I've spent the past year working on a product running on a relatively large Rails codebase, spending about half my time on the back-end and half in the UI layer. As the codebase has grown in size on the back-end, I've noticed an increasingly prevalent source of frustration in my day-to-day: slow request times when writing code for the UI.

But slow Rails should be a thing of the past. What's up?

With this particular codebase, we're using Require.js for a large part of our JS functionality. In production, the associated scripts are being compiled into a single file via R.js, but in development, Require.js will make a request for each script in your require directive. Unless you're running Rails behind Apache or Nginx (or some other reverse-proxy), Rails is being asked to serve all asset requests the browser issues, including assets. In our case, without completely realizing it ended up asking Rails for a lot of stuff. Based on this, it would stand to reason that if we're asking too much of a single Rails process, adding processes might help the problem, right?

Unicorn: Let's Add Some Concurrency

By starting a single master process that proxies requests through to one or more worker processes, Unicorn can split the work up for you relatively seamlessly. Essentially, as the browser is issuing requests, the master process is delegating the fulfillment of each request to a different process. That means that if you've specified that the Unicorn master process should start three worker processes, each of those processes should handle roughly a third of the requests the master receives.*

After adding Unicorn and scaling up the number of worker processes, I expected to see a significant decrease in request times. What I found was disappointing - there wasn't really any difference to speak of. I didn't measure any benchmarks at the time but I did have an idea: I knew that Rails handles class loading and caching differently in development than in production, so I started playing around with some of Rails' configuration.

The Solution

A default Rails development configuration (config/environments/development.rb) has a couple of configuration lines, one of which is:

config.cache_classes = false

This configuration is the part that allows us to change some of our application code and see those changes reflected the next time we hit the server. This is because Rails will reload the code in its autoload paths everytime a request is received. In my case, our large codebase meant the reloading of lots of files. After setting this to true, the speed gains were astounding - this one configuration change made the app feel nearly as snappy locally as it did in production.

With the application server responding much faster, there was one more line in the configuration that stood out:

config.assets.debug = true

When assets from the Asset Pipeline, this configuration essentially expands all of our //= require ... directives into actual server requests. While this helps in tracking bugs down to an individual file, it's really not always necessary. Even though a large portion of the codebase used Require.js in stead //= require ... directives, we still had a non-insignificant number of files that did. Flipping the switch on this configuration reduced the number of HTTP requests going out to our server on each page load, which added to our performance gains from the previous config change.

One Problem - What If I Need Code Reloading?

In my example, this was great if the UI structure was already built out and I was just working on JS / CSS code, but this happened frequently enough to render this solution insufficient. Restarting a server every time application code or views were modified was just too slow in process. However, I decided to take one more dive into the Unicorn docs just for kicks, and I found something.

Unicorn was built on strong principles lying in the foundation of Unix (see I like Unicorn because it's Unix by Ryan Tomayko). As such, it's capable of receiving and reacting to various signals. More specifically, Unicorn reacts to receiving a SIGHUP by reloading its configuration file and restarting all of its workers. Sending a signal to the process for code reloading is only mildly faster in-process, the gains weren't enough to justify doing this manually. However, there are lots of utilities out there that will watch a file system node and execute some code on change. My current favorite is rerun because of its simplicity. Using Rerun is what makes this all come together.

When you start a Unicorn server, you'll see output that looks something like this:

started with pid 25747
[2013-08-25T20:14:38.044546 #25747]  INFO -- : Refreshing Gem list
[2013-08-25T20:14:45.094800 #25747]  INFO -- : listening on addr= fd=12
[2013-08-25T20:14:45.099519 #25747]  INFO -- : master process ready
[2013-08-25T20:14:45.135473 #25751]  INFO -- : worker=1 ready
[2013-08-25T20:14:45.135675 #25750]  INFO -- : worker=0 ready
[2013-08-25T20:14:45.141830 #25752]  INFO -- : worker=2 ready

Since we know the PID, we can send a signal to it via:

kill -1 25757

Running this manually gets us our code reload, so to bring Rerun into the mix, all we need is:

rerun -- kill -1 25757

With a little Rerun tuning, you can restrict the types of files it watches to a path pattern, making it ignore anything in app/assets and watch for changes in application code.

* In reality it doesn't actually work out to be a third because of varying amounts of work being done per-request. I've simplified for the sake of discussion.