[FIXED] Nginx load balancer not working as expected when using Docker Compose

Issue

I have a simple Flask application that runs on port 5000 that I want to scale up to 3 instances. I want the Nginx load balancer to balance clients based on their associated key provided in the url (i.e. "localhost/get?key=1" will always route to server 1 and "...key=2" always routes to server 2). However, when I use docker-compose to build and scale my app, it is using the Round Robin load balancing strategy instead of the hash $arg_key consistent strategy I have specified in the nginx.conf file. I am not sure why this is happening.

nginx.conf:

events {}

http {
    # Define the group of servers available
 
    upstream backend {
        hash $arg_key consistent;
        server app:5000;
    }
    server {
        # Server group will respond to port 80
        listen 80;
        location / {
            proxy_pass http://backend;
 
            proxy_set_header Host $http_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-Proto $scheme;
        }
    }
}

Docker-compose:

version: "3.7"

services:
    app:
        build:
            context: components
            dockerfile: Dockerfile
        environment:
            - DB_USERNAME=${DB_USERNAME}
            - DB_PASSWORD=${DB_PASSWORD}
        networks:
            - kvnetwork
    nginx:
        build:
            context: nginx
            dockerfile: Dockerfile
        volumes:
            - './nginx/nginx.conf:/etc/nginx/nginx.conf'
        ports:
            - "80:80"
        networks:
            - kvnetwork
        depends_on:
            - app
networks:
    kvnetwork:

I would appreciate any help.

Solution

To scale your application to 3 instances while using Docker Compose, you should use the scale command after bringing up your services with docker-compose up. Here's an example command to scale your app service to 3 instances:

docker-compose up -d --scale app=3

nginx then needs to be configured to use the different instances. i.e.

upstream backend {
hash $arg_key consistent;

server app1:5000;
server app2:5000;
server app3:5000;
}

Answered By - Gabriel Ramuglia