Improving Rails boot time with Zeus

Dan Croak

Zeus improves Rails boot time. Saving seconds is most important when running focused tests:

rspec spec/models/user_spec.rb
rspec spec/models/user_spec.rb:123

Those are times when a tight feedback loop make a meaningful difference.


Install the Zeus gem on your machine:

gem install zeus

Do not include it in your Gemfile. It is an external piece of software.

Set up


zeus init

This will create two files in your Rails app’s directory. Ignore them globally in ~/.gitignore:


Edit zeus.json to include only the tasks for which you’ll use Zeus. Mine looks like this:

  "command": "ruby -rubygems -r./custom_plan -eZeus.go",

  "plan": {
    "boot": {
      "default_bundle": {
        "development_environment": {
          "prerake": {"rake": []},
          "console": ["c"],
          "generate": ["g"]
        "test_environment": {
          "test_helper": {"test": ["rspec"]}

I remove cucumber in favor of RSpec and Capybara.

I remove server in favor of Foreman and Pow.

Force the test environment

In spec/spec_helper.rb, change:

ENV['RAILS_ENV'] ||= 'test'


ENV['RAILS_ENV'] = 'test'

Remove auto-running code

The goal is to run tests in the context of Zeus. So, remove other similar systems.

From the RSpec docs:

Generally, life is simpler if you just use the rspec command. If you must use the ruby command, however, you’ll want to do the following:

require 'rspec/autorun'

This tells RSpec to run your examples.

We don’t need this behavior and can cause bugs when used with Zeus.

Remove either of these lines in spec/spec_helper.rb if they exist:

require 'rspec/autorun'
require 'rspec/autotest'

Remove Spork and Guard

For the same reasons, delete Spork and Guard from your Gemfile, delete your Guardfile, and delete any related Spork code in spec/spec_helper.rb or spec/support/.

Start Zeus

Zeus will need to be running before you can use its commands:

zeus start

I usually run this, and other long-running processes in a tmux session.

Now, those original commands will have the benefit of Rails boot time in under a second:

zeus rspec spec/models/user_spec.rb
zeus rspec spec/models/user_spec.rb:123

Bonus: run specs from vim

Many of us are running specs directly from vim. If you edit your ~/.vimrc to use Zeus like in this commit, you can run focused specs with: