Go

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 "v0.21.x" 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))
 }

1. 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"`
 }

1. 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 official 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:1.14 as builder

 # Copy local code to the container image.
 WORKDIR /app

 # Retrieve application dependencies using go modules.
 # Allows container builds to reuse downloaded dependencies.
 COPY go.* ./
 RUN go mod download

 # Copy local code to the container image.
 COPY . ./

 # Build the binary.
 # -mod=readonly ensures immutable go.mod and go.sum in container builds.
 RUN CGO_ENABLED=0 GOOS=linux go build -mod=readonly  -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:3
 RUN apk add --no-cache ca-certificates

 # Copy the binary to the production image from the builder stage.
 COPY --from=builder /app/helloworld /helloworld

 # Run the web service on container startup.
 CMD ["/helloworld"]

1. 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
   apiVersion: v1
   kind: Namespace
   metadata:
     name: knative-samples
   ---
   # A default broker
   apiVersion: eventing.knative.dev/v1
   kind: Broker
   metadata:
     name: default
     namespace: knative-samples
   spec: {}
   ---
   # Helloworld-go app deploment
   apiVersion: apps/v1
   kind: Deployment
   metadata:
     name: helloworld-go
     namespace: knative-samples
   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
     namespace: knative-samples
   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/v1
   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
   

2. Use the go tool to create a go.mod manifest.

go mod init github.com/knative/docs/docs/serving/samples/hello-world/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

1. 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 create a default Broker it. Verify using the following command:

   kubectl get broker --namespace knative-samples
   

   Note: you can also use injection based on labels with the Eventing sugar controller. For how to install the Eventing sugar controller, see Install optional Eventing extensions.

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. Get the Broker URL

   kubectl --namespace knative-samples get broker default
   

3. Run the following in the SSH terminal. Please replace the URL with the URL of the default broker.

curl -v "http://broker-ingress.knative-eventing.svc.cluster.local/knative-samples/default" \
    -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

   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/main/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/v1
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

1. Send a CloudEvent to the Broker

2. 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: 1.0
       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