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Hello World - .NET Core sample

A simple web app written in C# using .NET Core 2.1 that you can use for testing. It reads in an env variable TARGET and prints "Hello World: ${TARGET}!". If TARGET is not specified, it will use "NOT SPECIFIED" as the TARGET.

Prerequisites

A Kubernetes Engine cluster with Knative installed. Follow the installation instructions if you need to create one.
The Google Cloud SDK is installed and initalized.
You have kubectl configured to connect to the Kubernetes cluster running Knative. If you created your cluster using the Google Cloud SDK, this has already be done. If you created your cluster from the Google Cloud Console, run the following command, replacing CLUSTER_NAME with the name of your cluster:
```
gcloud containers clusters get-credentials CLUSTER_NAME
```
You have installed .NET Core SDK 2.1.

Recreating 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.

From the console, create a new empty web project using the dotnet command:
```
dotnet new web -o helloworld-csharp
```

Update the CreateWebHostBuilder definition in Program.cs by adding .UseUrls("http://0.0.0.0:8080") to define the serving port:

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>().UseUrls("http://0.0.0.0:8080");

Update the app.Run(...) statement in Startup.cs to read and return the TARGET environment variable:

app.Run(async (context) =>
{
    var target = Environment.GetEnvironmentVariable("TARGET") ?? "NOT SPECIFIED";
    await context.Response.WriteAsync($"Hello World: {target}");
});

In your project directory, create a file named Dockerfile and copy the code block below into it. For detailed instructions on dockerizing a .NET core app, see dockerizing a .NET core app.

FROM microsoft/dotnet:2.1-sdk
WORKDIR /app

# copy csproj and restore as distinct layers
COPY *.csproj ./
RUN dotnet restore

# copy and build everything else
COPY . ./
RUN dotnet publish -c Release -o out
ENTRYPOINT ["dotnet", "out/helloworld-csharp.dll"]

Create a new file, service.yaml and copy the following service definition into the file. Make sure to replace {PROJECT_ID} with the ID of your Google Cloud project. If you are using docker or another container registry instead, replace the entire image path.

apiVersion: serving.knative.dev/v1alpha1
kind: Service
metadata:
  name: helloworld-python
  namespace: default
spec:
  runLatest:
    configuration:
      revisionTemplate:
        spec:
          container:
            image: gcr.io/{PROJECT_ID}/helloworld-csharp
            env:
            - name: TARGET
              value: "C# 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.

For this example, we'll use Google Cloud Container Builder to build the sample into a container. To use container builder, execute the following gcloud command:
```
gcloud container builds submit --tag gcr.io/${PROJECT_ID}/helloworld-csharp
```
After the build has completed, 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 URL and IP address for your service, use kubectl get ing to list the ingress points in the cluster. It may take a few seconds for the ingress point to be created.

kubectl get ing

NAME                        HOSTS                                       ADDRESS        PORTS     AGE
helloworld-csharp-ingress   helloworld-csharp.default.demo-domain.com   35.232.134.1   80        1m

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-csharp.default.demo-domain.com" http://{IP_ADDRESS}
Hello World!
```

Removing 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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