Hello World - Golang

A simple web app written in Go that you can use to test knative eventing. It shows how to consume a CloudEvent in Knative eventing, and optionally how to respond back with another CloudEvent in the http response, using the Go SDK for CloudEvents

We will deploy the app as a Kubernetes Deployment along with a Kubernetes Service. However, you can also deploy the app as a Knative Serving Service.

Follow the steps below to create the sample code and then deploy the app to your cluster. You can also download a working copy of the sample, by running the following commands:

git clone -b "{{< branch >}}" https://github.com/knative/docs knative-docs
cd knative-docs/docs/eventing/samples/helloworld/helloworld-go

Before you begin

  • A Kubernetes cluster with Knative Eventing installed.
  • Docker installed and running on your local machine, and a Docker Hub account configured (we’ll use it for a container registry).

Recreating the sample code

  1. Create a new file named helloworld.go and paste the following code. This code creates a basic web server which listens on port 8080:

     import (
         "context"
         "log"
    
         cloudevents "github.com/cloudevents/sdk-go/v2"
         "github.com/google/uuid"
     )
    
     func receive(ctx context.Context, event cloudevents.Event) (*cloudevents.Event, cloudevents.Result) {
         // Here is where your code to process the event will go.
         // In this example we will log the event msg
         log.Printf("Event received. \n%s\n", event)
         data := &HelloWorld{}
         if err := event.DataAs(data); err != nil {
             log.Printf("Error while extracting cloudevent Data: %s\n", err.Error())
             return nil, cloudevents.NewHTTPResult(400, "failed to convert data: %s", err)
         }
         log.Printf("Hello World Message from received event %q", data.Msg)
    
         // Respond with another event (optional)
         // This is optional and is intended to show how to respond back with another event after processing.
         // The response will go back into the knative eventing system just like any other event
         newEvent := cloudevents.NewEvent()
         newEvent.SetID(uuid.New().String())
         newEvent.SetSource("knative/eventing/samples/hello-world")
         newEvent.SetType("dev.knative.samples.hifromknative")
         if err := newEvent.SetData(cloudevents.ApplicationJSON, HiFromKnative{Msg: "Hi from helloworld-go app!"}); err != nil {
             return nil, cloudevents.NewHTTPResult(500, "failed to set response data: %s", err)
         }
         log.Printf("Responding with event\n%s\n", newEvent)
         return &newEvent, nil
     }
    
     func main() {
         log.Print("Hello world sample started.")
         c, err := cloudevents.NewDefaultClient()
         if err != nil {
             log.Fatalf("failed to create client, %v", err)
         }
         log.Fatal(c.StartReceiver(context.Background(), receive))
     }
    
  2. Create a new file named eventschemas.go and paste the following code. This defines the data schema of the CloudEvents.

     package main
    
     // HelloWorld defines the Data of CloudEvent with type=dev.knative.samples.helloworld
     type HelloWorld struct {
       // Msg holds the message from the event
       Msg string `json:"msg,omitempty,string"`
     }
    
     // HiFromKnative defines the Data of CloudEvent with type=dev.knative.samples.hifromknative
     type HiFromKnative struct {
       // Msg holds the message from the event
       Msg string `json:"msg,omitempty,string"`
     }
    
  3. In your project directory, create a file named Dockerfile and copy the code block below into it. For detailed instructions on dockerizing a Go app, see Deploying Go servers with Docker.

    # Use the offical Golang image to create a build artifact.
    # This is based on Debian and sets the GOPATH to /go.
    # https://hub.docker.com/_/golang
    FROM golang:1.12 as builder
    
    # Copy local code to the container image.
    WORKDIR /go/src/github.com/knative/docs/helloworld
    COPY . .
    
    # Build the command inside the container.
    # (You may fetch or manage dependencies here,
    # either manually or with a tool like "godep".)
    RUN CGO_ENABLED=0 GOOS=linux go build -v -o helloworld
    
    # Use a Docker multi-stage build to create a lean production image.
    # https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
    FROM alpine
    RUN apk add --no-cache ca-certificates
    
    # Copy the binary to the production image from the builder stage.
    COPY --from=builder /go/src/github.com/knative/docs/helloworld/helloworld /helloworld
    
    # Run the web service on container startup.
    CMD ["/helloworld"]
    
  4. Create a new file, sample-app.yaml and copy the following service definition into the file. Make sure to replace {username} with your Docker Hub username.

