Why we need uWSGI and uwsgi

Question 

I'm looking at the WSGI specification and I'm trying to figure out how servers like uWSGI fit into the picture. I understand the point of the WSGI spec is to separate web servers like nginx from web applications like something you'd write using Flask. What I don't understand is what uWSGI is for. Why can't nginx directly call my Flask application? Can't flask speak WSGI directly to it? Why does uWSGI need to get in between them?

There are two sides in the WSGI spec: the server and the web app. Which side is uWSGI on?

Because nginx doesn't support the WSGI spec. Technically nginx could implement the WSGI spec if they wanted, they just haven't.

That being the case, we need a web server that does implement the spec, which is what the uWSGI server is for.

Note that uWSGI is a full fledged http server that can and does work well on its own. I've used it in this capacity several times and it works great. If you need super high throughput for static content, then you have the option of sticking nginx in front of your uWSGI server. When you do, they will communicate over a low level protocol known as uwsgi.

"What the what?! Another thing called uwsgi?!" you ask. Yeah, it's confusing. When you reference uWSGI you are talking about an http server. When you talk about uwsgi (all lowercase) you are talking about a binary protocol that the uWSGI server uses to talk to other servers like nginx. They picked a bad name on this one.

For anyone who is interested, I wrote a blog article about it with more specifics, a bit of history, and some examples.

Btw, what is WSGI? 

The Web Server Gateway Interface (WSGI, pronounced whiskey^[1][2] or WIZ-ghee^[3]) is a simple calling convention for web servers to forward requests to web applications or frameworks written in the Python programming language. The current version of WSGI, version 1.0.1, is specified in Python Enhancement Proposal (PEP) 3333.^[4]

WSGI was originally specified as PEP-333 in 2003.^[5] PEP-3333, published in 2010, updates the specification for Python 3.

References:

https://stackoverflow.com/questions/38601440/what-is-the-point-of-uwsgi