docs/community/samples/serving/helloworld-dart

Hello World - Dart sample

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. 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
   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.

   pub get
   

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

   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:

   # Use Google's official Dart image.
   # https://hub.docker.com/r/google/dart-runtime/
   FROM google/dart-runtime
   
   # Service must listen to $PORT environment variable.
   # This default value facilitates local development.
   ENV PORT 8080
   

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/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:

   # 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 IP address for your service, use these commands 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.

   # In Knative 0.2.x and prior versions, the `knative-ingressgateway` service was used instead of `istio-ingressgateway`.
   INGRESSGATEWAY=knative-ingressgateway
   
   # The use of `knative-ingressgateway` is deprecated in Knative v0.3.x.
   # Use `istio-ingressgateway` instead, since `knative-ingressgateway`
   # will be removed in Knative v0.4.
   if kubectl get configmap config-istio -n knative-serving &> /dev/null; then
       INGRESSGATEWAY=istio-ingressgateway
   fi
   
   kubectl get svc $INGRESSGATEWAY --namespace istio-system
   
   NAME                     TYPE           CLUSTER-IP     EXTERNAL-IP      PORT(S)                                      AGE
   xxxxxxx-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

   kubectl get ksvc helloworld-dart  --output=custom-columns=NAME:.metadata.name,DOMAIN:.status.domain
   NAME                DOMAIN
   helloworld-dart   helloworld-dart.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.

   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:

kubectl delete --filename service.yaml