cloud native, knative

Knative Eventing: Part 3 – Replying to broker

In part two of my Knative eventing tutorials, we streamed websocket data to a broker and then subscribed to the events with an event display service that displayed the events in real-time via a web UI.

In this post, I am going to show how to create reply events to a broker and then subscribe to them from a different service. Once we are done, we will have something that should look like the following:

In this scenario, we are receiving events from a streaming source (in this case a websocket) and each time we receive an event, we want to send another event as a reply back to the broker. In this example, we are just going to do this very simply in that every time we get an event, we send another.

In the next tutorial, we will look to receive the events and then perform some analysis on the fly, for example analyse the value of a transaction and assign a size variable. We could then implement some logic like if size = XL, send a reply back to the broker, which could then be listened for by an alerting application.


I am running Kubernetes on Docker Desktop for mac. You will also need Istio and Knative eventing installed.

Deploy a namespace:

kubectl create namespace knative-eventing-websocket-source

apply the knative-eventing label:

kubectl label namespace knative-eventing-websocket-source knative-eventing-injection=enabled

Ensure you have the cluster local gateway set up.

Adding newEvent logic:

In our application code, we are adding some code for sending a new reply event every time an event is received. The code is here:

newEvent := cloudevents.NewEvent()
newEvent.SetData("Hi from Knative!")
response.RespondWith(200, &newEvent)

This code creates a new event, with the following information:

source = ""

type = "dev.knative.eventing.jsaladas.transaction.classify"

data = "Hi from Knative!"

I can use whichever values I want for the above, this is just the values I decided on, feel free to change them. We can then use these fields later to filter for the reply events only. Our code will now, aside from receiving an event and displaying it, generate a new event that will enter the knative eventing ecosystem.

Initial components to run the example

The main code for this tutorial is already in the github repo for part 2. If you already followed this and have this running then you will need to redeploy the event-display service with a new image. For those who didn’t join for part 2, this part shows you how to deploy the components.

Deploy the websocket source application:

kubectl apply -f 010-deployment.yaml

Here is the yaml below:

apiVersion: apps/v1
kind: Deployment
  name: wseventsource
  namespace: knative-eventing-websocket-source
  replicas: 1
    matchLabels: &labels
      app: wseventsource
      labels: *labels
        - name: wseventsource
          - name: SINK
            value: "http://default-broker.knative-eventing-websocket-source.svc.cluster.local"

Next we will apply the trigger that will set up a subscription for the event display service to subscribe to events from the broker that have a source equal to "wss://". We can also filter on type or even another cloudEvent variable or, if we left them both empty, then it would filter on all events.

kubectl apply -f 040-trigger.yaml

Here is the trigger yaml:

kind: Trigger
  name: wsevent-trigger
  namespace: knative-eventing-websocket-source
  broker: default
      type: ""
      source: "wss://"
      apiVersion: v1
      kind: Service
      name: event-display

Next we will deploy the event-display service, which is the specified subscriber of the blockchain events in our trigger.yaml. This application is where we create our reply events.

This is a Kubernetes service so we need to apply the following yaml files:

apiVersion: apps/v1
kind: Deployment
  name: event-display
  namespace: knative-eventing-websocket-source
  replicas: 1
    matchLabels: &labels
      app: event-display
      labels: *labels
        - name: event-display
apiVersion: v1
kind: Service
  name: event-display
  namespace: knative-eventing-websocket-source
  type: NodePort
  - port: 80
    protocol: TCP
    targetPort: 8080
    name: consumer
  - port: 9080
    protocol: TCP
    targetPort: 9080
    nodePort: 31234
    name: dashboard
    app: event-display

If you head to localhost:31234, you should see the stream of events.

Subscribe to the reply events

Now we need to add another trigger, this time subscribing only to the reply events (that’s our newEvent that we set up in the go code). You can see that, in this case, we specify the source as "".

Here is the trigger yaml we apply:

kind: Trigger
  name: reply-trigger-test
  namespace: knative-eventing-websocket-source
  broker: default
      type: ""
      source: ""
      kind: Service
      name: test-display

This time, our subscriber is a Knative service called test-display, which we still need to deploy.

