c. Docker Compose

Two changes are needed in compose.yaml: add the lb service (Nginx) as the new entry point, and update the gateway service to run as multiple replicas without an exposed port.

Add Nginx

  lb:
    image: nginx:latest
    ports:
      - "8080:80"
    volumes:
      - ./setup/nginx.conf:/etc/nginx/nginx.conf:ro
    depends_on:
      - gateway

The volumes entry mounts the configuration file created in the previous step (setup/nginx.conf, relative to compose.yaml) read-only over Nginx's default configuration. The host port 8080 now belongs to lb (Nginx listens on 80 inside the container), so the clients keep calling http://localhost:8080.

depends_on makes Compose create the gateway first: Nginx resolves the name gateway when it starts, and exits with host not found in upstream if that name does not exist yet.

Scale the Gateway

Remove the ports mapping from the gateway service and raise the replica count in its deploy block (the repository keeps replicas: 1; use 3 to watch the balancing):

  gateway:
    build:
      context: ./gateway-service
      dockerfile: Dockerfile
    # ports removed — gateway is only reachable through lb
    deploy:
      replicas: 3