Understand the purpose of Uvicorn in FastAPI applications
David Y.
—The Problem
I’m learning to build applications with FastAPI and I keep reading about this thing called Uvicorn that I’m supposed to use. I understand that it needs to be installed alongside FastAPI and I need to use it to run my application, but I don’t get why.
What is Uvicorn doing and why do I need to use it with FastAPI?
The Solution
Uvicorn is a web server. It handles network communication - receiving requests from client applications such as users’ browsers and sending responses to them. It communicates with FastAPI using the Asynchronous Server Gateway Interface (ASGI), a standard API for Python web servers that run asynchronous code. This leaves FastAPI and our application code free to concentrate on the HTTP layer and application logic, without worrying about the low-level details of network connections.
ASGI is the successor to the Web Server Gateway Interface (WSGI), which only supported synchronous code. ASGI-compatible web servers support both asynchronous and synchronous code.
Uvicorn is not the only ASGI-compatible web server implementation. Alternatives include Daphne (used by Django) and Hypercorn. Both these servers can replace Uvicorn, but since FastAPI is built on Uvicorn, Uvicorn is the recommended web server in most situations.
Uvicorn can be used on its own for development, but production deployments should incorporate Gunicorn with Uvicorn workers. You can read more about the optimal number of these workers in our answer here.
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