How do you use Uvicorn with a Unix domain socket instead of a TCP port?

Instead of binding to a TCP host:port, Uvicorn can listen on a Unix domain socket (UDS) - a special file on the filesystem used for inter-process communication. This is faster than TCP loopback for communication between processes on the same machine.

# Bind to a Unix domain socket
uvicorn main:app --uds /tmp/uvicorn.sock

# Programmatic:
uvicorn.run("main:app", uds="/tmp/uvicorn.sock")

# Nginx config to proxy to the Unix socket:
# upstream app {
#     server unix:/tmp/uvicorn.sock;
# }
# server {
#     location / {
#         proxy_pass http://app;
#     }
# }

# Bind to a file descriptor (for socket activation, e.g. systemd):
uvicorn main:app --fd 3
# systemd passes the pre-bound socket as fd 3

Performance: Unix domain sockets bypass the TCP/IP stack entirely for same-machine communication, reducing latency. The improvement is measurable but modest - typically a few percent. The main practical benefit is avoiding port number management when running multiple services on the same host.

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