# Hello World - Kotlin sample

A simple web app written in Kotlin using [Ktor](https://ktor.io/) 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).

## Steps to recreate 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 directory and cd into it:

    ```shell
    mkdir hello
    cd hello
    ```
2. Create a file named `Main.kt` at `src/main/kotlin/com/example/hello` and copy the code block below into it:

    ```shell
    mkdir -p src/main/kotlin/com/example/hello
    ```
	
    ```kotlin
    package com.example.hello

    import io.ktor.application.*
    import io.ktor.http.*
    import io.ktor.response.*
    import io.ktor.routing.*
    import io.ktor.server.engine.*
    import io.ktor.server.netty.*

    fun main(args: Array<String>) {
        val target = System.getenv("TARGET") ?: "World"
        val port = System.getenv("PORT") ?: "8080"
        embeddedServer(Netty, port.toInt()) {
            routing {
                get("/") {
                    call.respondText("Hello $target", ContentType.Text.Html)
                }
            }
        }.start(wait = true)
    }
    ```
3. Switch back to `hello` directory

4. Create a new file, `build.gradle` and copy the following setting

   ```groovy
   buildscript {
       ext.kotlin_version = '1.2.61'
       ext.ktor_version = '0.9.4'

       repositories {
           mavenCentral()
       }
       dependencies {
           classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
       }
   }

   apply plugin: 'java'
   apply plugin: 'kotlin'
   apply plugin: 'application'

   sourceCompatibility = 1.8
   compileKotlin {
       kotlinOptions.jvmTarget = "1.8"
   }
   
   compileTestKotlin {
       kotlinOptions.jvmTarget = "1.8"
   }

   repositories {
       jcenter()
       maven { url "https://dl.bintray.com/kotlin/ktor" }
   }

   mainClassName = 'com.example.hello.MainKt'

   jar {
       manifest {
           attributes 'Main-Class': mainClassName
       }
       from { configurations.compile.collect { it.isDirectory() ? it : zipTree(it) } }
   }

   dependencies {
       compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
       compile "io.ktor:ktor-server-netty:$ktor_version"
       testCompile group: 'junit', name: 'junit', version: '4.12'
   }
   ```

5. Create a file named `Dockerfile` and copy the code block below into it.

    ```docker
    # Use the official gradle image to create a build artifact.
    # https://hub.docker.com/_/gradle
    FROM gradle as builder

    # Copy local code to the container image.
    COPY build.gradle .
    COPY src ./src

    # Build a release artifact.
    RUN gradle clean build --no-daemon

    # Use the Official OpenJDK image for a lean production stage of our multi-stage build.
    # https://hub.docker.com/_/openjdk
    # https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
    FROM openjdk:8-jre-alpine

    # Copy the jar to the production image from the builder stage.
    COPY --from=builder /home/gradle/build/libs/gradle.jar /helloworld.jar

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

    # Run the web service on container startup.
    CMD [ "java", "-jar", "-Djava.security.egd=file:/dev/./urandom", "/helloworld.jar" ]
    ```

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.

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

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

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

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`:

    ```shell
    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 IP address for your service, use
   `kubectl get service 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 assigned
   an external IP address.

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

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

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

    ```shell
    curl -H "Host: helloworld-kotlin.default.example.com" http://{IP_ADDRESS}
    ```
    ```shell
    Hello Kotlin Sample v1
    ```

## Remove the sample app deployment

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

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