docker-compose --scale X nginx.conf configuration

Nginx

Dynamic upstreams are possible in Nginx (normal, sans Plus) but with tricks and limitations.

1. You give up on upstream directive and use plain proxy_pass.

   It gives round robin load balancing and failover, but no extra feature of the directive like weights, failure modes, timeout, etc.

2. Your upstream hostname must be passed to proxy_pass by a variable and you must provide a resolver.

   It forces Nginx to re-resolve the hostname (against Docker networks' DNS).

3. You lose location/proxy_pass behaviour related to trailing slash.

   In the case of reverse-proxying to bare / like in the question, it does not matter. Otherwise you have to manually rewrite the path (see the references below).

Let's see how it works.

docker-compose.yml

version: '2.2'services:  reverse-proxy:    image: nginx:1.15-alpine    volumes:      - ./nginx.conf:/etc/nginx/nginx.conf    ports:      - 8080:8080  app:    # A container that exposes an API to show its IP address    image: containous/whoami    scale: 4

nginx.conf

worker_processes  1;events {  worker_connections  1024;}http {  access_log /dev/stdout;  error_log /dev/stderr;  server {    listen 8080;    server_name localhost;    resolver 127.0.0.11 valid=5s;    set $upstream app;    location / {      proxy_pass http://$upstream:80;    }  }}

Then...

docker-compose up -dseq 10 | xargs -I -- curl -s localhost:8080 | grep "IP: 172"

...produces something like the following which indicates the requests are distributed across 4 app containers:

IP: 172.30.0.2IP: 172.30.0.2IP: 172.30.0.3IP: 172.30.0.3IP: 172.30.0.6IP: 172.30.0.5IP: 172.30.0.3IP: 172.30.0.6IP: 172.30.0.5IP: 172.30.0.5

References:

1. Nginx with dynamic upstreams
2. Using Containers to Learn Nginx Reverse Proxy
3. Dynamic Nginx configuration for Docker with Python

Traefik

Traefik relies on Docker API directly and may be a simpler and more configurable option. Let's see it in action.

docker-compose.yml

version: '2.2'services:  reverse-proxy:    image: traefik      # Enables the web UI and tells Traefik to listen to docker    command: --api --docker      ports:      - 8080:80            - 8081:8080  # Traefik's web UI, enabled by --api    volumes:      # So that Traefik can listen to the Docker events      - /var/run/docker.sock:/var/run/docker.sock    app:    image: containous/whoami    scale: 4    labels:      - "traefik.frontend.rule=Host:localhost"

Then...

docker-compose up -dseq 10 | xargs -I -- curl -s localhost:8080 | grep "IP: 172"

...also produces something the output that indicates the requests are distributed across 4 app containers:

IP: 172.31.0.2IP: 172.31.0.5IP: 172.31.0.6IP: 172.31.0.4IP: 172.31.0.2IP: 172.31.0.5IP: 172.31.0.6IP: 172.31.0.4IP: 172.31.0.2IP: 172.31.0.5

In the Traefik UI (http://localhost:8081/dashboard/ in the example) you can see it recognised the 4 app containers:

References:

1. The Traefik Quickstart (Using Docker)

It's not possible with your current config since it's static. You have two options -

1. Use docker engine swarm mode - You can define replicas & swarm internal DNS will automatically balance the load across those replicas.
Ref - https://docs.docker.com/engine/swarm/

2. Use famous Jwilder nginx proxy - This image listens to the docker sockets, uses templates in GO to dynamically change your nginx configs when you scale your containers up or down.
Ref - https://github.com/jwilder/nginx-proxy