    # Namespace for sample application with eventing enabled
     apiVersion: v1
     kind: Namespace
     metadata:
       name: knative-samples
       labels:
           knative-eventing-injection: enabled
     ---
     # Helloworld-go app deploment
     apiVersion: apps/v1
     kind: Deployment
     metadata:
       name: helloworld-go
     spec:
       replicas: 1
       selector:
         matchLabels: &labels
           app: helloworld-go
       template:
         metadata:
           labels: *labels
         spec:
           containers:
             - name: helloworld-go
               image: docker.io/{username}/helloworld-go
    
     ---
    
     # Service that exposes helloworld-go app.
     # This will be the subscriber for the Trigger
       kind: Service
       apiVersion: v1
       metadata:
         name: helloworld-go
       spec:
         selector:
           app: helloworld-go
         ports:
         - protocol: TCP
           port: 80
           targetPort: 8080
     ---
     # Knative Eventing Trigger to trigger the helloworld-go service
     apiVersion: eventing.knative.dev/v1alpha1
     kind: Trigger
     metadata:
       name: helloworld-go
       namespace: knative-samples
     spec:
       broker: default
       filter:
         attributes:
           type: dev.knative.samples.helloworld
           source: dev.knative.samples/helloworldsource
       subscriber:
         ref:
           apiVersion: v1
           kind: Service
           name: helloworld-go
    

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-go .
    
    # Push the container to docker registry
    docker push {username}/helloworld-go
    
  2. After the build has completed and the container is pushed to docker hub, you can deploy the sample application into your cluster. Ensure that the container image value in sample-app.yaml matches the container you built in the previous step. Apply the configuration using kubectl:

    kubectl apply --filename sample-app.yaml
    
    1. Above command created a namespace knative-samples and labelled it with knative-eventing-injection=enabled, to enable eventing in the namespace. Verify using the following command:

      kubectl get ns knative-samples --show-labels
      
    2. It deployed the helloworld-go app as a K8s Deployment and created a K8s service names helloworld-go. Verify using the following command.

      kubectl --namespace knative-samples get deployments helloworld-go
      
      kubectl --namespace knative-samples get svc helloworld-go
      
    3. It created a Knative Eventing Trigger to route certain events to the helloworld-go application. Make sure that Ready=true

      kubectl --namespace knative-samples get trigger helloworld-go
      

Send and verify CloudEvents

Once you have deployed the application and verified that the namespace, sample application and trigger are ready, let’s send a CloudEvent.

Send CloudEvent to the Broker

We can send an http request directly to the Broker with correct CloudEvent headers set.

  1. Deploy a curl pod and SSH into it

    kubectl --namespace knative-samples run curl --image=radial/busyboxplus:curl -it
    
  2. Run the following in the SSH terminal

    curl -v "default-broker.knative-samples.svc.cluster.local" \
        -X POST \
        -H "Ce-Id: 536808d3-88be-4077-9d7a-a3f162705f79" \
        -H "Ce-Specversion: 1.0" \
        -H "Ce-Type: dev.knative.samples.helloworld" \
        -H "Ce-Source: dev.knative.samples/helloworldsource" \
        -H "Content-Type: application/json" \
        -d '{"msg":"Hello World from the curl pod."}'
    
    exit
    

Verify that event is received by helloworld-go app

Helloworld-go app logs the context and the msg of the above event, and replies back with another event.