Run the following to deploy the knative service that subscribes to reply events:

kubectl --namespace knative-eventing-websocket-source apply --filename - << END
kind: Service
  name: test-display
        - image:

We can now get the logs of the test-display service and you should only see the reply messages:

kubectl logs -l -c user-container --tail=100 -n knative-eventing-websocket-source

Next time we will look at classifying the events and use transaction size as a reply to the broker.

cloud native

What are CloudEvents?

CloudEvents is a design specification for sending events in a common and uniform way. They are an interesting proposal for standardising the way we send events in an event-driven ecosystem. The specification is an open and a versatile approach for sending and consuming.

CloudEvents is currently an ‘incubating’ project with the CNCF. On the cloudevents website, they specify that the advantages of using cloud events are:

  • Consistency
  • Accessibility
  • Portability

Metadata about an event is contained within a CloudEvent, through a number of required (and optional) attributes including:

  • id
  • source
  • specversion
  • type

For more information about the attributes, you can take a look at the cloudevents spec.

Here is an example of a CloudEvent from my previous eventing example:

You can see the required attributes are:

  • id: 8e3cf8fb-88bb-4a00-a3fe-0635e221ce92
  • source: wss://
  • specversion: 0.3
  • type: websocket-event

There are also some extension attributes such as knativearrivaltime, knativehistory and traceparent. We then also have the body of the message in Data.

Having these set attributes means they can be used for filtering (e.g through a Knative eventing trigger) and also for capturing key information that can be used by other services that subscribe to the events. I can, for example, filter for events that are only from a certain source or of a certain type.

CloudEvents are currently supported by Knative, Azure Event Grid and Open FaaS.

There are number of libraries for CloudEvents inlcuding for Python, Go and Java. I’ve used the go-sdk for CloudEvents a lot lately and will be running through some of this in some future posts.

knative, kubernetes

Step by Step: Deploy and interact with a Knative Service

In this post, I will show how to deploy a Knative service and interact with it through curl and via the browser. I’ll go over some of the useful stuff to know as I found this kind of confusing at first.

I’m running this on a mac using the Kubernetes that’s built in to Docker Desktop, so things will be a bit different if you are running another flavor of Kubernetes. You will need Istio and the Knative serving components installed to follow along with this.

For the service, we are deploying a simple web app example from, which by default prints out “Hi there, I love (word of your choice)”. The code is at the link above, or I have a simple test image on Docker hub, which just prints out “Hi there, I love test” (oh the lack of creativity!)

Deploying a Knative Service

First we need to create a namespace, in which our Knative service will be deployed. For example:

kubectl create namespace web-service

Here is the Knative service deployment, which is a file called service.yaml.

kind: Service
  name: event-display
        - image:

Deploy the service yaml by running the following command:

kubectl apply -f service.yaml -n web-service

Now run the following in order to view the Knative service and some details we will need:

kubectl get ksvc -n web-service

There are a few fields, including:

NAME: The name of the service

URL: The url of the service, which we will need to interact with it. By default the URL will be “<your-service-name>.<namespace>” however you can also have a custom domain.

READY: This should say “True”, if not it will say “False” and there will be a reason in the REASON field.

After a little while, you might notice the service will disappear as it scales down to zero. More on that in a while.


To interact with the service we just deployed, we need to understand a bit about the IngressGateway. By default, Knative uses the istio-ingressgateway as its gateway service. We need to understand this in order to expose our service outside of the local cluster.

We can look at the istio-ingressgateway using the following command:

kubectl get service istio-ingressgateway --namespace istio-system

This will return the following:

Within the gateway configuration, there are a number of ports and NodePorts specified as default including the one we will use to communicate with our service:

port: number: 80, name: http2 protocol: HTTP

To find the port for accessing the service you can run the following:

kubectl -n istio-system get service istio-ingressgateway -o jsonpath='{.spec.ports[?("http2")].port}'   

You can customise the Gateway configuration. Details and the different ports can be found here in the Istio documentation. I’d also recommend running through the Istio httpbin example to understand a bit more about istio and ingressgateway.

To interact with our service we will need to combine both the URL ( and the EXTERNAL-IP (localhost) which we saw for the istio-ingressgateway. Depending on your set up, these will not be the same as mine.

