Deploy a Rails and PostgreSQL app to Dokku

Dokku is a “Docker powered mini-Heroku in around 100 lines of Bash”. It’s really awesome as it allows you to get your own application platform up and running under an hour! In writing this post and trying out Dokku, I used an existing Rails 4 application that is currently running on Heroku. Because the application adheres to most of the concepts from the 12 factor methodology, I didn’t have to change a single thing in the application to get it running on Dokku. If you find your application relies too much on server specifics, this might be a good time to take a look at the 12 factor methodology.

This post covers the issues I met when deploying an existing Rails 4.1 application using PostgreSQL to a server running Dokku.

Installing Dokku

You need a server with Dokku installed, I recommend following this guide. Alternatively, you can follow the steps on the Dokku wiki.

In order to get up and running fast, I recommend using the Dokku image provided by DigitalOcean. It’s dead easy to get going, and can easily run on a small droplet for $5 per month.

Installing PostgreSQL

You can add a PostgreSQL container to your server by installing a Dokku plugin. Once you have followed the steps on the wiki, you’re almost ready to deploy your application.

Running the following commands on the server worked for me:

# installing the plugin
cd /var/lib/dokku/plugins
git clone postgresql
dokku plugins-install

# Create and setup the db
dokku postgresql:create <db-name>

# Once you've deployed your app once, you can link the db to the app like:
dokku postgresql:link <app-name> <db-name>

When you have linked you database with the application, a DATABASE_URL is set and Dokku will automatically change your Rails configuration on deploy to use that.

In order to install the pg gem, you need to ensure you have the PostgreSQL libraries installed. On Ubuntu you can install them with this command: sudo apt-get install libpq-dev.

Preparing your Rails application

First you need to add your Dokku server as a git remote: git remote add dokku dokku@<server>:<app-name>, replacing server and app-name with your Dokku server and application name.

You need to add a Procfile for Dokku to know how your application is started. This is an example Procfile for a Rails application using Unicorn:

web: bundle exec unicorn -p $PORT -c ./config/unicorn.rb

Setting application configuration variables

I’m using Dokku to setup a staging environment, so I need the server to have specific environment variables set. This is easily done on the server by running: dokku config:set <app-name> rails_env=staging

Deploying to Dokku

You can now deploy your application via git push dokku master. This should look very familiar if you’ve used Heroku before.

Once your app is deployed, you can migrate the database by SSH’ing onto the server and running dokku run <app-name> bundle exec rake db:migrate.

By now you should have your app running on Dokku! The url is printed at the end of the deployment.

Dealing with memory issues on deploy

If you get an error like cannot allocate memory when installing gems, you need to either upgrade your server to have more memory, or create a swap drive. I found creating a swap drive the easiest approach:

dd if=/dev/zero of=/swapfile bs=1024 count=1024000
mkswap /swapfile
swapon /swapfile


I hope this gave you a quick overview on how easy it is to get your own mini-Heroku running using Dokku. For more advanced setups I’d recommend looking at the documentation, and please share anything you find in the comments below.

photo of Jesper Kjeldgaard

Software Engineer
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