Oga: a new XML/HTML parser for Ruby

In the Ruby ecosystem there are plenty of HTTP libraries. Net::HTTP, HTTParty, HTTPClient, Patron, Curb, Excon, Tyhpoeus, just to name a few. There are so many of them it's almost as if it's required that one writes an HTTP client in order to call themselves a Ruby developer.

When it comes to XML/HTML parsing on the other hand the options are quite limited. The two most common libraries are Nokogiri and REXML. Both these libraries however have various flaws that makes working with them less than pleasant. REXML is generally quite slow, only supports XML and can use quite a chunk of memory when parsing data.

Nokogiri on the other hand is quite fast, but in turn is not thread-safe and in certain places has a bit of an odd API. Nokogiri also vendors its own copy of libxml which greatly increases install sizes and times. Most important of all, Nokogiri simply doesn't work on Rubinius.

So what exactly is the problem with Nokogiri and Rubinius? Well, on MRI and Rubinius Nokogiri will use a C extension. This extension in turn uses libxml. Due to MRI having a GIL everything might appear to be working as expected, however on Rubinius all hell breaks loose. To be exact, at certain points in time bogus data (e.g. null pointers) are sent to the garbage collector, this in turn crashes Rubinius. Both I and Brian Shirai (brixen) have spent quite some time trying to figure out what the heck is going on, without any success so far. The exact details of all this can be found in the following Nokogiri issue: https://github.com/sparklemotion/nokogiri/issues/1047.

This particular problem is thus severe that some of the production applications I've tested (that use Nokogiri heavily) consistently crash around 30 seconds into the process' lifetime. As a result it's impossible for me to run these applications on Rubinius. If a process were to crash once every few days I might be able to live with it while searching for a solution, every 30 seconds however is just not an option.

All of this prompted me to start working on an alternative, an alternative that doesn't require complicated system libraries or Ruby implementation specific codebases. For the past 8 months I've been working on exactly that. I've called the project Oga, and it can be found on GitLab.com: https://gitlab.com/yorickpeterse/oga. Today, 199 days after the first Git commit, I'll be releasing the first version on RubyGems.

Oga is primarily written in Ruby (91% Ruby according to Github), with a small native extension for the XML lexer. It supports parsing of XML and HTML, comes with XPath expressions, support for XML namespaces and much more. It works on MRI, Rubinius and JRuby and doesn't require large system libraries. This in turn means smaller Gem sizes and much faster installation times. For more information, see the Oga README.

Oga can be installed from RubyGems as following (the installation process should only take a few seconds):

gem install oga

Once installed you can start parsing XML and HTML documents. For example, lets parse the Reddit frontpage and get all article titles:

require 'oga'
require 'net/http'

body     = Net::HTTP.get(URI.parse('http://www.reddit.com/'))
document = Oga.parse_html(body)
titles   = document.xpath('//div[contains(@class, "entry")]/p[@class="title"]/a/text()')

titles.each do |title|
  puts title.text

Because Oga is a very young library there is a big chance you'll bump into bugs or other issues (I'm going to be honest here). For example, HTML parsing is not yet as solid as it should be (https://gitlab.com/yorickpeterse/oga/issues/20), Oga also does not yet honor the encoding set in the document itself (https://gitlab.com/yorickpeterse/oga/issues/29). If you happen to run into any problems/bugs, please report these at the issue tracker. Feedback and questions are also more than welcome.

Personally I'm really excited about what Oga currently is and what it will become (it also seems other share that sentiment). I was not expecting it to take nearly 8 months to write such a library, but looking back at everything it was more than worth the effort.

And last, I'd like to thank the following people: