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Deployment

Server deployment is a complex area, that will depend on what kind of service you're deploying Uvicorn onto.

As a general rule, you probably want to:

  • Run uvicorn --reload from the command line for local development.
  • Run gunicorn -k uvicorn.workers.UvicornWorker for production.
  • Additionally run behind Nginx for self-hosted deployments.
  • Finally, run everything behind a CDN for caching support, and serious DDOS protection.

Running from the command line

Typically you'll run uvicorn from the command line.

$ uvicorn example:app --reload --port 5000

The ASGI application should be specified in the form path.to.module:instance.path.

When running locally, use --reload to turn on auto-reloading.

To see the complete set of available options, use uvicorn --help:

$ uvicorn --help
Usage: uvicorn [OPTIONS] APP

Options:
  --host TEXT                     Bind socket to this host.  [default:
                                  127.0.0.1]
  --port INTEGER                  Bind socket to this port.  [default: 8000]
  --uds TEXT                      Bind to a UNIX domain socket.
  --fd INTEGER                    Bind to socket from this file descriptor.
  --reload                        Enable auto-reload.
  --reload-dir TEXT               Set reload directories explicitly, instead
                                  of using 'sys.path'.
  --workers INTEGER               Number of worker processes. Not valid with
                                  --reload.
  --loop [auto|asyncio|uvloop]    Event loop implementation.  [default: auto]
  --http [auto|h11|httptools]     HTTP protocol implementation.  [default:
                                  auto]
  --ws [none|auto|websockets|wsproto]
                                  WebSocket protocol implementation.
                                  [default: auto]
  --lifespan [auto|on|off]        Lifespan implementation.  [default: auto]
  --interface [auto|asgi3|agsi2|wsgi]
                                  Select ASGI3, ASGI2, or WSGI as the
                                  application interface.
  --log-level [critical|error|warning|info|debug]
                                  Log level.  [default: info]
  --no-access-log                 Disable access log.
  --proxy-headers                 Use X-Forwarded-Proto, X-Forwarded-For,
                                  X-Forwarded-Port to populate remote address
                                  info.
  --root-path TEXT                Set the ASGI 'root_path' for applications
                                  submounted below a given URL path.
  --limit-concurrency INTEGER     Maximum number of concurrent connections or
                                  tasks to allow, before issuing HTTP 503
                                  responses.
  --limit-max-requests INTEGER    Maximum number of requests to service before
                                  terminating the process.
  --timeout-keep-alive INTEGER    Close Keep-Alive connections if no new data
                                  is received within this timeout.  [default:
                                  5]
  --ssl-keyfile TEXT              SSL key file
  --ssl-certfile TEXT             SSL certificate file
  --ssl-version INTEGER           SSL version to use (see stdlib ssl module's)
                                  [default: 2]
  --ssl-cert-reqs INTEGER         Whether client certificate is required (see
                                  stdlib ssl module's)  [default: 0]
  --ssl-ca-certs TEXT             CA certificates file
  --ssl-ciphers TEXT              Ciphers to use (see stdlib ssl module's)
                                  [default: TLSv1]
  --help                          Show this message and exit.

See the settings documentation for more details on the supported options for running uvicorn.

Running programmatically

To run directly from within a Python program, you should use uvicorn.run(app, **config). For example:

import uvicorn

class App:
    ...

app = App()

if __name__ == "__main__":
    uvicorn.run(app, host="127.0.0.1", port=5000, log_level="info", reload=True)

The set of configuration options is the same as for the command line tool.

Using a process manager

Running Uvicorn using a process manager ensures that you can run multiple processes in a resilient manner, and allows you to perform server upgrades without dropping requests.

A process manager will handle the socket setup, start-up multiple server processes, monitor process aliveness, and listen for signals to provide for processes restarts, shutdowns, or dialing up and down the number of running processes.

Uvicorn provides a lightweight way to run multiple worker processes, for example --workers 4, but does not provide any process monitoring.

Gunicorn

Gunicorn is probably the simplest way to run and manage Uvicorn in a production setting. Uvicorn includes a gunicorn worker class that means you can get set up with very little configuration.

