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Run a Ruby on Rails app

In this guide we create and deploy a Ruby on Rails application. To run this example, follow these steps:

  1. Install the kraft CLI tool and a container runtime engine, e.g. Docker.

  2. Clone the examples repository and cd into the examples/ruby3.2-rails/ directory:

Terminal window
git clone
cd examples/ruby3.2-rails/

Make sure to log into KraftCloud by setting your token and a metro close to you. We use fra0 (Frankfurt, 🇩🇪) in this guide:

Terminal window
# Set KraftCloud access token
# Set metro to Frankfurt, DE

When done, invoke the following command to deploy this application on KraftCloud:

Terminal window
kraft cloud deploy -M 1024 -p 443:3000 -e GEM_HOME=/usr/local/bundle -e BUNDLE_APP_CONFIG=/usr/local/bundle .

The output shows the instance URL and other details:

Terminal window
[] Deployed successfully!
────────── name: ruby32-rails-apa93
────────── uuid: 2f85b9db-94f8-45d2-8e38-ed9b56cb8695
───────── state: running
─────────── url:
───────── image: ruby32-rails@sha256:fdd46011408fdee05644665ad59b24115737e3fdb352169ec2f3f16a45d4f31d
───── boot time: 577.34 ms
──────── memory: 1024 MiB
service group: aged-waterfall-qraz0s7d
── private fqdn: ruby32-rails-apa93.internal
──── private ip:
────────── args: /usr/bin/ruby /app/bin/rails server -b

In this case, the instance name is ruby32-rails-apa93 and the URL is They are different for each run.

Use curl to query the KraftCloud instance of the Python-based HTTP web server:

Terminal window
<h1>Hello World</h1>
Hello, World!

At any point in time, you can list information about the instance:

Terminal window
kraft cloud instance list
ruby32-rails-apa93 running 2 minutes ago ruby32-rails@sha256:fdd46011408fdee... 1.0 GiB /usr/bin/ruby /app/bin/rails server -b 577.34 ms

When done, you can remove the instance:

Terminal window
kraft cloud instance remove ruby32-rails-apa93

Customize your Application

To customize the application, update the files in the repository, listed below:

  • app/ and config/: the contents to update the Rails setup
  • Kraftfile: the KraftCloud specification
  • Dockerfile: the Docker-specified application filesystem
spec: v0.6
runtime: ruby:3.2
rootfs: ./Dockerfile
cmd: ["/usr/bin/ruby", "/app/bin/rails", "server", "-b", ""]

The app/ and config/ directories contain files that are to overwrite generated Rails files:

Terminal window
tree app/ config/
|-- controllers/
| `-- hello_controller.rb
`-- views/
`-- hello/
`-- index.html.erb
|-- environments/
| `-- development.rb
`-- routes.rb

These files add the configuration (controller, view, route) to print the Hello, World! message in Rails. They overwrite the generated Rails configuration to provide the application setup. Update these files, and other files, with required contents for your own application.

Lines in the Kraftfile have the following roles:

  • spec: v0.6: The current Kraftfile specification version is 0.6.

  • runtime: ruby:3.2: The Unikraft runtime kernel to use is Ruby 3.2.

  • rootfs: ./Dockerfile: Build the application root filesystem using the Dockerfile.

  • cmd: ["/usr/bin/ruby", "/app/bin/rails", "server", "-b", ""]: Use /usr/bin/ruby /app/bin/rails server -b as the starting command of the instance.

Lines in the Dockerfile have the following roles:

  • The RUN lines install Rails and generate the default application setup for a hello application.

  • COPY . /app/: Copies the local files (from app/ and config/) to the application, overwriting generated contents to provide the user-specified application.

  • FROM scratch: Build the filesystem from the scratch container image, to create a base image.

  • COPY --from=build /app /app: Copy the application directory to the filesystem.

The following options are available for customizing the application:

  • If only updating the implementation in the app/ or config/ directories, or adding new directories or files to overwrite Rails generated files, no other change is required.

  • If changing the application name, change the hello name in RUN rails generate controller hello to a new one. The file names in the app/ and config/ directories have to be similarly updated.

  • More extensive changes may require expanding the Dockerfile with additional Dockerfile commands.

Learn More

Use the --help option for detailed information on using KraftCloud:

Terminal window
kraft cloud --help

Or visit the CLI Reference.