Expose a WebSocket server to Application Gateway

As outlined in the Application Gateway v2 documentation - it provides native support for the WebSocket and HTTP/2 protocols. Both Application Gateway and the Kubernetes Ingress don't have a user-configurable setting to selectively enable or disable WebSocket support.

Tip

Also see What is Application Gateway for Containers.

The following Kubernetes deployment YAML shows the minimum configuration used to deploy a WebSocket server, which is the same as deploying a regular web server:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: websocket-server
spec:
  selector:
    matchLabels:
      app: ws-app
  replicas: 2
  template:
    metadata:
      labels:
        app: ws-app
    spec:
      containers:
        - name: websocket-app
          imagePullPolicy: Always
          image: your-container-repo.azurecr.io/websockets-app
          ports:
            - containerPort: 8888
      imagePullSecrets:
        - name: azure-container-registry-credentials

---

apiVersion: v1
kind: Service
metadata:
  name: websocket-app-service
spec:
  selector:
    app: ws-app
  ports:
  - protocol: TCP
    port: 80
    targetPort: 8888

---

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: websocket-repeater
  annotations:
    kubernetes.io/ingress.class: azure/application-gateway
spec:
  rules:
    - host: ws.contoso.com
      http:
        paths:
          - backend:
              serviceName: websocket-app-service
              servicePort: 80

Given that all the prerequisites are fulfilled, and you have an Application Gateway controlled by a Kubernetes Ingress in your AKS, the deployment shown previously would result in a WebSockets server exposed on port 80 of your Application Gateway's public IP and the ws.contoso.com domain.

The following cURL command would test the WebSocket server deployment:

curl -i -N -H "Connection: Upgrade" \
        -H "Upgrade: websocket" \
        -H "Origin: http://localhost" \
        -H "Host: ws.contoso.com" \
        -H "Sec-Websocket-Version: 13" \
        -H "Sec-WebSocket-Key: 123" \
        http://1.2.3.4:80/ws

WebSocket Health Probes

If your deployment doesn't explicitly define health probes, Application Gateway would attempt an HTTP GET on your WebSocket server endpoint. Depending on the server implementation (here's one we love) WebSocket specific headers may be required (Sec-Websocket-Version for instance). Since Application Gateway doesn't add WebSocket headers, the Application Gateway's health probe response from your WebSocket server is most likely 400 Bad Request. As a result, Application Gateway marks your pods as unhealthy. This status eventually results in a 502 Bad Gateway for the consumers of the WebSocket server. To avoid the bad gateway error, you might need to add an HTTP GET handler for a health check to your server (/health for instance, which returns 200 OK).