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What is the point of using Gunicorn when hosting a Django project on Linux VM

I have an Linux instance running on Google Compute Engine. I installed pip and django on it and cloned a Django project that I worked on locally. Like I would on localhost I ran my app like so: python3 manage.py runserver 0.0.0.0:8080, and my server was up and running with no problems. I read online on how WSGI servers are required for python apps to run well on servers however I don't see why I would need something like gunicorn to run my app

Here's what the documentation for runserver says:
DO NOT USE THIS SERVER IN A PRODUCTION SETTING. It has not gone through security audits or performance tests. (And that’s how it’s gonna stay. We’re in the business of making Web frameworks, not Web servers, so improving this server to be able to handle a production environment is outside the scope of Django.)
Django's runserver is itself a WSGI server, but it's aimed at being easy for developers to use.
Production WSGI servers like uWSGI and Gunicorn have performance and production environments in mind. They handle concurrency better, they are faster, and are built to withstand malicious users, not just developers.

Should you deploy django with wsgi?

Do you need to deploy django with wsgi? I am running Django on a Docker instance and it seems like often the recommended solution is just to use Django's development server, i.e. the command python manage.py runserver. When exactly is a web server such as wsgi needed -- and in this instance, in a containerized application, is the django development server enough for production applications?

You answer your own question:
is the django development server enough for production applications ?
In the django documentation, you can read the following:
Now’s a good time to note: don’t use this server in anything resembling a production environment. It’s intended only for use while developing. (We’re in the business of making Web frameworks, not Web servers.)
And also this part:
DO NOT USE THIS SERVER IN A PRODUCTION SETTING. It has not gone through security audits or performance tests. (And that’s how it’s gonna stay. We’re in the business of making Web frameworks, not Web servers, so improving this server to be able to handle a production environment is outside the scope of Django.)
So no. Don't use the Django development server in production. Security risks, poor performances, etc.

The development server is never recommended as an option for production server. It has a number of security and performance issues.
The solution which is working well for us is Gunicorn behind an Nginx reverse proxy (so that multiple people can connect smoothly.)
The method mentioned in this tutorial is a good beginners guide to a Ubuntu setup with nginx and gunicorn. When bringing docker into the mix use this tutorial.

You can use Django Channels to deploy in production without using WSGI.
You can set things up in one of two ways; either route all traffic through a HTTP/WebSocket interface server, removing the need to run a WSGI server at all; or, just route WebSockets and long-poll HTTP connections to the interface server, and leave other pages served by a standard WSGI server.

Are a WSGI server and HTTP server required to serve a Flask app?

Setting up Flask with uWSGI and Nginx can be difficult. I tried following this DigitalOcean tutorial and still had trouble. Even with buildout scripts it takes time, and I need to write instructions to follow next time.
If I don't expect a lot of traffic, or the app is private, does it make sense to run it without uWSGI? Flask can listen to a port. Can Nginx just forward requests?
Does it make sense to not use Nginx either, just running bare Flask app on a port?

When you "run Flask" you are actually running Werkzeug's development WSGI server, and passing your Flask app as the WSGI callable.
The development server is not intended for use in production. It is not designed to be particularly efficient, stable, or secure. It does not support all the possible features of a HTTP server.
Replace the Werkzeug dev server with a production-ready WSGI server such as Gunicorn or uWSGI when moving to production, no matter where the app will be available.
The answer is similar for "should I use a web server". WSGI servers happen to have HTTP servers but they will not be as good as a dedicated production HTTP server (Nginx, Apache, etc.).
Flask documents how to deploy in various ways. Many hosting providers also have documentation about deploying Python or Flask.

First create the app:
import flask
app = flask.Flask(__name__)
Then set up the routes, and then when you want to start the app:
import gevent.pywsgi
app_server = gevent.pywsgi.WSGIServer((host, port), app)
app_server.serve_forever()
Call this script to run the application rather than having to tell gunicorn or uWSGI to run it.
I wanted the utility of Flask to build a web application, but had trouble composing it with other elements. I eventually found that gevent.pywsgi.WSGIServer was what I needed. After the call to app_server.serve_forever(), call app_server.stop() when to exit the application.
In my deployment, my application is listening on localhost:port using Flask and gevent, and then I have Nginx reverse-proxying HTTPS requests to it.

You definitely need something like a production WSGI server such as Gunicorn, because the development server of Flask is meant for ease of development without much configuration for fine-tuning and optimization.
Eg. Gunicorn has a variety of configurations depending on the use case you are trying to solve. But the development flask server does not have these capabilities. In addition, these development servers show their limitations as soon as you try to scale and handle more requests.
With respect to needing a reverse proxy server such as Nginx is concerned it depends on your use case.
If you are deploying your application behind the latest load balancer in AWS such as an application load balancer(NOT classic load balancer), that itself will suffice for most use cases. No need to take effort into setting up NGINX if you have that option.
The purpose of a reverse proxy is to handle slow clients, meaning clients which take time to send the request. These reverse load balancers buffer the requests till the entire request is got from the clients and send them async to Gunicorn. This improves the performance of your application considerably.

Flask using Nginx?

I am a .net developer coming over to python. I have recently started using Flask and have some quick questions about serving files.
I noticed a lot of tutorials focused on nginix and flask. However, I am able to run flask without nginx. I'm just curious as to why this is used together (nginx and flask). Is nginx only for static files?

Nginx is a proxy server, imagine your apps have multiples microservices on differents languagues.
For more info NGINX REVERSE PROXY

On a development machine flask can be run without a webserver (nginx, apache etc) or an application container (eg uwsgi, gunicorn etc).
Things are different when you want to handle the load on a production server. For starters python is relatively very slow when it comes to serving static content where as apache / nginx do that very well.
When the application becomes big enough to be broken into multiple separate services or has to be horizontally scaled, the proxy server capabilities of nginx come in very handy.
In the architectures I build, nginx serves as the entry point where ssl is terminates and the rest of the application is behind a VPN and firewall.
Does this help?

From http://flask.pocoo.org/docs/1.0/deploying/ :
"While lightweight and easy to use, Flask’s built-in server is not suitable for production as it doesn’t scale well. Some of the options available for properly running Flask in production are documented here."

What is the correct way to leave a Django server running on an EC2 instance?

I'm currently using screen and doing
sudo python manage.py runserver 0.0.0.0:80
Then closing the terminal. Seems like a bit of a hack. What is the correct way to do it?

runserver is a development server. You shouldn't use it in production, as explained at https://docs.djangoproject.com/en/1.4/ref/django-admin/#runserver-port-or-address-port where it says:
DO NOT USE THIS SERVER IN A PRODUCTION SETTING. It has not gone through security audits or performance tests. (And that's how it's gonna stay. We're in the business of making Web frameworks, not Web servers, so improving this server to be able to handle a production environment is outside the scope of Django.)
You should use one of the methods given in https://docs.djangoproject.com/en/1.4/howto/deployment/ for deploying a Django project in production. I have used mod_wsgi with Apache, gunicorn with nginx - the precise solution is up to you and the requirements of your project, but the deployment section of the Django manual goes through various options.

This really isn't a good idea; the built in development server shouldn't be used outside of your local development machine. Look at the docs:
DO NOT USE THIS SERVER IN A PRODUCTION SETTING. It has not gone through security audits or performance tests. (And that's how it's gonna stay. We're in the business of making Web frameworks, not Web servers, so improving this server to be able to handle a production environment is outside the scope of Django.)
Instead you should set up nginx+gunicorn/uwsgi or just apache+mod_wsgi.
If you ec2 instance is totally blocked from all possible communication from the outside world (which is unlikely) you can use the screen command