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 "v0.23.0" 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¶

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))
 }

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"`
 }

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"]

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

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.

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

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

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.
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
```
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.

Deploy a curl pod and SSH into it

kubectl --namespace knative-samples run curl --image=radial/busyboxplus:curl -it

Get the Broker URL

kubectl --namespace knative-samples get broker default

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.

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

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/tree/main/cmd/event_display
          image: gcr.io/knative-releases/knative.dev/eventing/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

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

Send a CloudEvent to the Broker

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