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Hello World - Ruby

A simple web app written in Ruby that you can use for testing. It reads in an env variable TARGET and prints “Hello ${TARGET}!”. If TARGET is not specified, it will use “World” as the TARGET.

Follow the steps below to create the sample code and then deploy the app to your cluster. You can also download a working copy of the sample, by running the following commands:

git clone -b "{{< branch >}}" knative-docs
cd knative-docs/docs/serving/samples/hello-world/helloworld-ruby

Before you begin

  • A Kubernetes cluster with Knative installed and DNS configured. Follow the installation instructions if you need to create one.
  • Docker installed and running on your local machine, and a Docker Hub account configured (we’ll use it for a container registry).

Recreating the sample code

  1. Create a new directory and cd into it:

    mkdir app
    cd app
  2. Create a file named app.rb and copy the code block below into it:

    require 'sinatra'
    set :bind, ''
    get '/' do
     target = ENV['TARGET'] || 'World'
     "Hello #{target}!\n"
  3. Create a file named Dockerfile and copy the code block below into it. See official Ruby docker image for more details.

    # Use the official lightweight Ruby image.
    FROM ruby:2.6-slim
    # Install production dependencies.
    WORKDIR /usr/src/app
    COPY Gemfile Gemfile.lock ./
    RUN gem install bundler && bundle install
    # Copy local code to the container image.
    COPY . ./
    # Run the web service on container startup.
    CMD ["ruby", "./app.rb"]
  4. Create a file named Gemfile and copy the text block below into it.

    source ''
    gem 'sinatra'
    gem 'rack', '>= 2.0.6'
  5. Run bundle. If you don’t have bundler installed, copy the Gemfile.lock to your working directory.

    bundle install
  6. Create a new file, service.yaml and copy the following service definition into the file. Make sure to replace {username} with your Docker Hub username.

    kind: Service
      name: helloworld-ruby
      namespace: default
            - image:{username}/helloworld-ruby
                - name: TARGET
                  value: "Ruby Sample v1"

Build and deploy this sample

Once you have recreated the sample code files (or used the files in the sample folder) you’re ready to build and deploy the sample app.

  1. Use Docker to build the sample code into a container. To build and push with Docker Hub, run these commands replacing {username} with your Docker Hub username:

    # Build the container on your local machine
    docker build -t {username}/helloworld-ruby .
    # Push the container to docker registry
    docker push {username}/helloworld-ruby
  2. After the build has completed and the container is pushed to docker hub, you can deploy the app into your cluster. Ensure that the container image value in service.yaml matches the container you built in the previous step. Apply the configuration using kubectl:

    kubectl apply --filename service.yaml
  3. Now that your service is created, Knative will perform the following steps:

    • Create a new immutable revision for this version of the app.
    • Network programming to create a route, ingress, service, and load balance for your app.
    • Automatically scale your pods up and down (including to zero active pods).
  4. To find the URL for your service, use

    kubectl get ksvc helloworld-ruby,URL:.status.url
    NAME                URL
  5. Now you can make a request to your app and see the result. Replace the URL below with the URL returned in the previous command.

    Hello Ruby Sample v1!

Remove the sample app deployment

To remove the sample app from your cluster, delete the service record:

kubectl delete --filename service.yaml