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Run a Ruby app

In this guide we create and deploy a simple Ruby-based HTTP web server. 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/http-ruby3.2/ directory:

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

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 -p 443:8080 -M 256 .

The output shows the instance URL and other details:

Terminal window
[] Deployed successfully!
────────── name: http-ruby32-s6l8n
────────── uuid: b1ebbbc0-5efa-476c-adb6-99866773245c
───────── state: running
─────────── url:
───────── image: http-ruby32@sha256:4cf3b341898e6ebff18ff2b68353ef872dded650c9d16a6f005a8fbe8a7cbb3d
───── boot time: 71.19 ms
──────── memory: 256 MiB
service group: silent-resonance-1jtz5c66
── private fqdn: http-ruby32-s6l8n.internal
──── private ip:
────────── args: /usr/bin/ruby /src/server.rb

In this case, the instance name is http-ruby32-s6l8n and the URL is They are different for each run.

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

Terminal window
Hello, World!

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

Terminal window
kraft cloud instance list
http-ruby32-s6l8n running 12 minutes ago 256 MiB /usr/bin/ruby /src/server.rb 71191us

When done, you can remove the instance:

Terminal window
kraft cloud instance remove http-ruby32-s6l8n

Customize your Application

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

  • server.rb: the actual Ruby HTTP server
  • Kraftfile: the KraftCloud specification
  • Dockerfile: the Docker-specified application filesystem
require 'socket'
socket =
loop do
client = socket.accept
request = client.gets
response = "HTTP/1.1 200 OK\r\n" \
"Content-type: text/html\r\n" \
"Connection: close\r\n" \
"\r\n" \
"Hello, World!"

The following options are available for customizing the application:

  • If only updating the implementation in the server.rb source file, no other change is required.

  • If new files are added, these have to be copied in the application filesystem, using the COPY command in the Dockerfile.

  • If a new Ruby source files is added, update the cmd line in the Kraftfile and replace server.rb to run that file when creating the instance.

  • 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.