Problem when making frontend and backend communicate - reactjs

Question

i'm working with Minikube to make a full stack K8s application using React as a frontend and ASP NET Core as a backend.
Here there are my configuration
Deployments and Services
apiVersion: v1
kind: ServiceAccount
metadata:
name: web-frontend
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: frontend-deployment
labels:
app: frontend
spec:
replicas: 1
selector:
matchLabels:
app: frontend
template:
metadata:
labels:
app:
frontend
spec:
serviceAccountName: web-frontend
containers:
- name: frontend
image: frontend
ports:
- containerPort: 80
---
apiVersion: v1
kind: Service
metadata:
name: frontend-svc
spec:
selector:
app: frontend
ports:
- protocol: TCP
port: 80
targetPort: 80
---
apiVersion: v1
kind: ServiceAccount
metadata:
name: backend
---
apiVersion: apps/v1
kind: Deployment
metadata:
name: backend-deployment
labels:
app: backend
spec:
replicas: 1
selector:
matchLabels:
app: backend
template:
metadata:
labels:
app:
backend
spec:
serviceAccountName: backend
containers:
- name: backend
image: backend
ports:
- containerPort: 5000
---
apiVersion: v1
kind: Service
metadata:
name: backend
spec:
selector:
app: backend
ports:
- protocol: TCP
port: 5000
targetPort: 5000
Dockerfiles for the frontend
FROM node:alpine as build-image
WORKDIR /app
COPY package.json ./
COPY package-lock.json ./
RUN npm i
COPY . .
CMD ["npm", "run", "start"]
This is instead my Ingress
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
name: frontend-backend-ingress
annotations:
# nginx.ingress.kubernetes.io/rewrite-target: /$1
spec:
ingressClassName: nginx
rules:
- http:
paths:
- path: /?(.*)
pathType: Prefix
backend:
service:
name: frontend-svc
port:
number: 80
- path: /api/?(.*)
pathType: Prefix
backend:
service:
name: backend
port:
number: 5000
However, when I type minikube tunnel to expose the ingress IP locally I can reach the frontend, but when the frontend tries to get a fetch request to /api/something in the browser console I get GET http://localhost/api/patients/ 404 (Not Found) and an error SyntaxError: Unexpected token < in JSON at position 0.
Moreover, If I change the Ingress in this way
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
name: frontend-backend-ingress
annotations:
# nginx.ingress.kubernetes.io/rewrite-target: /$1
spec:
ingressClassName: nginx
rules:
- http:
paths:
- path: /
pathType: Prefix
backend:
service:
name: frontend-svc
port:
number: 80
- path: /api/
pathType: Prefix
backend:
service:
name: backend
port:
number: 5000
Then I can issue curl localhost/api/something and I get the JSON result, but when the frontend tries to contact the backend I get
GET http://localhost/api/patients/ 500 (Internal Server Error)
SyntaxError: Unexpected end of JSON input
at main.358f50ad.js:2:473736
at s (main.358f50ad.js:2:298197)
at Generator._invoke (main.358f50ad.js:2:297985)
at Generator.next (main.358f50ad.js:2:298626)
at Al (main.358f50ad.js:2:439869)
at a (main.358f50ad.js:2:440073)
This looks strange because if I try the frontend and the backend outside kubernetes everything works fine and from the React application the result from the backend is correctly fetched (of course using the proxy inside the package.json)

Answer 1

To contact or make links between apps you could use their kubernetes native FQDN ( try to ping or telnet it if you want to test the connection but here is how it works:
Thr default FQDN of any service is:
<service-name>.<namespace>.svc.cluster.local.
In your above example, you should be able to contact you backend service from your frontend one with:
backend.YOURNAMESPACENAME.svc.cluster.local:5000
For services in same namespace, you don't need to use the FQDN to access services, just the service name would be enough:
backend:5000
I don't know where you exactly configure the links between the frontend and backend but however, you should "variabilize" this link and add the variable definition in kubernetes manifest.