This is part 5 of my ongoing series on Ruby on Rails performance. Today’s topic is a bit more complicated that some of the previous parts.
I’m going to explore an alternative to using the method 1ActiveRecord::Base.to_xml.
Before I get into that, I probably should explain why. After all, 1ActiveRecord::Base.to_xml is really easy to use. The alternative I’m going to demonstrate isn’t very easy to use.
The problem is that 1ActiveRecord::Base.to_xml can be really slow—even after making the optimizations I’ve explored in the previous parts to this series. For fairly simple AR objects, this won’t be a problem. However, when you have a deep object hierarchy and need substantial portions of it to be included in the XML response, it becomes a problem.
One instance where I encountered this was a nested has_many tree. That is, the object tree looks something like this:
ParentObject has_many ChildObjects
Each ChildObject has_many OtherObjects
The XML output is often 100k or more, and representing a few dozen or more total objects.
In part 3 of my Ruby on Rails Performance Series we’ll look at how to improve the parsing speed of XML content.
Rails will automatically parse XML in two key places:
1. When XML content is sent to a controller/action pair via POST or PUT. (Your app is an API server.)
2. When XML content is received from an ActiveResource request. (Your app is an API client.)
The default XML parser is REXML. It’s written in pure Ruby which makes it very portable. And slow. Very slow.
So, we need to replace REXML with something faster. The key is the 1libxml-ruby gem which is just a binding to the very fast libxml2 C library.
libxml-ruby has a different set of methods from REXML and is not drop in compatible. If you’re planning on moving your own REXML parsing code to libxml-ruby, it will take some work. However, it’s worth it. In one particular project, I had about a 100x speed improvement!
Rails, as it often does, makes the switch to libxml-ruby super easy. First, install the gem (requires a functioning compiler, the libxml2 library, and headers for libxml2):
1gem install libxml-ruby
Then, just add a new file to 1config/initializers/. Let’s call it 1config/initializers/xml_config.rb. In that file, put this:
1ActiveSupport::XmlMini.backend = 'LibXML'
This was only added in recent versions of Rails, I believe in 2.3.
The standard way to build XML in Ruby is with the Builder gem.
Builder iterates over each character in the output text to determine if it needs to escape it. When generating larger XML documents, this can be quite slow.
Thankfully, the solution is already available via the 1fast_xs gem. This gem does compile an extension, so you’ll need a working compiler.
1gem install fast_xs
Then just require it after Builder:
1require 'builder'2require 'fast_xs'
It will be used automatically by Builder going forward.
If your app is built on Rails, it’s even easier. Just install the gem—that’s it! Recent versions of Rails will auto-detect the existence of 1fast_xs and enable it.