The following will start Gunicorn with four worker processes:

gunicorn -w 4 -k uvicorn.workers.UvicornWorker

The UvicornWorker implementation uses the uvloop and httptools implementations. To run under PyPy you'll want to use pure-python implementation instead. You can do this by using the UvicornH11Worker class.

gunicorn -w 4 -k uvicorn.workers.UvicornH11Worker

Gunicorn provides a different set of configuration options to Uvicorn, so some options such as --limit-concurrency are not yet supported when running with Gunicorn.

Supervisor

To use supervisor as a process manager you should either:

  • Hand over the socket to uvicorn using its file descriptor, which supervisor always makes available as 0, and which must be set in the fcgi-program section.
  • Or use a UNIX domain socket for each uvicorn process.

A simple supervisor configuration might look something like this:

supervisord.conf:

[supervisord]

[fcgi-program:uvicorn]
socket=tcp://localhost:8000
command=venv/bin/uvicorn --fd 0 example:App
numprocs=4
process_name=uvicorn-%(process_num)d
stdout_logfile=/dev/stdout
stdout_logfile_maxbytes=0

Then run with supervisord -n.

Circus

To use circus as a process manager, you should either:

  • Hand over the socket to uvicorn using its file descriptor, which circus makes available as $(circus.sockets.web).
  • Or use a UNIX domain socket for each uvicorn process.

A simple circus configuration might look something like this:

circus.ini:

[watcher:web]
cmd = venv/bin/uvicorn --fd $(circus.sockets.web) example:App
use_sockets = True
numprocesses = 4

[socket:web]
host = 0.0.0.0
port = 8000

Then run circusd circus.ini.

Running behind Nginx

Using Nginx as a proxy in front of your Uvicorn processes may not be neccessary, but is recommended for additional resiliance. Nginx can deal with serving your static media and buffering slow requests, leaving your application servers free from load as much as possible.

In managed environments such as Heroku, you wont typically need to configure Nginx, as your server processes will already be running behind load balancing proxies.

The recommended configuration for proxying from Nginx is to use a UNIX domain socket between Nginx and whatever the process manager that is being used to run Uvicorn.

When fronting the application with a proxy server you want to make sure that the proxy sets headers to ensure that application can properly determine the client address of the incoming connection, and if the connection was over http or https.

You should ensure that the X-Forwarded-For and X-Forwarded-Proto headers are set by the proxy, and that Uvicorn is run using the --proxy-headers setting. This ensure that the ASGI scope includes correct client and scheme information.

Here's how a simple Nginx configuration might look. This example includes setting proxy headers, and using a UNIX domain socket to communicate with the application server.

http {
  server {
    listen 80;
    client_max_body_size 4G;

    server_name example.com;

    location / {
      proxy_set_header Host $http_host;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header X-Forwarded-Proto $scheme;
      proxy_redirect off;
      proxy_buffering off;
      proxy_pass http://uvicorn;
    }

    location /static {
      # path for static files
      root /path/to/app/static;
    }
  }

  upstream uvicorn {
    server unix:/tmp/uvicorn.sock;
  }

}

Uvicorn's --proxy-headers behavior may not be sufficient for more complex proxy configurations that use different combinations of headers, or where the application is running behind more than one intermediary proxying service.

In those cases you might want to use an ASGI middleware to set the client and scheme dependant on the request headers.

Running behind a CDN

Running behind a content delivery network, such as Cloudflare or Cloud Front, provides a serious layer of protection against DDOS attacks. Your sevice will be running behind huge clusters of proxies and load balancers that are designed for handling huge amounts of traffic, and have capabilities for detecting and closing off connections from DDOS attacks.

Proper usage of cache control headers can mean that a CDN is able to serve large amounts of data without always having to forward the request on to your server.

Content Delivery Networks can also be a low-effort way to provide HTTPS termination.

Running with HTTPS

To run uvicorn with https, a certificate and a private key are required. The recommended way to get them is using Let's Encrypt.

For local development with https, it's possible to use mkcert to generate a valid certificat and private key.

$ uvicorn example:app --port 5000 --ssl-keyfile=./key.pem --ssl-certfile=./cert.pem

Running gunicorn worker

It also possible to use certificates with uvicorn's worker for gunicorn

$ gunicorn --keyfile=./key.pem --certfile=./cert.pem -k uvicorn.workers.UvicornWorker example:app