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:

    package main
   
    import (
      "context"
      "fmt"
      "log"
      "net/http"
      "os"
   
      cloudevents "github.com/cloudevents/sdk-go"
      "github.com/google/uuid"
    )
   
    type eventData struct {
      Message string `json:"message,omitempty,string"`
    }
   
    func receive(ctx context.Context, event cloudevents.Event, response *cloudevents.EventResponse) error {
      // Here is where your code to process the event will go.
      // In this example we will log the event msg
      log.Printf("Event Context: %+v\n", event.Context)
      data := &HelloWorld{}
      if err := event.DataAs(data); err != nil {
        log.Printf("Error while extracting cloudevent Data: %s\n", err.Error())
        return err
      }
      log.Printf("Hello World Message %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")
      newEvent.SetData(HiFromKnative{Msg: "Hi from Knative!"})
      response.RespondWith(200, &newEvent)
   
      log.Printf("Responded with event %v", newEvent)
   
      return nil
    }
   
    func handler(w http.ResponseWriter, r *http.Request) {
      log.Print("Hello world received a request.")
      target := os.Getenv("TARGET")
      if target == "" {
        target = "World"
      }
      fmt.Fprintf(w, "Hello %s!\n", target)
    }
   
    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: 0.3" \
   -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

   kubectl --namespace knative-samples logs -l app=helloworld-go --tail=50
   

   You should see something similar to:

   Event received. Context: Context Attributes,
     specversion: 0.3
     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
   
   Hello World Message "Hello World from the curl pod."
   Responded with event Validation: valid
   Context Attributes,
     specversion: 0.2
     type: dev.knative.samples.hifromknative
     source: knative/eventing/samples/hello-world
     id: 37458d77-01f5-411e-a243-a459bbf79682
   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
             image: gcr.io/knative-releases/github.com/knative/eventing-sources/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