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Hello World Eventing

As of v0.5, Knative Eventing defines Broker and Trigger to receive and filter messages. This is explained in more detail on Knative Eventing page:

Broker and Trigger

Under the covers, Knative creates Channels and Subscriptions and supports direct and fanout delivery options:


Knative Eventing has a few different types of event sources (Kubernetes, GitHub, GCP Pub/Sub etc.). In this tutorial, we will focus on GCP Pub/Sub events.

Install Knative Eventing

You probably installed Knative Eventing when you installed Knative. If not, follow the Knative installation instructions and take a look at the installation section in Knative Eventing page. In the end, you should have pods running in knative-eventing and knative-sources namespaces. Double check that this is the case:

kubectl get pods -n knative-eventing
kubectl get pods -n knative-sources

Configuring outbound network access

In Knative, the outbound network access is disabled by default. This means that you cannot even call Google Cloud APIs from Knative.

In our samples, we want to call Google Cloud APIs, so make sure you follow instructions on Configuring outbound network access page to enable access.

Setup Google Cloud Pub/Sub event source and default Broker

Follow the instructions on GCP Cloud Pub/Sub source page to setup Google Cloud Pub/Sub event source and also have a Broker injected in the default namespace. But don't create the trigger, we'll do that here.

In the end, you should have a GCP Pub/Sub source setup:

kubectl get gcppubsubsource

NAME             AGE
testing-source   1d

And a default broker as well:

kubectl get broker

NAME      READY   REASON   HOSTNAME                                   
default   True             default-broker.default.svc.cluster.local   

Hello World - .NET Core sample

Let's now create a .NET Core version of that sample. Create an empty ASP.NET Core app:

dotnet new web -o message-dumper-csharp

Inside the message-dumper-csharp folder, update Startup.cs to have a logger to print the contents of the event:

using System.IO;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;

namespace message_dumper_csharp
    public class Startup
        private readonly ILogger _logger;

        public Startup(ILogger<Startup> logger)
            _logger = logger;

        public void ConfigureServices(IServiceCollection services)

        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
            if (env.IsDevelopment())

            app.Run(async (context) =>
                using (var reader = new StreamReader(context.Request.Body))
                    var content = reader.ReadToEnd();
                    _logger.LogInformation("C# Message Dumper received message: " + content);
                    await context.Response.WriteAsync(content);

Change the log level in appsettings.json to Information:

  "Logging": {
    "LogLevel": {
      "Default": "Information"
  "AllowedHosts": "*"

Build and push Docker image

Before building the Docker image, make sure the app has no compilation errors:

dotnet build

Create a Dockerfile for the image:

FROM microsoft/dotnet:2.2-sdk

COPY *.csproj .
RUN dotnet restore

COPY . .

RUN dotnet publish -c Release -o out



CMD ["dotnet", "out/message-dumper-csharp.dll"]

Build and push the Docker image (replace meteatamel with your actual DockerHub):

docker build -t meteatamel/message-dumper-csharp:v1 .

docker push meteatamel/message-dumper-csharp:v1

Deploy the service and create a trigger

Create a trigger.yaml file.

kind: Service
  name: message-dumper-csharp
            # Replace meteatamel with your actual DockerHub
            # Disable scale to zero with a minScale of 1.
kind: Trigger
  name: gcppubsub-source-sample-csharp
      kind: Service
      name: message-dumper-csharp

This defines the Knative Service that will run our code and Trigger to connect to Pub/Sub messages to the Service.

kubectl apply -f trigger.yaml

Check that the service and trigger are created:

kubectl get ksvc,trigger

Test the service

We can now test our service by sending a message to Pub/Sub topic:

gcloud pubsub topics publish testing --message="Hello World"

Wait a little and check that a pod is created:

kubectl get pods --selector

You can inspect the logs of the subscriber (replace <podid> with actual pod id):

kubectl logs --follow -c user-container <podid>

You should see something similar to this:

Hosting environment: Production
Content root path: /app
Now listening on:
Application started. Press Ctrl+C to shut down.
Application is shutting down...
Hosting environment: Production
Content root path: /app
Now listening on:
Application started. Press Ctrl+C to shut down.
info: Microsoft.AspNetCore.Hosting.Internal.WebHost[1]
      Request starting HTTP/1.1 POST http://message-dumper-csharp.default.svc.cluster.local/ application/json 108
info: message_dumper_csharp.Startup[0]
      C# Message Dumper received message: {"ID":"198012587785403","Data":"SGVsbG8gV29ybGQ=","Attributes":null,"PublishTime":"2019-01-21T15:25:58.25Z"}
info: Microsoft.AspNetCore.Hosting.Internal.WebHost[2]
      Request finished in 29.9881ms 200 

Finally, if you decode the Data field, you should see the "Hello World" message:

echo "SGVsbG8gV29ybGQ=" | base64 -D
Hello World

What's Next?

Integrate with Translation API

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