Running Rails with Puma and JRuby on Heroku

Last post I demonstrated setting up a Ruby on Rails app on Heroku,
running with the Puma server. As I mentioned before, in order to
get the most out of Puma you should use a Ruby implementation with real
threads like JRuby or Rubinius 2. In this post we'll repeat
most of the previous steps, but this time we'll use a Third-Party Buildpack
and get setup with JRuby.

Create a new Rails 3.x app with JRuby

Note: I'm not covering the specifics other than telling you to use RVM or rbenv to setup JRuby (I'm running

First create a new rails app

~$ jruby -S rails new heroku-jruby-puma
~$ cd heroku-jruby-puma
~$ git init

If you check your Gemfile, you should see some key differences:

  • instead of sqlite3 gem you have activerecord-jdbcsqlite3-adapter
  • gem jruby-openssl included
  • assets group includes therubyrhino gem

Configure Rails to work with JRuby

Some adjustments are needed to get this new Rails app to play nice on
Heroku with Puma. First, we can't just use the pg gem; You need to use
the activerecord-jdbcpostgresql-adapter instead. Add puma and set
some configuration variables for Rails and we're set.


gem 'activerecord-jdbcpostgresql-adapter'
gem 'puma'
group :development do 
  gem 'foreman'
  gem 'activerecord-jdbcsqlite3-adapter'

Note: here I left activerecord-jdbcsqlite3-adapter in the
development group just as an example.


config.assets.initialize_on_precompile = false


config.serve_static_assets = true

# Heroku log stuff
STDOUT.sync = true
config.logger =

Create Heroku app with JRuby buildpack

Heroku's Cedar stack has no native language or framework; instead your
sever is setup via a set of scripts called Buildpacks. Heroku
provides buildpacks for Ruby, Node.js, Python and a handful of others,
but you can also create or use your own Buildpack by using the --buildpack flag and providing a URL. Here we'll use JRuby's Heroku Buildpack from

~$ heroku create heroku-jruby-puma \ 

  Creating heroku-jruby-puma ... done, stack is cedar  

Now when you push your code up, Heroku's slug compiler knows how to
package things an build them to use JRuby.

Add a Procfile

As I mentioned last time, a Procfile tells Heroku how to run your app:

web: bin/puma -p $PORT -e $RACK_ENV

Thats it! Commit your changes and push to Heroku like before:

~$ git push
  -----> Heroku receiving push
  -----> Fetching custom buildpack... done
  -----> JRuby app detected
  -----> Vendoring JRuby into slug
  -----> Installing dependencies with Bundler
  -----> Writing config/database.yml to read from DATABASE_URL
  -----> Precompiling assets
  -----> Discovering process types
       Procfile declares types -> web
  -----> Compiled slug size is 44.5MB
  -----> Launching... done, v7 deployed to Heroku

     6bced4f..383a7cc  master -> master

Within a few moments you should be running on JRuby/Puma on Heroku.
Notice that your slug size is significantly larger than a normal stock
Rails app running on MRI. That's because of 4th item you see above,
"Vendoring JRuby into slug".

Congratulations! You're now running Rails with JRuby and Puma on Heroku.

See Also:



My name is Clint Shryock. I develop things in Go and Ruby. I live in central Missouri, where the weather is beautiful 4 months of the year.
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