Nginx config for a Flask endpoint that does internal redirect via X-Accel-Redirect and passes a flash message to an OpenResty / Lua endpoint

app.py

'''
Flask server that *internal*-redirects to another Nginx endpoint without generating additional HTTP traffic.
Also passes an internaly-only 'flash message' via headers.
'''

from flask import Flask, request, make_response

application = Flask(__name__)

@application.route('/from/flask')
def fromFlask():
  response = make_response('This response body is ignored! Only the headers are used by downstream Nginx location(s).')
  response.headers.set('X-Accel-Redirect', '/intermediate')
  response.headers.set('X-Flash-Message', 'This is the flash message from Flask')
  return response

if __name__ == "__main__":
  application.run(host='0.0.0.0')

nginx.conf

# This Gist lets you do 2 cool things:
# 1) internally-redirect from an upstream app-server endpoint written in one language/framework (Python/Flask) to another (Lua/OpenResty), without generating additional HTTP traffic.
# 2) send an internal-only 'flash message' from one endpoint to another without storing it in the user session.
#
# This technique depends on the `X-Accel-Redirect` header, and some Nginx helpers to pass data between locations.
# The 'intermediate location' (below) is based on @agentzh's post: https://groups.google.com/forum/#!msg/openresty-en/vPxQFntAfCc/sQBBTSVq7xYJ
#
# Example:
# $ curl http://localhost/from
# This is the flash message from Flask, in a response generated by a Lua endpoint
#
# In practice, you'll likely want to use `ngx.location.capture` instead, for simplicity's sake.
# Even so, this technique can still be useful in rare scenarios.

http {
  upstream app_server_flask {
    # Out of scope for this Gist: you'll need to start Gunicorn / etc. to setup this Unix-domain socket.
    server unix:/run/app_server_flask.sock;
  }

  # Proxy the Flask app server.
  location = '/from/flask' {
#    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
#    proxy_set_header X-Forwarded-Proto $scheme;
#    proxy_set_header Host $host;
#    proxy_redirect off;
    proxy_pass http://app_server_flask;
  }

  # Note: this is an extra, optional endpoint to illustrate how to use this technique without any (Flask) code.
  location = '/from/nginx' {
    more_set_headers 'X-Accel-Redirect: /intermediate';
    more_set_headers 'X-Flash-Message: This is the flash message from Nginx';
  }

  # This helper converts headers set by the originating location into Nginx variables to be read by the destination location.
  # See agentzh's post for details: https://groups.google.com/forum/#!msg/openresty-en/vPxQFntAfCc/sQBBTSVq7xYJ
  location = '/intermediate' {
    internal;
    set $X_Flash_Message_From_Upstream $upstream_http_x_flash_message;
#    set $X_Flash_Message 'oh boy'; # Debugging helper when `$upstream_http_x_flash_message` isn't provided / correctly specified.
    rewrite ^/(.+) /to;
  }

  # The final destination, a Lua endpoint that displays the 'flash message' from the original location(s).
  location = '/to' {
    content_by_lua_block {
      local flashMessage = ngx.var['X_Flash_Message']
      ngx.say(string.format('%s, in a response generated by a Lua endpoint', flashMessage))
    }
  }
}