Knative Serving: Using Ambassador Gateway

This is a continuation of my experimentation with Knative serving, this time around, building a gateway on top of a Knative serving applications. This builds on two of my previous posts — on using Knative to deploy a Spring Boot App and making a service-to-service call in Knative.

Why a Gateway on Top of Knative Application

To explain this, let me touch on my previous post. Assuming that Knative serving is already available in a Kubernetes environment, the way to deploy an application is using a manifest, which looks like this:

apiVersion: serving.knative.dev/v1alpha1
kind: Service
metadata:
  name: sample-boot-knative-service
  namespace: default
spec:
  runLatest:
    configuration:
      revisionTemplate:
        spec:
          container:
            image: bijukunjummen/sample-boot-knative-app:0.0.3-SNAPSHOT
            env:
            - name: ASAMPLE_ENV
              value: "sample-env-val"

Now, to invoke this application, I have to make the call via an ingress created by Knative serving, which can be obtained the following way in a minikube environment:

export GATEWAY_URL=$(echo $(minikube ip):$(kubectl get svc knative-ingressgateway -n istio-system -o 'jsonpath={.spec.ports[?(@.port==80)].nodePort}'))

The request now has to go through the ingress and the ingress uses a Host HTTP header to then route the request to the app. The host header for the deployed service can be obtained using the following bash script:

export APP_DOMAIN=$(kubectl get services.serving.knative.dev sample-boot-knative-service  -o="jsonpath={.status.domain}")

and then a call via the knative ingress gateway made the following way, using CURL:

curl -X "POST" "http://${GATEWAY_URL}/messages" \
     -H "Accept: application/json" \
     -H "Content-Type: application/json" \
     -H "Host: ${APP_DOMAIN}" \
     -d $'{
  "id": "1",
  "payload": "one",
  "delay": "300"
}'

http http://${GATEWAY_URL}/messages Host:"${APP_DOMAIN}" id=1 payload=test delay=1

There are too many steps involved in making a call to the application via the knative ingress:

My objective in this post is to simplify the users' experience in making a call to the app by using a Gateway like Ambassador.

Integrating Ambassador to Knative

There is nothing special about installing Ambassador into a Knative environment. The excellent instructions, provided here, worked cleanly in my minikube environment.

My objective with the gateway is summarized in this picture:

With Ambassador in place, all the user has to do is to send a request to Ambassador Gateway, and it would take care of plugging in the Host header before making a request to the Knative Ingress.

So how does this work? Fairly easily! Assuming Ambassador is in place, all it needs is a configuration that piggybacks on a Kubernetes service the following way:

---
apiVersion: v1
kind: Service
metadata:
  name: sample-knative-app-gateway
  annotations:
    getambassador.io/config: |
      ---
      apiVersion: ambassador/v0
      kind:  Mapping
      name: sample-boot-knative-app
      prefix: /messages
      rewrite: /messages
      service: knative-ingressgateway.istio-system.svc.cluster.local 
      host_rewrite: sample-boot-knative-service.default.example.com
spec:
  type: LoadBalancer
  ports:
  - name: ambassador
    port: 80
    targetPort: 80
  selector:
    service: ambassador

Here, I am providing configuration via a Service annotation, intercepting any calls to /messages uri, and forwarding these request to the knative ingressgatway service (knative-ingressgateway.istio-system.svc.cluster.local) and adding the host header of "sample-boot-knative-service.default.example.com".

The interaction from a user perspective is far simpler because all I have to do is get the URL for this new service and make the API call in a minikube environment using the following bash script:

export AMB_URL=$(echo $(minikube ip):$(kubectl get svc sample-knative-app-gateway -n default -o 'jsonpath={.spec.ports[?(@.port==80)].nodePort}'))

http http://${AMB_URL}/messages id=1 payload=test delay=1

It may be easier to try this on a real code, which is available my GitHub repo here.