I’m developing a web server in Rust and it works fine using just Docker and defining two services in compose. As follows:

x-common_auth_service: &common_auth_service
  container_name: auth
  build: 
    context: ./auth
    dockerfile: Dockerfile
  network_mode: host
  restart: always
  deploy:
    resources:
      limits:
        memory: 40M
      reservations:
        memory: 20M
  depends_on:
    - redis

services:
  
  auth:
    <<: *common_auth_service
    container_name: auth
    environment: 
      APP_PORT: 3002 //specific port
  
  auth2:
    <<: *common_auth_service
    container_name: auth2
    environment: 
      APP_PORT: 3003 //specific port

But now I need to implement these services using Docker Swarm.

Here I came across the first problem when using replicas.

  compose.yml
  auth_service:
    image: service/auth_app_rust
    build: 
      context: ./auth
      dockerfile: Dockerfile
    networks:
      - network_overlay
    ports:
      - "5000-5001:3002"
    deploy:
      mode: replicated
      replicas: 2

  nginx:
    image: nginx:latest
    volumes:
      - ./nginx/auth.conf:/etc/nginx/nginx.conf
    networks:
      - host
    depends_on:
      - auth_service

nginx.conf

upstream auth_server {
    server 127.0.0.1:5000;
    server 127.0.0.1:5001;
    keepalive 200;
}

server {
    listen 9999;
    location / {
        proxy_buffering off;
        proxy_set_header Connection "";
        proxy_http_version 1.1;
        proxy_set_header Keep-Alive "";
        proxy_set_header Proxy-Connection "keep-alive";
        proxy_pass http://auth_server;
    }
}

Services that previously defined ports manually used the same port across replicas, so I decided to use “port range mapping”.

And at first it seemed to work fine, but I noticed that Docker Swarm doesn’t assign each port to the container, it actually balances between the replicas.
In other words, if I run:

curl http://localhost:5000 

It won’t necessarily run in container 1.

Although I have found it very convenient to scale services using replicas, and the model also makes it easy to upgrade services, how do I solve this problem?

Because I would still like to use replicas, instead of defining several services, but I would not like to use the Docker Swarm Load Balancer, as I already use it through Nginx.

In short, I want to scale the number of containers running the web server in Rust, without having to create several of them in docker-compose, and also not use the load balancer, as the load balancer alignment would be bad.