  1. Display helloworld-go app logs shell kubectl --namespace knative-samples logs -l app=helloworld-go --tail=50 You should see something similar to:

    Event received.
    Validation: valid
    Context Attributes,
     specversion: 1.0
     type: dev.knative.samples.helloworld
     source: dev.knative.samples/helloworldsource
     id: 536808d3-88be-4077-9d7a-a3f162705f79
     time: 2019-10-04T22:35:26.05871736Z
     datacontenttype: application/json
    Extensions,
     knativearrivaltime: 2019-10-04T22:35:26Z
     knativehistory: default-kn2-trigger-kn-channel.knative-samples.svc.cluster.local
     traceparent: 00-971d4644229653483d38c46e92a959c7-92c66312e4bb39be-00
    Data,
      {"msg":"Hello World from the curl pod."}
    
    Hello World Message "Hello World from the curl pod."
    Responded with event
    Validation: valid
    Context Attributes,
      specversion: 1.0
      type: dev.knative.samples.hifromknative
      source: knative/eventing/samples/hello-world
      id: 37458d77-01f5-411e-a243-a459bbf79682
      datacontenttype: application/json
    Data,
      {"msg":"Hi from Knative!"}
    
    

    Play around with the CloudEvent attributes in the curl command and the trigger specification to understand how Triggers work.

Verify reply from helloworld-go app

helloworld-go app replies back with an event of type= dev.knative.samples.hifromknative, and source=knative/eventing/samples/hello-world. This event enters the eventing mesh via the Broker and can be delivered to other services using a Trigger

  1. Deploy a pod that receives any CloudEvent and logs the event to its output.

    kubectl --namespace knative-samples apply --filename - << END
    # event-display app deploment
    apiVersion: apps/v1
    kind: Deployment
    metadata:
      name: event-display
      namespace: knative-samples
    spec:
      replicas: 1
      selector:
        matchLabels: &labels
          app: event-display
      template:
        metadata:
          labels: *labels
        spec:
          containers:
            - name: helloworld-go
              # Source code: https://github.com/knative/eventing-contrib/tree/master/cmd/event_display
              image: gcr.io/knative-releases/knative.dev/eventing-contrib/cmd/event_display
    ---
    # Service that exposes event-display app.
    # This will be the subscriber for the Trigger
    kind: Service
    apiVersion: v1
    metadata:
      name: event-display
      namespace: knative-samples
    spec:
      selector:
        app: event-display
      ports:
        - protocol: TCP
          port: 80
          targetPort: 8080
    END
    
  2. Create a trigger to deliver the event to the above service

    kubectl --namespace knative-samples apply --filename - << END
    apiVersion: eventing.knative.dev/v1alpha1
    kind: Trigger
    metadata:
      name: event-display
      namespace: knative-samples
    spec:
      broker: default
      filter:
        attributes:
          type: dev.knative.samples.hifromknative
          source: knative/eventing/samples/hello-world
      subscriber:
        ref:
          apiVersion: v1
          kind: Service
          name: event-display
    END
    
  3. Send a CloudEvent to the Broker

  4. Check the logs of event-display service

    kubectl --namespace knative-samples logs -l app=event-display --tail=50
    

    You should see something similar to:

      cloudevents.Event
      Validation: valid
      Context Attributes,
        specversion: 0.3
        type: dev.knative.samples.hifromknative
        source: knative/eventing/samples/hello-world
        id: 8a7384b9-8bbe-4634-bf0f-ead07e450b2a
        time: 2019-10-04T22:53:39.844943931Z
        datacontenttype: application/json
      Extensions,
        knativearrivaltime: 2019-10-04T22:53:39Z
        knativehistory: default-kn2-ingress-kn-channel.knative-samples.svc.cluster.local
        traceparent: 00-4b01db030b9ea04bb150b77c8fa86509-2740816590a7604f-00
      Data,
        {
          "msg": "Hi from helloworld-go app!"
        }
    

Note: You could use the above approach to test your applications too.

Removing the sample app deployment

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

kubectl delete --filename sample-app.yaml