Uvicorn

Uvicorn

Overview

Uvicorn is a lightning-fast ASGI server for Python. It's the recommended way to run FastAPI, Starlette, and Django ASGI in production. For multi-core utilization, pair it with Gunicorn as a process manager.

Instructions

Step 1: Development

pip install uvicorn[standard]

# Run with auto-reload
uvicorn app.main:app --reload --port 8000

Step 2: Production with Gunicorn

pip install gunicorn

# Gunicorn manages multiple Uvicorn worker processes
gunicorn app.main:app \
  --workers 4 \                    # CPU cores × 2 + 1
  --worker-class uvicorn.workers.UvicornWorker \
  --bind 0.0.0.0:8000 \
  --timeout 120 \
  --graceful-timeout 30 \
  --access-logfile - \
  --error-logfile -

Step 3: Docker Production

FROM python:3.12-slim
WORKDIR /​app

COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

COPY . .

# Non-root user
RUN adduser --system --uid 1001 app
USER app

# Production command
CMD ["gunicorn", "app.main:app", \
     "--workers", "4", \
     "--worker-class", "uvicorn.workers.UvicornWorker", \
     "--bind", "0.0.0.0:8000", \
     "--timeout", "120"]

Step 4: Programmatic Configuration

# run.py — Uvicorn with programmatic config
import uvicorn

if __name__ == "__main__":
    uvicorn.run(
        "app.main:app",
        host="0.0.0.0",
        port=8000,
        workers=4,
        log_level="info",
        access_log=True,
        proxy_headers=True,        # trust X-Forwarded-* from reverse proxy
        forwarded_allow_ips="*",
    )

Guidelines

• Development: uvicorn --reload (single process, auto-reload on file changes).
• Production: gunicorn with UvicornWorker class (multi-process, no reload).
• Workers formula: (2 × CPU cores) + 1 — e.g., 4 cores → 9 workers.
• Always use behind a reverse proxy (nginx, Caddy) for TLS termination and static files.
• Set proxy_headers=True when behind a load balancer to get real client IPs.