I see people are running setups like Nginx + Gunicorn + Flask.
Can someone explain what is the benefit of having Gunicorn in front of Flask? Why not just have Flask running? Doesn't it consume more resources having Gunicorn + Flask running? Is it able to reboot the flask instance when it fails to respond?
What's also the purpose of having nginx on top of gunicorn? isn't gunicorn enough? Again, more resources being spent?
I think you may be confused, Flask is not a web server, it is a framework and needs some sort of web server, such as Gunicorn, Nginx or Apache, to accept HTTP requests which it will then operate on. The reason why people run Nginx and Gunicorn together is that in addition to being a web server, Nginx can also proxy connections to Gunicorn which brings certain performance benefits, here is a pretty good answer that elaborates on those benefits: http://serverfault.com/questions/220046/why-is-setting-nginx-as-a-reverse-proxy-a-good-idea
EDIT: Added link containing information about performance benefits of running Nginx as a proxy.