Docker load balance using NGINX proxy

Question

I'm trying to load balance an API server using nginx and docker's native DNS.

I was hoping nginx will round-robin API calls to all available servers. But even when I specify docker's DNS server as the resolver nginx forward the request to only one server.

Relevant section from docker-compose.yml

proxy:
  restart: always
  build: ./src/nginx/.
  ports:
    - "8080:8080"
  links:
    - api:servers.api

nginx.conf

worker_processes 2;

events { worker_connections 1024; }

http {
    sendfile on;

    server {
        listen 8080;

        location / {
            resolver_timeout 30s;
            resolver 127.0.0.11 ipv6=off valid=10s;
            set $backend http://servers.api:80;
            proxy_pass          $backend;
            proxy_redirect      off;
        }
    }
}

NGINX round-robin load balancer works if I manually specify each server, which I don't want to do since it can't scale automatically.

worker_processes 2;

events { worker_connections 1024; }

http{
    sendfile on;

    upstream api_servers{
        server project_api_1:80;
        server project_api_2:80;
        server project_api_3:80;
    }

    server{
        listen 8080;

        location / {
            proxy_pass          http://api_servers;
            proxy_redirect      off;
        }
    }
}

How to configure nginx in such a way that it can detect new containers added and include them in the round-robin?

Answer 1

Docker's DNS is the responsible to do the round robin in this case. Don't use links option in your compose, it's not necessary. Look, I'm using this example:

docker-compose.yml:

version: '3'
  services:
    api:
      image: my-api-image
    client:
      image: ubuntu:latest

So I start my application with docker-compose up -d api and then scale it: docker-compose scale api=10. Now, inside the client (docker-compose run client bash):

root@ce3857690292:/# dig api
...
;; QUESTION SECTION:
;api.               IN  A

;; ANSWER SECTION:
api.            600 IN  A   172.19.0.6
api.            600 IN  A   172.19.0.9
api.            600 IN  A   172.19.0.7
api.            600 IN  A   172.19.0.8
api.            600 IN  A   172.19.0.11
api.            600 IN  A   172.19.0.2
api.            600 IN  A   172.19.0.10
api.            600 IN  A   172.19.0.3
api.            600 IN  A   172.19.0.5
api.            600 IN  A   172.19.0.4

With curl you can see the round robin:

root@1719c10f864a:/# curl -vI api
* Rebuilt URL to: api/
*   Trying 172.19.0.6...
* Connected to api (172.19.0.6) port 80 (#0)
...
root@1719c10f864a:/# curl -vI api
* Rebuilt URL to: api/
*   Trying 172.19.0.7...
* Connected to api (172.19.0.7) port 80 (#0)
...
root@1719c10f864a:/# curl -vI api
* Rebuilt URL to: api/
*   Trying 172.19.0.8...
* Connected to api (172.19.0.8) port 80 (#0)

In your case you need to replace the client service in my docker-compose with your nginx and use your api as upstream (without links)