Uvicorn server basics in FastAPI - Step-by-Step Execution

Concept Flow - Uvicorn server basics

Write FastAPI app code

↓

Run Uvicorn server command

↓

Uvicorn loads app

↓

Server listens on host:port

↓

Client sends HTTP request

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Uvicorn routes request to FastAPI app

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App processes request and returns response

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Uvicorn sends response back to client

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Repeat for next requests or stop server

This flow shows how you write a FastAPI app, run it with Uvicorn, and how Uvicorn handles requests and responses.

Execution Sample

FastAPI

from fastapi import FastAPI
import uvicorn

app = FastAPI()

@app.get("/")
async def read_root():
    return {"message": "Hello World"}

if __name__ == "__main__":
    uvicorn.run(app, host="127.0.0.1", port=8000)

This code creates a simple FastAPI app and runs it with Uvicorn on localhost port 8000.

Execution Table

Step	Action	Evaluation	Result
1	Import FastAPI and Uvicorn	Modules loaded	Ready to create app
2	Create FastAPI app instance	app = FastAPI()	App object created
3	Define GET endpoint '/'	Function read_root registered	Endpoint ready
4	Check if __name__ == '__main__'	True when run directly	Proceed to run server
5	Call uvicorn.run(app, host, port)	Uvicorn starts server	Server listens on 127.0.0.1:8000
6	Client sends GET request to '/'	Uvicorn receives request	Routes to read_root function
7	read_root executes	Returns {'message': 'Hello World'}	Response prepared
8	Uvicorn sends response to client	Client receives JSON message	Request complete
9	Wait for next request or stop	Server running	Loop continues or server stops

💡 Server stops when process is terminated or interrupted

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	After Step 7	Final
app	undefined	FastAPI instance	FastAPI with endpoint	Running server reference	N/A	N/A
read_root	undefined	undefined	Function registered	N/A	Function executed	N/A

Key Moments - 3 Insights

Why do we check if __name__ == '__main__' before running uvicorn?

This check ensures the server runs only when the script is executed directly, not when imported. See execution_table step 4.

What happens if the client sends a request to a path not defined in the app?

Uvicorn returns a 404 Not Found response because no matching endpoint exists. This is part of Uvicorn routing after step 6.

Why is the read_root function defined as async?

FastAPI supports async functions for better performance with I/O. Uvicorn can handle async endpoints efficiently, as shown in step 7.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result after step 5?

A App object created

B Server listens on 127.0.0.1:8000

C Function read_root registered

D Client receives JSON message

💡 Hint

Check the 'Result' column for step 5 in execution_table

At which step does the server send the response back to the client?

A Step 8

B Step 7

C Step 6

D Step 9

💡 Hint

Look for 'Uvicorn sends response to client' in execution_table

If we remove the if __name__ == '__main__' check, what changes in the execution?

A App object is not created

B Endpoints are not registered

C Server runs even when imported as a module

D Client requests fail

💡 Hint

Refer to key_moments about the purpose of the __name__ check

Concept Snapshot

Uvicorn runs FastAPI apps by loading the app instance and listening on a host and port.
Use uvicorn.run(app, host, port) inside if __name__ == '__main__' to start the server.
Uvicorn routes HTTP requests to FastAPI endpoints and sends back responses.
Async functions in FastAPI improve performance with Uvicorn.
Stop the server by interrupting the process (Ctrl+C).

Full Transcript

This visual execution shows how a FastAPI app is created and run using Uvicorn. First, the FastAPI app instance is created and endpoints are defined. Then, the script checks if it is run directly and starts the Uvicorn server on localhost port 8000. When a client sends a request, Uvicorn receives it and routes it to the matching FastAPI endpoint function. The function executes asynchronously and returns a JSON response. Uvicorn sends this response back to the client. The server keeps running until stopped. Key points include the importance of the __name__ check to avoid running the server on import, and how Uvicorn handles async endpoints efficiently.