How to Use Uvicorn Workers in Production with FastAPI

To use uvicorn workers in production with FastAPI, run uvicorn your_app:app --host 0.0.0.0 --port 8000 --workers 4. The --workers option starts multiple worker processes to handle requests concurrently, improving performance and reliability.

Syntax

The basic syntax to run a FastAPI app with multiple uvicorn workers is:

• uvicorn your_app:app: Runs the FastAPI app where your_app is the Python module and app is the FastAPI instance.
• --host 0.0.0.0: Makes the server accessible externally.
• --port 8000: Sets the port number.
• --workers 4: Starts 4 worker processes to handle requests concurrently.

bash

uvicorn your_app:app --host 0.0.0.0 --port 8000 --workers 4

Example

This example shows how to run a simple FastAPI app with 2 workers using uvicorn. It demonstrates how multiple workers can serve requests concurrently.

python

from fastapi import FastAPI
import time

app = FastAPI()

@app.get("/slow")
async def slow_endpoint():
    time.sleep(3)  # Simulate slow processing
    return {"message": "This was slow"}

# Run this command in terminal:
# uvicorn example:app --host 0.0.0.0 --port 8000 --workers 2

Common Pitfalls

1. Using --workers with reload: The --reload option is for development and does not work well with multiple workers. Avoid combining them.

2. Not using a process manager: In production, use a process manager like systemd or supervisor to keep workers running and restart on failure.

3. Overloading workers: Setting too many workers can exhaust CPU and memory. A good rule is 2-4 workers per CPU core.

bash

## Wrong way (development reload with workers):
# uvicorn example:app --reload --workers 4

## Right way (production):
# uvicorn example:app --host 0.0.0.0 --port 8000 --workers 4

Quick Reference

Option	Description	Example
--host	Set server host address	--host 0.0.0.0
--port	Set server port	--port 8000
--workers	Number of worker processes	--workers 4
--reload	Auto reload on code changes (dev only)	--reload (avoid with workers)

Key Takeaways

Use the --workers option with uvicorn to run multiple worker processes for FastAPI in production.

Avoid using --reload with multiple workers; it is meant for development only.

Choose the number of workers based on CPU cores to balance performance and resource use.

Use a process manager to keep uvicorn workers running reliably in production.

Run uvicorn with --host 0.0.0.0 to make your FastAPI app accessible externally.