Understanding the Relation Between FastAPI and Uvicorn

If you're coming from a JavaScript ecosystem (like I did) and are trying to develop apps in Python, you’ll likely find yourself comparing the components to better understand how they map across. Well, I’ve done that comparison for you — thank me later! Let’s first take a look at the Python components and see how they compare to their JavaScript counterparts. FastAPI and Uvicorn are two essential building blocks when developing high-performance Python APIs. FastAPI is the web framework. It helps you define routes, handle requests, perform validation, and build APIs easily. It follows the ASGI (Asynchronous Server Gateway Interface) standard. Uvicorn is the ASGI server. It is responsible for running your FastAPI app, handling incoming HTTP connections, and managing the event loop efficiently. To illustrate this, think of FastAPI as the chef preparing the food (your API responses), and Uvicorn as the waiter delivering that food to the customer (handling requests and responses). In simple terms: FastAPI builds the application, Uvicorn serves it to the world. You develop your API with FastAPI, then use Uvicorn to launch it like this: uvicorn main:app --reload Here, main is your Python file, and app is your FastAPI instance. Together, FastAPI and Uvicorn enable you to build modern, fast, and scalable Python APIs with minimal effort. Comparison Aspect FastAPI + Uvicorn Node.js Ecosystem (Express.js/Fastify) Framework FastAPI (Python web framework) Express.js, Fastify (JavaScript frameworks) Server Uvicorn (ASGI server) Node.js HTTP server (Express.js, Fastify) Asynchronous Support Natively asynchronous (async/await) Natively asynchronous (async/await, Promises) Performance High performance, handles many concurrent requests High performance with event-driven architecture Serving the App uvicorn main:app --reload node server.js Use Case Ideal for high-performance, async I/O-bound APIs Ideal for handling many concurrent connections Chef/Waiter Analogy FastAPI is the chef, Uvicorn is the waiter Express/Fastify is the chef, Node.js is the waiter

Apr 29, 2025 - 01:57

Understanding the Relation Between FastAPI and Uvicorn

If you're coming from a JavaScript ecosystem (like I did) and are trying to develop apps in Python, you’ll likely find yourself comparing the components to better understand how they map across. Well, I’ve done that comparison for you — thank me later!

Let’s first take a look at the Python components and see how they compare to their JavaScript counterparts.

FastAPI and Uvicorn are two essential building blocks when developing high-performance Python APIs.

FastAPI is the web framework. It helps you define routes, handle requests, perform validation, and build APIs easily. It follows the ASGI (Asynchronous Server Gateway Interface) standard.
Uvicorn is the ASGI server. It is responsible for running your FastAPI app, handling incoming HTTP connections, and managing the event loop efficiently.

To illustrate this, think of FastAPI as the chef preparing the food (your API responses), and Uvicorn as the waiter delivering that food to the customer (handling requests and responses).

In simple terms:

FastAPI builds the application,

Uvicorn serves it to the world.

You develop your API with FastAPI, then use Uvicorn to launch it like this:

uvicorn main:app --reload

Here, main is your Python file, and app is your FastAPI instance. Together, FastAPI and Uvicorn enable you to build modern, fast, and scalable Python APIs with minimal effort.

Comparison

Aspect	FastAPI + Uvicorn	Node.js Ecosystem (Express.js/Fastify)
Framework	FastAPI (Python web framework)	Express.js, Fastify (JavaScript frameworks)
Server	Uvicorn (ASGI server)	Node.js HTTP server (Express.js, Fastify)
Asynchronous Support	Natively asynchronous (async/await)	Natively asynchronous (async/await, Promises)
Performance	High performance, handles many concurrent requests	High performance with event-driven architecture
Serving the App	`uvicorn main:app --reload`	`node server.js`
Use Case	Ideal for high-performance, async I/O-bound APIs	Ideal for handling many concurrent connections
Chef/Waiter Analogy	FastAPI is the chef, Uvicorn is the waiter	Express/Fastify is the chef, Node.js is the waiter