# Hello World - Clojure sample

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.

## Prerequisites

* A Kubernetes cluster with Knative installed. Follow the
  [installation instructions](https://github.com/knative/docs/blob/master/install/README.md) if you need
  to create one.
* [Docker](https://www.docker.com) 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:

    ```clojure
    (ns helloworld.core
      (:use ring.adapter.jetty)
      (:gen-class))

    (defn handler [request]
      {:status 200
       :headers {"Content-Type" "text/html"}
       :body (str "Hello "
                  (if-let [target (System/getenv "TARGET")]
                    target
                    "World")
                  "!\n")})

    (defn -main [& args]
      (run-jetty handler {:port (if-let [port (System/getenv "PORT")]
                                  (Integer/parseInt port)
                                  8080)}))
    ```
    
1. 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.
   
   ```clojure
   (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)
   ```

1. 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](https://github.com/docker-library/docs/tree/master/clojure).

    ```docker
    # 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

    # Configure and document the service HTTP port.
    ENV PORT 8080
    EXPOSE $PORT

    # Run the web service on container startup.
    CMD ["java", "-jar", "app-standalone.jar"]
    ```

1. 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.

    ```yaml
    apiVersion: serving.knative.dev/v1alpha1
    kind: Service
    metadata:
      name: helloworld-clojure
      namespace: default
    spec:
      runLatest:
        configuration:
          revisionTemplate:
            spec:
              container:
                image: docker.io/{username}/helloworld-clojure
                env:
                - 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:

    ```shell
    # Build the container on your local machine
    docker build -t {username}/helloworld-clojure .

    # Push the container to docker registry
    docker push {username}/helloworld-clojure
    ```

1. 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`:

    ```shell
    kubectl apply --filename service.yaml
    ```

1. 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).

1. To find the IP address for your service, use
   `kubectl get svc knative-ingressgateway --namespace istio-system` to get the ingress IP for your
   cluster. If your cluster is new, it may take sometime for the service to get asssigned
   an external IP address.

    ```shell
    kubectl get svc knative-ingressgateway --namespace istio-system

    NAME                     TYPE           CLUSTER-IP     EXTERNAL-IP      PORT(S)                                      AGE
    knative-ingressgateway   LoadBalancer   10.23.247.74   35.203.155.229   80:32380/TCP,443:32390/TCP,32400:32400/TCP   2d

    ```

1. To find the URL for your service, use
    ```
    kubectl get ksvc helloworld-clojure --output=custom-columns=NAME:.metadata.name,DOMAIN:.status.domain
    NAME                DOMAIN
    helloworld-clojure  helloworld-clojure.default.example.com
    ```

1. Now you can make a request to your app to see the results. Replace
   `{IP_ADDRESS}` with the address you see returned in the previous step.

    ```shell
    curl -H "Host: helloworld-clojure.default.example.com" http://{$IP_ADDRESS}
    Hello World: Clojure Sample v1!
    ```

## Removing the sample app deployment

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

```shell
kubectl delete --filename service.yaml
```
