clear you’d really need something “else” to orchestrate this entire process. After some poking around, NodeJS, everyone’s favorite hipster runtime, seemed to be the obvious choice. There’s a handful of node modules that basically “bridge” node with phantom and allow a node script to asynchronously manipulate a PhantomJS instance. Based solely on the funny description I decided to run with phantomjs-node and was off to the races.

Getting everything setup was straightforward enough but then as I started looking at the phantomjs-node examples I started realizing this was a one way trip to callback soup. I’ve been doing some PhoneGap work recently and using jQuery’s Deferreds has significantly help keep the project from becoming a mess of callbacks. On the NodeJS side, it looks like there’s two functionally equivalent implementations but I decided to run with Q since the “wrapper” function names are shorter.

The Code

Anyway, the main problem we’re trying to address is that with multiple nested callbacks code becomes particularly difficult to follow. It’s hard to read, hard to trace control flow, and obviously hard to debug. Take for example, the phantomjs-node example:

It’s already THREE callbacks deep and all it’s done is initialize PhantomJS and load a page. Imagine layering on a few more asynchronous operations, doing all of this in a loop, and then some post-processing. Enter Deferreds. How To Node has an eloquent explanation of what deferreds are and how Node impliments them but in a nutshell they’re useful for making asynchronous code easier to follow.

The main issue I ran into using Q was that “Q.ninvoke” and “Q.npost” wrapper functions kept causing exceptions while using them with phantomjs-node. What I ended up doing instead was creating my own Deferreds in separate functions and then resolving them inside a normal callback.

My PhantomJS-node code ended up looking like:

It’s without a doubt easier to follow and would also make it much easier to do more complicated PhantomJS related tasks.

So how did screenshotting a few thousand domains go? Well there’s a story about that as well…

Posted In: Javascript

Tags: , ,