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Expose a WebSocket server

As outlined in the Application Gateway v2 documentation - it provides native support for the WebSocket and HTTP/2 protocols. Please note, that for both Application Gateway and the Kubernetes Ingress - there is no user-configurable setting to selectively enable or disable WebSocket support.

The Kubernetes deployment YAML below 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:
              service:
                name: websocket-app-service
                port:
                  number: 80

Given that all the prerequisites are fulfilled, and you have an App Gateway controlled by a K8s Ingress in your AKS, the deployment above would result in a WebSockets server exposed on port 80 of your App 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 does not explicitly define health probes, App Gateway would attempt an HTTP GET on your WebSocket server endpoint. Depending on the server implementation (here is one we love) WebSocket specific headers may be required (Sec-Websocket-Version for instance). Since App Gateway does not add WebSocket headers, the App Gateway's health probe response from your WebSocket server will most likely be 400 Bad Request. As a result App Gateway will mark your pods as unhealthy, which will eventually result in a 502 Bad Gateway for the consumers of the WebSocket server. To avoid this you may need to add an HTTP GET handler for a health check to your server (/health for instance, which returns 200 OK).