My website has a separate server for the front-end and back-end, and so my back-end server needs to open up CORS permissions so that the front-end can request data from it.

I am using Flask-Cors successfully in development, but it doesn't work when I deploy to production. (please note that I have looked at other flask-cors questions on SO, but none of them fit my situation)

Here is the relevant code that is working in development:

# 3rd party imports
import flask
from flask import Flask, request, redirect, send_from_directory, jsonify
from flask_cors import CORS

# Create the app
app = Flask(__name__)
CORS(app, origins=[
  'http://localhost:5001',
])

# Define the routes
@app.route('/')
def index():
  # no CORS code was necessary here
  app.logger.info(f'request is: {flask.request}')

What I've tried:

• Adding my server's ip address 'http://162.243.168.182:5001' to the CORS list is not enough to resolve the issue, although I understand it should be there.
• It seems that using '*' to allow ALL origins does not work either. (very suspicious!)

Please note that I am using a Docker container, so my environment between development and prod are almost identical. But what's different is that I'm on a different server and I've modified the front-end to send the request to the new IP address (resulting in the famous “Access-Control-Allow-Origin” header missing CORS error).

Now I'm wondering if the flask.request object is somehow missing information, and this causes Flask-Cors to not send the Access-Control-Allow-Origin header like it's supposed to. I can provide that logging info if you think it would help!

More information!

The Dockerfile I am using in PROD is:

# base image
FROM tiangolo/uwsgi-nginx-flask:python3.8-2020-12-19

# install deps
RUN pip3 install ediblepickle==1.1.3
# RUN pip3 install flask==1.1.2 # pre-installed on tiangolo/uwsgi-nginx-flask
RUN pip3 install flask-cors==3.0.9
RUN pip3 install numpy==1.19.2
RUN pip3 install scipy==1.5.2
RUN pip3 install pandas==1.1.2
RUN pip3 install networkx==2.5

# pull in files for deployment
COPY ./app /app

# Note that there is no CMD to run because the CMD set in the base image is what we already wanted.  As long as the Flask app is called `app`, the python file is named `main.py`, the parent directory is named `app`, and that same directory gets copied into `/app`, then the base image is designed to make our app work out-of-the-box.

and the command I use to kick it off is:

docker build -t mvlancellotti/tennis-backend:prod -f prod.Dockerfile . && docker run --rm -p 5000:80 --name tennis-backend-container mvlancellotti/tennis-backend:prod

Going into the /app directory of the container, there is the file uwsgi.ini with contents:

[uwsgi]
module = main
callable = app

which seems to work, and the file /etc/nginx/nginx.conf has contents:

user  nginx;
worker_processes 1;
error_log  /var/log/nginx/error.log warn;
pid        /var/run/nginx.pid;
events {
    worker_connections 1024;
}
http {
    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;
    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';
    access_log  /var/log/nginx/access.log  main;
    sendfile        on;
    keepalive_timeout  65;
    include /etc/nginx/conf.d/*.conf;
}
daemon off;

and the file /etc/nginx/conf.d/nginx.conf has contents:

server {
    listen 80;
    location / {
        try_files $uri @app;
    }
    location @app {
        include uwsgi_params;
        uwsgi_pass unix:///tmp/uwsgi.sock;
    }
    location /static {
        alias /app/static;
    }
}