Hello World - Dart

A simple web app written in the Dart 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 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).
  • dart-sdk 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:

    name: hello_world_dart
publish_to: none # let's not accidentally publish this to pub.dartlang.org

environment:
  sdk: ">=2.12.0 <3.0.0"

dependencies:
  shelf: ^1.0.0

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

    > pub get

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

    import 'dart:io';

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

Future main() async {
  // Find port to listen on from environment variable.
  final port = int.parse(Platform.environment['PORT'] ?? '8080');

  // Read $TARGET from environment variable.
  final target = Platform.environment['TARGET'] ?? 'World';

  Response handler(Request request) => Response.ok('Hello $target');

  // Serve handler on given port.
  final server = await serve(
    const Pipeline().addMiddleware(logRequests()).addHandler(handler),
    InternetAddress.anyIPv4,
    port,
  );
  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:

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

  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.

    apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: helloworld-dart
  namespace: default
spec:
  template:
    spec:
      containers:
        - 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:

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

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

  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-dart  --output=custom-columns=NAME:.metadata.name,URL:.status.url
NAME             URL
helloworld-dart  http://helloworld-dart.default.1.2.3.4.xip.io

  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-dart.default.1.2.3.4.xip.io
Hello Dart Sample v1

Removing the sample app deployment

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

kubectl delete --filename service.yaml