# Hello World - Python
A simple web app written in Python 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, by adding the Cloud Eventing headers outlined in the Cloud Events standard definition.
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
# Clone the relevant branch version such as "release-0.13"
git clone https://github.com/knative/docs.git knative-docs
cd knative-docs/code-samples/eventing/helloworld/helloworld-python
```
## 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.py` and paste the following code. This
code creates a basic web server which listens on port 8080:
```python
from flask import Flask, request, make_response
import uuid
app = Flask(__name__)
@app.route('/', methods=['POST'])
def hello_world():
app.logger.warning(request.data)
# Respond with another event (optional)
response = make_response({
"msg": "Hi from helloworld-python app!"
})
response.headers["Ce-Id"] = str(uuid.uuid4())
response.headers["Ce-specversion"] = "0.3"
response.headers["Ce-Source"] = "knative/eventing/samples/hello-world"
response.headers["Ce-Type"] = "dev.knative.samples.hifromknative"
return response
if __name__ == '__main__':
app.run(debug=True, host='0.0.0.0', port=8080)
```
1. Add a `requirements.txt` file containing the following contents:
```bash
Flask==2.0.3
```
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
FROM python:3.9-alpine
COPY . /app
WORKDIR /app
RUN pip install -r requirements.txt
EXPOSE 8080
ENTRYPOINT [ "python" ]
CMD [ "helloworld.py" ]
```
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 with eventing enabled
apiVersion: v1
kind: Namespace
metadata:
name: knative-samples
labels:
eventing.knative.dev/injection: enabled
---
# 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-python app deployment
apiVersion: apps/v1
kind: Deployment
metadata:
name: helloworld-python
namespace: knative-samples
spec:
replicas: 1
selector:
matchLabels: &labels
app: helloworld-python
template:
metadata:
labels: *labels
spec:
containers:
- name: helloworld-python
image: docker.io/{username}/helloworld-python
imagePullPolicy: IfNotPresent
---
# Service that exposes helloworld-python app.
# This will be the subscriber for the Trigger
apiVersion: v1
kind: Service
metadata:
name: helloworld-python
namespace: knative-samples
spec:
selector:
app: helloworld-python
ports:
- protocol: TCP
port: 80
targetPort: 8080
---
# Knative Eventing Trigger to trigger the helloworld-python service
apiVersion: eventing.knative.dev/v1
kind: Trigger
metadata:
name: helloworld-python
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-python
```
## 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 and push the container on your local machine.
docker buildx build --platform linux/arm64,linux/amd64 -t "{username}/helloworld-python" --push .
```
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`:
```bash
kubectl apply -f sample-app.yaml
```
1. The previous command creates a namespace `knative-samples` and labels it with `knative-eventing-injection=enabled`, to enable eventing in the namespace. Verify using the following command:
```bash
kubectl get ns knative-samples --show-labels
```
1. It deployed the `helloworld-python` app as a K8s Deployment and created a K8s service names helloworld-python. Verify using the following command:
```bash
kubectl --namespace knative-samples get deployments helloworld-python
kubectl --namespace knative-samples get svc helloworld-python
```
1. It created a Knative Eventing Trigger to route certain events to the helloworld-python application. Make sure that `Ready=true`:
```bash
kubectl -n knative-samples get trigger helloworld-python
```
## Send and verify CloudEvents
After you have deployed the application, and have verified that the namespace, sample application and trigger are ready, you can send a CloudEvent.
### Send CloudEvent to the Broker
You can send an HTTP request directly to the Knative [Broker](https://knative.dev/docs/eventing/broker/) if the correct CloudEvent headers are set.
1. Deploy a curl pod and SSH into it:
```bash
kubectl --namespace knative-samples run curl --image=radial/busyboxplus:curl -it
```
1. Run the following command in the SSH terminal:
```bash
curl -v "broker-ingress.knative-eventing.svc.cluster.local/knative-samples/default" \
-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-python app
The Helloworld-python app logs the context and the msg of the event you created earlier, and replies with another event.
1. Display helloworld-python app logs
```bash
kubectl --namespace knative-samples logs -l app=helloworld-python --tail=50
```
You should see something similar to:
```{ .bash .no-copy }
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!"}
```
Try the CloudEvent attributes in the curl command and the trigger specification to understand how
[Triggers](https://knative.dev/docs/eventing/broker/triggers/) work.
## Verify reply from helloworld-python app
The `helloworld-python` app replies with an event type `type= dev.knative.samples.hifromknative`, and source `source=knative/eventing/samples/hello-world`. The event enters the eventing mesh through the broker, and can be delivered to event sinks using a trigger
1. Deploy a Pod that receives any CloudEvent and logs the event to its output.
1. Copy the following YAML into a file:
```yaml
# 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-python
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. Create a trigger to deliver the event to the previously created service.
1. Copy the following YAML into a file:
```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
```
Example output:
```{ .bash .no-copy }
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- 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
```