I’m developing a web server that uses nodejs react for the frontend and java spring boot in the backend.
I’m creating a container for my application with docker and i’m using nginx to do reverse-proxy between the frontend and the backend. The problem is that, when i try to communicate from the frontend with the backend i have the error 502 Bad Gateway and in the nginx server console:

connect() failed (111: Connection refused) while connecting to upstream, client: 172.20.0.1, server: , request: "POST /api/authenticate HTTP/1.1", upstream: "http://172.20.0.3:9093/authenticate", host: "localhost:3050", referrer: "http://localhost:3050/"   

This is my default.conf:

upstream backend{
    server b-applicationgateway:9093;
}

server {
    listen 80;

    location /api/ {
        proxy_pass http://backend/;
    }

    location / {
        proxy_pass http://f-frontend:3000;
    }

}

These are the nginx and frontend dockerfiles:
nginx

FROM nginx
COPY ./default.conf /etc/nginx/conf.d/default.conf

frontend

FROM node:14.14.0-alpine
WORKDIR /app
COPY ./package.json ./
RUN npm i
COPY . .
CMD ["npm", "run", "start"]

Finally, this is the docker-compose.yaml file:

version: '3.5'

services:

  f-frontend:
    stdin_open: true
    environment:
      - CHOKIDAR_USEPOLLING=true
    build:
      dockerfile: Dockerfile.dev
      context: ./Frontend/my-app
    networks:
      n-frontend:
        aliases:
          - f-frontend
    ports:
      - "3000:3000"

  b-applicationgateway:
    build:
      context: Backend/
      dockerfile: applicationGateway.containerfile
    hostname: b-applicationgateway
    image: b-applicationgateway:latest
    ports:
      - "9093:9093"
    networks:
      backend:
      int-net:
      n-frontend:
        aliases:
          - b-applicationgateway
    depends_on:
      - b-datamanager
      - b-authenticationmanager
      - b-assetmanager

  nginx:
    depends_on:
      - b-applicationgateway
      - f-frontend
    restart: always
    build:
      context: ./nginx
      dockerfile: Dockerfile.dev
    ports:
      - "3050:80"
    networks:
      n-frontend:
        aliases:
          - nginx



networks:
  backend:
  n-frontend:
    external: true
    name: n-frontend
  int-net:
    external: true
    name: int-net


volumes:
  data:

I intentionally deleted some other backend services because i think they are useless for this question, since if i run only the backend part everything works.

The gateway controller (in the backend) has these annotations:

@RestController
@RequiredArgsConstructor
@Slf4j
@CrossOrigin(origins = {"http://localhost:3000", "http://localhost:3050"}, allowCredentials = "true")

I checked the docker network n-frontend with the command ‘docker network inspect n-frontend’ , and all the containers are present

"Containers": {
            "3343cfd269f670df0c9d8abfaa102cf0662d4a10cb7fb8eeb92b71f98917df2d": {
                "Name": "webplatform-f-frontend-1",
                "EndpointID": "ca720661c9b5fd6b72d7c95c5ae9f81ccb8712c124407f69a5564050bc5ceba2",
                "MacAddress": "02:42:ac:14:00:02",
                "IPv4Address": "172.20.0.2/16",
                "IPv6Address": ""
            },
            "548a82a6daf4e6cb9645330c2e5c210cf0071b937e4bdf2668818d4a60218462": {
                "Name": "webplatform-b-applicationgateway-1",
                "EndpointID": "9f6efba31f2055969d10f5badebf1ec9386f1743fe0dbf703b013139edacd122",
                "MacAddress": "02:42:ac:14:00:03",
                "IPv4Address": "172.20.0.3/16",
                "IPv6Address": ""
            },
            "b3dd4141e266277b440320c36685dcdfce9a8ab17c899b06b90ba59e102d1354": {
                "Name": "webplatform-nginx-1",
                "EndpointID": "e0f616f2d75c829956be1a61814cff6db23c7ecd3786a0143e4f90747f68750a",
                "MacAddress": "02:42:ac:14:00:04",
                "IPv4Address": "172.20.0.4/16",
                "IPv6Address": ""
            }
        },

Does anyone have a solution? Thanks a lot in advance.