It will be something like the following:

curl -H "Host:"

Scaling our Service

Your initial pod has probably disappeared right now because when a service is idle, it will scale down to zero after around 90 seconds. You should see the pod start ‘Terminating’ and then disappear.

Knative uses the KPA (Knative Pod Autoscaler), which runs as a Kubernetes deployment. The KPA scales based on requests (concurrency), however it is also possible to use the HPA (Horizontal Pod Autoscaler), which allows scaling based on CPU.

You can find out more detailed information about autoscaling here but for now just note that you can change the parameters in the ConfigMap.

To see the autoscaler config you can run the following command:

kubectl describe configmap config-autoscaler -n knative-serving

To edit the ConfigMap:

kubectl edit configmap config-autoscaler -n knative-serving 

In the result you will see some fields including:

scale-to-zero-grace-period: 30s
stable-window: 60s

The scale-to-zero-grace-period specifies how long it will wait until it scales an inactive service down to zero. The autoscaler takes a 60 second window to assess activity. If it is determined that within that 60 seconds stable-window, there are no events, it will then wait another 30 seconds before scaling to zero. This is why it takes around 90 seconds to terminate an inactive service.

If desired, these can be amended so that your service will scale down faster or slower. There is also a field called enable-scale-to-zero, which (if you want to be able to scale to zero) must be set to “true”.

Test using curl

Once you curl the service again you should see the pod spin up again.

curl -H "Host:"

Should return:

Hi there, I love test!

Access Knative Service through browser

If you are using Docker Desktop on a mac, to access through a browser you could add the host to the hostfile on your mac.

sudo vi /etc/hosts

Add to the file and save it.

Alternatively, if you don’t want to (or can’t) change the host file, I used the “Simple Modify Headers” browser plugin. Then click on the icon once installed and select ‘configure’. Input the parameters as follows and then click the start button.

Now open http://localhost/test and you should see:

knative, kubernetes

Knative Eventing Example: Part 1

In my last post I shared some methods for getting up and running with Knative eventing. In this post, we are going to step back a little to try and understand how the eventing component works. It gives a high level overview of a demo you can follow along with on GitHub.

This will be the first of a series of posts, which walk through an example, that we will build out in complexity over the coming weeks.

First, lets go over a few reasons why we might look to use eventing.

What capabilities does Knative Eventing bring?

  • Ability to decouple producers and consumers (this means, for example, that consumers can be subscribed to an event type before any of those event types have been produced).
  • Events are published as CloudEvents (this is a topic I would like to cover separately in more detail).
  • Push-based messaging

There are a number of key components that I will describe below, which together will make up the initial example. Channels and subscriptions will not be included in this post, we’ll discuss those another time.

What are we building?

Let’s first take a look at the diagram below to get a picture of how these components fit and interact together. This diagram shows the type of demo scenario we are looking to recreate over the next few posts.

Each of these components are deployed using yaml files, except for the broker, which is automatically created once the knative injection is enabled within the namespace. You can deploy a custom broker if you wish but I won’t include that in this post.

In this simple example, we use a Kubernetes app deployment as the source and a Knative Service as the consumer, which will subscribe to the events.

The code that I use to stream the events to the broker is available here on GitHub and gives more detailed instructions if you want to build it yourself. It also contains the yaml files used in this tutorial.


Our source is the producer of the events. It could be an application, a web socket, a process etc. It produces events that other services may or may not be interested in subscribing to.

There are a number of different types of sources, each one is a custom resource. The range of sources available can be seen in the Knative documentation here. You can create your own event source if you need to.

The following yaml shows a simple Kubernetes app deployment which is our source:

In the above example, the source is a go application, which streams messages via a web socket connection. It sends them as CloudEvents, which our service will consume. It is available to view here.

Broker and Trigger are CRDs, which will manage the delivery of events and abstract away the details of these from the related services.


The broker is where events get received. It is like a holding area, from where they can be consumed by those interested. As mentioned above, a default broker is automatically created when you label your namespace with kubectl label namespace my-event-namespace knative-eventing-injection=enabled


Our trigger provides a filter, by which it is determines which events should be delivered to a given consumer.

Here below is an example trigger, which just defines that the event-display service subscribes to all events from the default broker.

