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Hello World - Kotlin sample

A simple web app written in Kotlin using Ktor 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 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).

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.

Create a new directory and cd into it:
```
mkdir hello
cd hello
```

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

mkdir -p src/main/kotlin/com/example/hello

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)
}

Switch back to hello directory

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

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'
}

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

FROM gradle

ADD build.gradle ./build.gradle
ADD src ./src
RUN gradle clean build
ENTRYPOINT ["java","-jar","-Djava.security.egd=file:/dev/./urandom","/home/gradle/build/libs/gradle.jar"]

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/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.

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-kotlin .

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

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 -f service.yaml
```
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).

To find the IP address for your service, use kubectl get service knative-ingressgateway -n 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.

kubectl get service knative-ingressgateway -n 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

To find the URL for your service, use

kubectl get services.serving.knative.dev helloworld-kotlin  -o=custom-columns=NAME:.metadata.name,DOMAIN:.status.domain

NAME                DOMAIN
helloworld-kotlin   helloworld-kotlin.default.example.com

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

Remove the sample app deployment

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

kubectl delete -f service.yaml

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