Suponiendo una aplicación donde se quiera distribuir el tráfico sobre distintas instancias para soportar la carga, es posible usar Nginx como servidor web y al mismo tiempo balanceador de carga.

# Arquitectura

  • 1 Nginx
  • 3 simple aplicaciones web como backends
# docker-compose.yml
---
version: '3'
services:
  nginx:
    build:
      context: .
    ports:
      - 8080:80
    links:
      - backend1
      - backend2
      - backend3
    depends_on:
      - backend1
      - backend2
      - backend3

  backend1:
    image: python
    command: python -m http.server 80

  backend2:
    image: python
    command: python -m http.server 80

  backend3:
    image: python
    command: python -m http.server 80
# Dockerfile
FROM nginx
LABEL maintainer "ariel.gerardo.rios@gmail.com"

COPY ["nginx.conf", "/etc/nginx/nginx.conf"]
# nginx.conf
events{}

http {
    upstream backend {
        server backend1;
        server backend2;
        server backend3;
    }

    log_format upstreamlog '[$time_local] upstream $upstream_addr: $request upstream_response_time $upstream_response_time msec $msec request_time $request_time';

    server {
        listen 80;
        access_log /dev/stdout upstreamlog;

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

# Simulación

El método de selección aplicado por defecto es Round Robin.

~$ docker-compose build && docker-compose up  # localhost:8080
...
nginx_1     | [17/May/2017:01:56:50 +0000] upstream 172.30.0.2:80: GET / HTTP/1.1 upstream_response_time 0.003 msec 1494986210.108 request_time 0.003
nginx_1     | [17/May/2017:01:56:50 +0000] upstream 172.30.0.3:80: GET /favicon.ico HTTP/1.1 upstream_response_time 0.001 msec 1494986210.130 request_time 0.001
nginx_1     | [17/May/2017:01:56:51 +0000] upstream 172.30.0.4:80: GET / HTTP/1.1 upstream_response_time 0.002 msec 1494986211.914 request_time 0.002
nginx_1     | [17/May/2017:01:56:52 +0000] upstream 172.30.0.2:80: GET / HTTP/1.1 upstream_response_time 0.004 msec 1494986212.142 request_time 0.004
nginx_1     | [17/May/2017:01:56:52 +0000] upstream 172.30.0.3:80: GET / HTTP/1.1 upstream_response_time 0.009 msec 1494986212.396 request_time 0.009
...

Se puede alterar el método de selección modificando el peso de los nodos:

upstream backend {
    server backend1 weight=5;
    server backend2;
    server backend3;
}

# Otros métodos

  • Persistencia de sesiones:
upstream backend {
    ip_hash;
    server backend1;
    server backend2;
    server backend3;
}
  • Menos conectado:
upstream backend {
    least_conn;
    server backend1;
    server backend2;
    server backend3;
}

Más información

Documentación de Nginx.