Unable to reverse proxy with Nginx docker with multiple flask container instances

Question

I am trying to use nginx container to act as a reverse proxy which would distribute the load across multiple flask app instances, I am using docker-composer to do that.

version: '3'
services:
  app:
    build:
      context:  ./app
      dockerfile: Dockerfile
    expose:
      - "8001"

  proxy:
    build:
      context:  ./nginx
      dockerfile: Dockerfile
    ports:
      - "80:80"
    links:
      - app

and my nginx docker file is

FROM nginx

COPY nginx.conf /etc/nginx/nginx.conf

and my app dockerfile is

FROM python:3-alpine

RUN apk update && apk upgrade && pip install -U pip
RUN apk add --update alpine-sdk make gcc python3-dev python-dev libxslt-dev libxml2-dev libc-dev openssl-dev libffi-dev zlib-dev py-pip \
    && rm -rf /var/cache/apk/*

RUN set -x \
    && VER="17.03.0-ce" \
    && curl -L -o /tmp/docker-$VER.tgz https://get.docker.com/builds/Linux/x86_64/docker-$VER.tgz \
    && tar -xz -C /tmp -f /tmp/docker-$VER.tgz \
    && mv /tmp/docker/* /usr/bin

WORKDIR /app
ADD . /app

RUN pip install --trusted-host pypi.python.org -r requirements.txt

EXPOSE 8001

CMD ["python3.6", "main.py"]

I am using the following command to build

docker-compose build

and the following command to start containers

docker-compose up -d --scale app=5

which shows

Starting testapp_app_1 ... done
Creating testapp_app_2 ... done
Creating testapp_app_3 ... done
Creating testapp_app_4 ... done
Creating testapp_app_5 ... done

but which I check docker logs, seems like all the request are server by testingapp_app_1

my nginx conf file is

#worker_processes 4;

events { worker_connections 1024; }

http {
#    sendfile on;

#    upstream app_servers {
#        server app:8001;
#    }

    server {
        listen 80;
        server_name localhost;

        resolver 127.0.0.11;
        set $backends app;
        location / {
            proxy_pass         http://$backends:8001;
            proxy_redirect     off;
            proxy_set_header   Host $host;
            proxy_set_header   X-Real-IP $remote_addr;
            proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header   X-Forwarded-Host $server_name;
        }
    }
}

Please explain how to dynamically round-robin requests to each of the app containers

See this stackoverflow.com/questions/50203408/… and github.com/willprice/… — Tarun Lalwani
– Tarun Lalwani, Commented Jun 26, 2018 at 13:08

Jibin Mathew · Accepted Answer · 2018-07-12 09:28:57Z

The method that you mentioned doesn't work, the server cannot detect a new instance with the default nginx server, you would need jwilder/docker-gen image to integrate dynamically created app instances to the nginx load balancer.

your dockerfile in the root should look something like:

version: '3'
services:
  nginx:
    image: nginx
    #restart: unless-stopped
    labels:
      com.github.jrcs.letsencrypt_nginx_proxy_companion.nginx_proxy: "true"
    container_name: nginx
    ports:
      - "80:80"
    volumes:
      - ./conf.d:/etc/nginx/conf.d
      - ./vhost.d:/etc/nginx/vhost.d

  nginx-gen:
    image: jwilder/docker-gen
    command: -notify-sighup nginx -watch -wait 5s:30s /etc/docker-gen/templates/nginx.tmpl /etc/nginx/conf.d/default.conf
    container_name: nginx-gen
    #restart: unless-stopped
    volumes:
      - ./conf.d:/etc/nginx/conf.d
      - ./vhost.d:/etc/nginx/vhost.d
      - /var/run/docker.sock:/tmp/docker.sock:ro
      - ./nginx.tmpl:/etc/docker-gen/templates/nginx.tmpl:ro
networks:
  default:
    external:
      name: nginx-proxy

and the dockerfile in the app directory would like :

version: '3'
services:
  app:
    build:
      context:  ./app
      dockerfile: Dockerfile
    expose:
      - "8001"
    environment:
      VIRTUAL_HOST: localhost

networks:
  default:
    external:
      name: nginx-proxy

It's important to have the same virtual hostname across all the instance and the jwilder/docker-gen instance will restart and set it in round-robin fashion.

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