I've created a sample spring boot application that exposes a websocket endpoint at localhost:8080/ws. Basically I followed this guide except for I am not using the .withSockJS Option.

When I run this application locally, my sample angular app can connect to the websocket.

Now I want to have both containers (spring boot app and angular app) in a single Kubernetes pod. They both spin up when I run them. Then I expose the angular frontend's port to be able to view the app. But the logs tell me that it is not able to connect to the websocket backend via ws://localhost:8080/ws

Even when I connect to the backend container, I can see that it is up and running, but my curl websocket test also always fails.

This is my pod def:

---
 apiVersion: v1
 kind: Pod
 metadata:
   name: my-app.example.org
   labels:
     app: my-app-system
 spec:
   containers:
     - name: backend
       image: test/my-app-backend
       ports:
         - containerPort: 8080
       env:
       - name: SPRING_PROFILES_ACTIVE
         value: "dev-docker-postgres"
       - name: JAVA_OPTIONS
         value: "-agentlib:jdwp=transport=dt_socket,address=5005,server=y,suspend=n"
     - name: frontend
       image: test/my-app-frontend
       ports:
         - containerPort: 4200
       imagePullPolicy: Always
       command: ["/bin/sh"]
       args: ["-c", "npm run kubstart"]

   imagePullSecrets:
     - name: registrykey 

One more thing:

When I additionally expose the backend container's port via NodePort type, and start the angular app locally on my machine with the service's url the websocket connection succeeds.

It seems I am not able to let both containers in my pod communicate with each other via ws://