# Hello World - Dart sample

A simple web app written in the [Dart](www.dartlang.org) programming language
that you can use for testing. It reads in the env variable `TARGET` and prints
`"Hello $TARGET"`. If `TARGET` is not specified, it will use `"World"` as `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).
* [dart-sdk](https://www.dartlang.org/tools/sdk#install) installed and configured
  if you want to run the program locally.

## Recreating the sample code

While you can clone all of the code from this directory, it is useful to know how
to build a hello world Dart application step-by-step. This application can be
created using the following instructions.

1. Create a new directory and write `pubspec.yaml` as follows:

    ```yaml
    name: hello_world_dart
    private: True  # let's not accidentally publish this to pub.dartlang.org
    description: >-
      Hello world server example in dart.
    dependencies:
      shelf: ^0.7.3
    environment:
      sdk: '>=2.0.0 <3.0.0'
    ```

2. If you want to run locally, install dependencies. If you only want to run in
   Docker or Knative, you can skip this step.

    ```shell
    pub get
    ```

3. Create a new file `bin/main.dart` and write the following code:

    ```dart
    import 'dart:io';

    import 'package:shelf/shelf.dart';
    import 'package:shelf/shelf_io.dart';

    void main() {
      // Find port to listen on from environment variable.
      var port = int.tryParse(Platform.environment['PORT']);
      
      // Read $TARGET from environment variable.
      var target = Platform.environment['TARGET'] ?? 'World';

      // Create handler.
      var handler = Pipeline().addMiddleware(logRequests()).addHandler((request) {
        return Response.ok('Hello $target');
      });

      // Serve handler on given port.
      serve(handler, InternetAddress.anyIPv4, port).then((server) {
        print('Serving at http://${server.address.host}:${server.port}');
      });
    }
    ```

4. Create a new file named `Dockerfile`, this file defines instructions for 
   dockerizing your applications, for dart apps this can be done as follows:

    ```Dockerfile
    # Use Google's official Dart image.
    # https://hub.docker.com/r/google/dart-runtime/
    FROM google/dart-runtime

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

5. 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-dart
      namespace: default
    spec:
      runLatest:
        configuration:
          revisionTemplate:
            spec:
              container:
                image: docker.io/{username}/helloworld-dart
                env:
                - name: TARGET
                  value: "Dart 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-dart .

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

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-dart  --output=custom-columns=NAME:.metadata.name,DOMAIN:.status.domain
    NAME                DOMAIN
    helloworld-dart   helloworld-dart.default.example.com
    ```

1. 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-dart.default.example.com" http://{IP_ADDRESS}
    Hello Dart 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
```

