rewrite deployment docs

docs/deploying/gevent.rst

@@ -0,0 +1,80 @@
+gevent
+======
+
+Prefer using :doc:`gunicorn` or :doc:`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``.
+
+:doc:`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.
+
+.. _gevent: https://www.gevent.org/
+.. _greenlet: https://greenlet.readthedocs.io/en/latest/
+
+
+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``.
+
+.. code-block:: text
+
+    $ 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.
+
+.. code-block:: python
+    :caption: ``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()
+
+.. code-block:: text
+
+    $ 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 :doc:`nginx` or :doc:`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.