Multiple Containers, Same Port, no Reverse Proxy

Disclaimer: In 2021, there is still a place for simple setups with just one machine serving all traffic. So, no Kubernetes and no cloud load balancers in this post. Just good old Docker and Podman.

Even when you have just one physical or virtual server, it's often a good idea to run multiple instances of your application on it. Luckily, when the application is containerized, it's actually relatively simple. With multiple application containers, you get horizontal scaling and a much-needed redundancy for a very little price. Thus, if there is a sudden need for handling more requests, you can adjust the number of containers accordingly. And if one of the containers dies, there are others to handle its traffic share, so your app isn't a SPOF anymore.

The tricky part here is how to expose such a multi-container application to the clients. Multiple containers mean multiple listening sockets. But most of the time, clients just want to have a single point of entry.

Benefits of exposing multiple Docker containers on the same port

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How to use Flask with gevent (uWSGI and Gunicorn editions)

Python is booming and Flask is a pretty popular web-framework nowadays. Probably, quite some new projects are being started in Flask. But people should be aware, it's synchronous by design and ASGI is not a thing yet. So, if someday you realize that your project really needs asynchronous I/O but you already have a considerable codebase on top of Flask, this tutorial is for you. The charming gevent library will enable you to keep using Flask while start benefiting from all the I/O being asynchronous. In the tutorial we will see:

  • How to monkey patch a Flask app to make it asynchronous w/o changing its code.
  • How to run the patched application using gevent.pywsgi application server.
  • How to run the patched application using Gunicorn application server.
  • How to run the patched application using uWSGI application server.
  • How to configure Nginx proxy in front of the application server.
  • [Bonus] How to use psycopg2 with psycogreen to make PostgreSQL access non-blocking.

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