What are Uvicorn workers and how do you configure multi-process deployments?

By default Uvicorn runs as a single process with a single event loop. To utilise multiple CPU cores and handle more concurrent requests, you can run multiple worker processes using the --workers flag.

# Run 4 worker processes
uvicorn main:app --workers 4

# Workers can also be set via environment variable:
export WEB_CONCURRENCY=4
uvicorn main:app

# Common formula for worker count:
# workers = (2 x CPU cores) + 1  for I/O-bound apps
# workers = CPU cores             for CPU-bound apps
import multiprocessing
workers = (2 * multiprocessing.cpu_count()) + 1

# IMPORTANT: --workers and --reload are mutually exclusive

How multi-worker mode works: Uvicorn spawns the specified number of separate OS processes, each running its own event loop and handling requests independently. There is no shared state between workers - each process is completely independent. This is true parallelism (bypasses the GIL) unlike threading.

Production recommendation: for production multi-worker deployments, use Gunicorn with UvicornWorker rather than Uvicorn's built-in --workers. Gunicorn provides superior process lifecycle management, graceful restarts, and health monitoring.

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