# 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](https://cloudevents.io/) in Knative
eventing, and optionally how to respond back with another CloudEvent in the http
response, using the
[Go SDK for CloudEvents](https://github.com/cloudevents/sdk-go)

We will deploy the app as a
[Kubernetes Deployment](https://kubernetes.io/docs/concepts/workloads/controllers/deployment/)
along with a
[Kubernetes Service](https://kubernetes.io/docs/concepts/services-networking/service/).
However, you can also deploy the app as a
[Knative Serving Service](https://knative.dev/docs/serving/).

Do the following steps 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:

```bash
git clone https://github.com/knative/docs.git knative-docs
cd knative-docs/code-samples/eventing/helloworld/helloworld-go
```

## Before you begin

- A Kubernetes cluster with [Knative Eventing](https://knative.dev/docs/install/eventing/install-eventing-with-yaml) installed.
- [Docker](https://www.docker.com) 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:

       ```go
        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.

       ```go
        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 following code
   block into it. For detailed instructions on dockerizing a Go app, see
   [Deploying Go servers with Docker](https://blog.golang.org/docker).

       ```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.

      ```yaml
      # 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
        annotations:
          # Note: you can set the eventing.knative.dev/broker.class annotation to change the class of the broker.
          # The default broker class is MTChannelBasedBroker, but Knative also supports use of the other class.
          eventing.knative.dev/broker.class: MTChannelBasedBroker
      spec: {}
      ---
      # Helloworld-go app deployment
      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
      ```

1. Use the go tool to create a
   [`go.mod`](https://github.com/golang/go/wiki/Modules#gomod) manifest.

       ```bash
       go mod init github.com/knative/docs/code-samples/serving/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:

       ```bash
       # 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 by running the following command:

       ```bash
       kubectl apply --filename sample-app.yaml
       ```

1. Verify that the command created the namespace `knative-samples` and a default Broker by running the command:

      ```bash
      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](https://knative.dev/docs/install/eventing/install-eventing-with-yaml/#install-optional-eventing-extensions).

1. It deployed the helloworld-go app as a K8s Deployment and created a K8s
      service names helloworld-go. Verify using the following command.

      ```bash
      kubectl --namespace knative-samples get deployments helloworld-go

      kubectl --namespace knative-samples get svc helloworld-go
      ```

1. It created a Knative Eventing Trigger to route certain events to the
      helloworld-go application. Make sure that Ready=true

      ```bash
      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](https://knative.dev/docs/eventing/broker/)
with correct CloudEvent headers set.

1. Deploy a curl pod and SSH into it

       ```bash
       kubectl -n knative-samples run curl --image=radial/busyboxplus:curl -it
       ```

1. Get the Broker URL

       ```bash
       kubectl -n knative-samples get broker default
       ```

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

    ```bash
    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

The Helloworld-go app logs the context and the msg of the event you created earlier, and replies
with another event.

1.  Display helloworld-go app logs

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

    You should see something similar to:

    ```{ .bash .no-copy }
    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](https://knative.dev/docs/eventing/broker/triggers/).

## 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. Using the following example, create a YAML file for a Pod that receives any
CloudEvent and logs the event to its output:

      ```yaml
      # event-display app deployment
      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
      ```

1. Apply the YAML file by running the command:

    ```bash
    kubectl apply -f <filename>.yaml
    ```
    Where `<filename>` is the name of the file you created in the previous step.

1. Using the following example, create a YAML file for a trigger to deliver the event
to the `event-display` Service:

       ```yaml
       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
       ```

1. Apply the YAML file by running the command:

    ```bash
    kubectl apply -f <filename>.yaml
    ```
    Where `<filename>` is the name of the file you created in the previous step.

1. [Send a CloudEvent to the Broker](#send-and-verify-cloudevents)

1. Check the logs of `event-display` service:

    ```bash
    kubectl -n knative-samples logs -l app=event-display --tail=50
    ```

    You should see something similar to:

    ```{ .bash .no-copy }
      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!"
        }
    ```

## Removing the sample app deployment

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

```bash
kubectl delete -f sample-app.yaml
```