# gevent

Prefer using Gunicorn or uWSGI with gevent workers rather than using gevent directly. Gunicorn and uWSGI provide much more configurable and production-tested servers.

gevent allows writing asynchronous, coroutine-based code that looks like standard synchronous Python. It uses greenlet to enable task switching without writing `async/await` or using `asyncio`.

eventlet is another library that does the same thing. Certain dependencies you have, or other considerations, may affect which of the two you choose to use.

gevent provides a WSGI server that can handle many connections at once instead of one per worker process. You must actually use gevent in your own code to see any benefit to using the server.

## Installing

When using gevent, greenlet>=1.0 is required, otherwise context locals such as `request` will not work as expected. When using PyPy, PyPy>=7.3.7 is required.

Create a virtualenv, install your application, then install `gevent`.

    $ cd hello-app
    $ python -m venv venv
    $ . venv/bin/activate
    $ pip install .  # install your application
    $ pip install gevent

## Running

To use gevent to serve your application, write a script that imports its `WSGIServer`, as well as your app or app factory.

`wsgi.py`

    from gevent.pywsgi import WSGIServer
    from hello import create_app

    app = create_app()
    http_server = WSGIServer(("127.0.0.1", 8000), app)
    http_server.serve_forever()

    $ python wsgi.py

No output is shown when the server starts.

## Binding Externally

gevent should not be run as root because it would cause your application code to run as root, which is not secure. However, this means it will not be possible to bind to port 80 or 443. Instead, a reverse proxy such as nginx or Apache httpd should be used in front of gevent.

You can bind to all external IPs on a non-privileged port by using `0.0.0.0` in the server arguments shown in the previous section. Don’t do this when using a reverse proxy setup, otherwise it will be possible to bypass the proxy.

`0.0.0.0` is not a valid address to navigate to, you’d use a specific IP address in your browser.