Under spec, I can also add filter: > attributes: and then include some CloudEvent attributes to filter by. We can then filter the CloudEvent fields, such as type or source etc in order to determine those to which a service subscribes. Here is another example, which filters on a specific event type:

You can also filter on an expression such as:

expression: ce.type == "com.github.pull.create"


We can have one or multiple consumers. These are the services that are interested (or not) in the events. If you have a single consumer, you can send straight from the source to consumer.

Here is the Knative Service deployment yaml:

Because I want to send events to a Knative Service, I need to have the cluster-local visibility label (this is explained in more detail below). For the rest, I am using a pre-built image from knative for a simple event display, the code for which can be found here.

Once you have all of these initial components, it looks like this:


I had some issues with getting the events to the consumer at first when trying this initial demo out. In the end I found out that in order to sink to a Knative Service, you need to add the cluster local gateway to the Istio installation. This is somewhat vaguely mentioned in the docs around installing Istio but could probably have been a bit clearer. Luckily, I found this great post, which helped me massively!

When you install Istio you will need to ensure that you see (at least) the following:

Handy tips and stuff I wish I had known before…

If you want to add a sink URL in your source file, see the example below for a broker sink:


default is the name of the broker and knative-eventing-websocket-source is the namespace.

In order to verify events are being sent/received/consumed then you can use the following examples as a reference:

//Getting the logs of the source app
kubectl --namespace knative-eventing-websocket-source logs -l app=wseventsource --tail=100 

//Getting the logs of the broker
kubectl --namespace knative-eventing-websocket-source logs -l --tail=100 

//Getting the logs of a Knative service
kubectl logs -l -c user-container --since=10m -n knative-eventing-websocket-source

You should see something like this when you get the service logs:

Next steps

Next time we will be looking at building out our service and embellishing it a little so we can transform, visualise and send new events back to a broker.

knative, kubernetes

Up and Running with Knative Eventing on Docker desktop

I’ve been playing around with Knative Eventing and wanted to write my own post on how to get it up and running on a Kubernetes cluster. The docs are pretty straight forward but I always like to keep a record for myself, just so that it’s all in one place.

Hopefully this guide will help someone who is new to the world of Knative eventing get up and running on their local machine.

So let’s get started with our install.

Get Docker Desktop

First you will need to install Docker Desktop. I’m using a Mac so I followed the instructions from the Docker website, they also have for Windows.

Once Docker desktop is installed, go to Preferences > Kubernetes > Enable Kubernetes (this post assumes you have kubectl). Then under ‘Advanced’, you will need to change the settings to increase the resources available:

Install an Ingress controller (Istio)

Knative needs Istio in order to run for it’s ingress controller. You can also use Gloo, but in this example we will use the former.

I followed the instructions from the official Istio site:

Install Istio

You can also install a lighter version of Istio for Knative, for which you can find the instructions here. The installation uses Helm, despite Istio starting to move away from Helm. Also, I needed to remove all the comment lines from the helm template command that makes the istio-lean.yaml file, otherwise it wouldn’t run for me.

Install Knative Serving

Then to install Knative, I first installed Knative serving (as recommended for Docker Desktop users) as per the instructions from their site:

Install Knative Serving

Install Knative Eventing CRDs

After this was installed, I then installed the CRDs for Knative eventing as per the instructions at the link below:

Install the CRDs for Knative Eventing

Check Install

Once you have installed everything, run the following command and you should have something like this:

kubectl get pods --all-namespaces

Create a namespace with Knative Eventing

Now create a test namespace:

kubectl create namespace my-event-namespace

Then we need to add the resources that will be needed from Knative to manage events into the namespace we just created. This is done using the following command:

kubectl label namespace my-event-namespace knative-eventing-injection=enabled

Cleaning Up

Rather than deleting everything, you can just scale your pods down to zero when not in use. This way you can spin them back up when you want to use them again. To do this use the following command:

kubectl get deploy -n <namespace> -o name | xargs -I % kubectl scale % --replicas=0 -n <namespace>

So, for example so scale down any Knative-eventing pods I would use:

kubectl get deploy -n knative-eventing -o name | xargs -I % kubectl scale % --replicas=0 -n knative-eventing

Next steps would be to try out some of the examples listed on the Knative website. In the next few weeks I will be posting some more on using Knative eventing so stay tuned 🙂