Nginx Blocking task with long processing time - docker-compose

Question

I'm trying to perform horizontal scaling of long processing task with docker compose.
To simulate that task, I used the following code in python:
#app.get("/test")
def test():
start = time.time()
print("Start time", time.asctime(time.localtime(start)))
time.sleep(5)
end = time.time()
print("End time", time.asctime(time.localtime(end)))
return {"start":start, "end":end}
In docker compose I created 2 containers for the same code above. Mainly back1 and back2
version: '3.8'
services:
back1:
build:
context: ./backend
dockerfile: DockerFile
volumes:
- ./backend:/usr/code
command: python main.py
ports:
- "8001"
back2:
build:
context: ./backend
dockerfile: DockerFile
volumes:
- ./backend:/usr/code
command: python main.py
ports:
- "8001"
nginx:
build:
context: ./backend/config
dockerfile: DockerFile
ports:
- "80:80"
And this is my nginx conf file:
upstream backend {
server back1:8001;
server back2:8001;
keepalive 32;
}
server {
listen 80;
server_name api.localhost;
location / {
proxy_pass http://backend;
}
}
I tried to simultaneously trigger the API http://api.localhost/test. However it seems that the second request was activated after the 1st request is completed. Even though it was processed by different instance of container.
Is nginx actually blocking?? I know that we can use queueing (eg: rabbitMQ, etc), but I thought that spinning up 2 docker container and using nginx as load balancer would achieve horizontal scalling. Am I wrong?