Hello World - Clojure

A simple web app written in Clojure 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.


  • 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

While you can clone all of the code from this directory, hello world apps are generally more useful if you build them step-by-step. The following instructions recreate the source files from this folder.

  1. Create a new file named src/helloworld/core.clj and paste the following code. This code creates a basic web server which listens on port 8080:

    (ns helloworld.core
      (:use ring.adapter.jetty)
    (defn handler [request]
      {:status 200
       :headers {"Content-Type" "text/html"}
       :body (str "Hello "
                  (if-let [target (System/getenv "TARGET")]
    (defn -main [& args]
      (run-jetty handler {:port (if-let [port (System/getenv "PORT")]
                                  (Integer/parseInt port)
  2. In your project directory, create a file named project.clj and copy the code below into it. This code defines the project’s dependencies and entrypoint.

    (defproject helloworld "1.0.0-SNAPSHOT"
      :description "Hello World - Clojure sample"
      :dependencies [[org.clojure/clojure "1.9.0"]
                     [ring/ring-core "1.6.3"]
                     [ring/ring-jetty-adapter "1.6.3"]]
      :main helloworld.core)
  3. In your project directory, create a file named Dockerfile and copy the code block below into it. For detailed instructions on dockerizing a Clojure app, see the clojure image documentation.

    # Use the official Clojure image.
    # https://hub.docker.com/_/clojure
    FROM clojure
    # Create the project and download dependencies.
    WORKDIR /usr/src/app
    COPY project.clj .
    RUN lein deps
    # Copy local code to the container image.
    COPY . .
    # Build an uberjar release artifact.
    RUN mv "$(lein uberjar | sed -n 's/^Created \(.*standalone\.jar\)/\1/p')" app-standalone.jar
    # Run the web service on container startup.
    CMD ["java", "-jar", "app-standalone.jar"]
  4. 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.

    apiVersion: serving.knative.dev/v1
    kind: Service
      name: helloworld-clojure
      namespace: default
            - image: docker.io/{username}/helloworld-clojure
                - name: TARGET
                  value: "Clojure Sample v1"

Building and deploying the 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-clojure .
    # Push the container to docker registry
    docker push {username}/helloworld-clojure
  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-clojure --output=custom-columns=NAME:.metadata.name,URL:.status.url
    NAME                URL
    helloworld-clojure  http://helloworld-clojure.default.
  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.

    curl http://helloworld-clojure.default.
    Hello World!

Removing the sample app deployment

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

kubectl delete --filename service.yaml