I know GIL blocks python from running its threads across cores. If it does so, why python is being used in webservers, how are the companies like youtube, instagram handling it.
PS: I know alternatives like multiprocessing can solve it. But it would be great if anyone can post it with a scenario that was handled by them.
Python is used for server-side handling in webservers, but not (usually) as webserver.
On normal setup: we have have Apache or other webserver to handles a lot of processes (server-side) (python uses usually wsgi). Note usually apache handles directly "static" files. So we have one apache server, many parallel apache processes (to handle connection and basic http) and many python processes which handles one connection per time.
Each of such process are independent each others (they just use the same resources), so you can program your server side part easily, without worrying about deadlocks. It is mostly a trade-off: performance of code, and easy and quickly to produce code without huge problems. But usually webserver with python scale very well (also on large sites), and servers are cheaper then programmers.
Note: security is also increased by having just one request in a process.
GIL exists in CPython, (Python interpreter made in C and most used), other interpreter versions such as Jython or IronPython don't have such problem, because they don't have GIL.
Even though, using CPython you can still have concurrency, just do your thing in C and then "link it" in your Python code, just like Numpy or similar do.
Other thing is, even though you have your page using Flask or Django, when you set up it in a production server, you have an Apache or Nginx, etc which has a real charge balancer (or load balancer, I can't remember the name in english now) that can serve the page to many people at the same time.
Take it from the Flask docs (link):
Flask’s built-in server is not suitable for production as it doesn’t scale well and by default serves only one request at a time. [...]
If you want to deploy your Flask application to a WSGI server not listed here, look up the server documentation about how to use a WSGI app with it. Just remember that your Flask application object is the actual WSGI application.
Although a bit late, but I will try to give a generic and useful answer.
@Giacomo Catenazzi's answer is a good one but some part of it is factually incorrect.
API requests (or other form of web requests) are served from an already running process. The creation of this 'already running' process is handled by some webserver like gunicorn which on startup creates specified number of processes that are running the code in your web application continuously waiting to serve any incoming request.
Needless to say, each of these processes are limited by the GIL to only run one thread at a time. But one process in its lifetime handles more than one (normally many) request. Here it would be better if we could understand the flow of a request.
We will take an example of flask but this is applicable to most web frameworks. When a request comes from Nginx, it is handed over to gunicorn which interacts with your web application via wsgi. When the request reaches to the framework, an app context is created and some variables are pushed into the app-context. Then it follows the normal route that mostly people are familiar with: routing, db calls, response creation and so on. The response is then handed back to the gunicorn via wsgi again. At the time of handing over the response, the app context is teared down. So it's the app context, not the process that is created on every new request.
Also, I have talked only about the sync worker in gunicorn but it also has an option of async worker which can handle multiple requests in parallel through coroutines. But thats a separate topic.
So answering your question:
Read more about gunicorn's design and how it can be used to help you achieve your requirements. This is a good thread about gunicorn with flask understanding. And this is a great resource to understand flask app context
