document using gevent for async (#5900)

docs/async-await.rst

@@ -23,12 +23,6 @@ method in views that inherit from the :class:`flask.views.View` class, as
 well as all the HTTP method handlers in views that inherit from the
 :class:`flask.views.MethodView` class.
 
-.. admonition:: Using ``async`` with greenlet
-
-    When using gevent or eventlet to serve an application or patch the
-    runtime, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is
-    required.
-
 
 Performance
 -----------
@@ -78,15 +72,15 @@ Flask based on the `ASGI`_ standard instead of WSGI. This allows it to
 handle many concurrent requests, long running requests, and websockets
 without requiring multiple worker processes or threads.
 
-It has also already been possible to run Flask with Gevent or Eventlet
-to get many of the benefits of async request handling. These libraries
-patch low-level Python functions to accomplish this, whereas ``async``/
-``await`` and ASGI use standard, modern Python capabilities. Deciding
-whether you should use Flask, Quart, or something else is ultimately up
-to understanding the specific needs of your project.
+It has also already been possible to :doc:`run Flask with Gevent </gevent>` to
+get many of the benefits of async request handling. Gevent patches low-level
+Python functions to accomplish this, whereas ``async``/``await`` and ASGI use
+standard, modern Python capabilities. Deciding whether you should use gevent
+with Flask, or Quart, or something else is ultimately up to understanding the
+specific needs of your project.
 
-.. _Quart: https://github.com/pallets/quart
-.. _ASGI: https://asgi.readthedocs.io/en/latest/
+.. _Quart: https://quart.palletsprojects.com
+.. _ASGI: https://asgi.readthedocs.io
 
 
 Extensions
@@ -120,6 +114,6 @@ implemented async support, or make a feature request or PR to them.
 Other event loops
 -----------------
 
-At the moment Flask only supports :mod:`asyncio`. It's possible to
-override :meth:`flask.Flask.ensure_sync` to change how async functions
-are wrapped to use a different library.
+At the moment Flask only supports :mod:`asyncio`. It's possible to override
+:meth:`flask.Flask.ensure_sync` to change how async functions are wrapped to use
+a different library. See :ref:`gevent-asyncio` for an example.

docs/deploying/eventlet.rst

@@ -1,80 +1,8 @@
+:orphan:
+
 eventlet
 ========
 
-Prefer using :doc:`gunicorn` with eventlet workers rather than using
-`eventlet`_ directly. Gunicorn provides a much more configurable and
-production-tested server.
+`Eventlet is no longer maintained.`__ Use :doc:`/deploying/gevent` instead.
 
-`eventlet`_ 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:`gevent` 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.
-
-eventlet provides a WSGI server that can handle many connections at once
-instead of one per worker process. You must actually use eventlet in
-your own code to see any benefit to using the server.
-
-.. _eventlet: https://eventlet.net/
-.. _greenlet: https://greenlet.readthedocs.io/en/latest/
-
-
-Installing
-----------
-
-When using eventlet, 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
-``eventlet``.
-
-.. code-block:: text
-
-    $ cd hello-app
-    $ python -m venv .venv
-    $ . .venv/bin/activate
-    $ pip install .  # install your application
-    $ pip install eventlet
-
-
-Running
--------
-
-To use eventlet to serve your application, write a script that imports
-its ``wsgi.server``, as well as your app or app factory.
-
-.. code-block:: python
-    :caption: ``wsgi.py``
-
-    import eventlet
-    from eventlet import wsgi
-    from hello import create_app
-
-    app = create_app()
-    wsgi.server(eventlet.listen(("127.0.0.1", 8000)), app)
-
-.. code-block:: text
-
-    $ python wsgi.py
-    (x) wsgi starting up on http://127.0.0.1:8000
-
-
-Binding Externally
-------------------
-
-eventlet 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 eventlet.
-
-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.
+__ https://eventlet.readthedocs.io

docs/deploying/gevent.rst

@@ -7,15 +7,12 @@ 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.
+switching without writing ``async/await`` or using ``asyncio``. This is
+not the same as Python's ``async/await``, or the ASGI server spec.
 
 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.
+instead of one per worker process. See :doc:`/gevent` for more
+information about enabling it in your application.
 
 .. _gevent: https://www.gevent.org/
 .. _greenlet: https://greenlet.readthedocs.io/en/latest/
@@ -24,8 +21,7 @@ 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,
+When using gevent, greenlet>=1.0 is required. When using PyPy,
 PyPy>=7.3.7 is required.
 
 Create a virtualenv, install your application, then install ``gevent``.

docs/deploying/gunicorn.rst

@@ -8,7 +8,7 @@ multiple worker implementations for performance tuning.
 *   It does not support Windows (but does run on WSL).
 *   It is easy to install as it does not require additional dependencies
     or compilation.
-*   It has built-in async worker support using gevent or eventlet.
+*   It has built-in async worker support using gevent.
 
 This page outlines the basics of running Gunicorn. Be sure to read its
 `documentation`_ and use ``gunicorn --help`` to understand what features
@@ -93,20 +93,19 @@ otherwise it will be possible to bypass the proxy.
 IP address in your browser.
 
 
-Async with gevent or eventlet
------------------------------
+Async with gevent
+-----------------
 
-The default sync worker is appropriate for many use cases. If you need
-asynchronous support, Gunicorn provides workers using either `gevent`_
-or `eventlet`_. This is not the same as Python's ``async/await``, or the
-ASGI server spec. You must actually use gevent/eventlet in your own code
-to see any benefit to using the workers.
+The default sync worker is appropriate for most use cases. If you need numerous,
+long running, concurrent connections, Gunicorn provides an asynchronous worker
+using `gevent`_. This is not the same as Python's ``async/await``, or the ASGI
+server spec. See :doc:`/gevent` for more information about enabling it in your
+application.
 
-When using either gevent or eventlet, 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.
+.. _gevent: https://www.gevent.org/
 
-To use gevent:
+When using gevent, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is
+required.
 
 .. code-block:: text
 
@@ -115,16 +114,3 @@ To use gevent:
     Listening at: http://127.0.0.1:8000 (x)
     Using worker: gevent
     Booting worker with pid: x
-
-To use eventlet:
-
-.. code-block:: text
-
-    $ gunicorn -k eventlet 'hello:create_app()'
-    Starting gunicorn 20.1.0
-    Listening at: http://127.0.0.1:8000 (x)
-    Using worker: eventlet
-    Booting worker with pid: x
-
-.. _gevent: https://www.gevent.org/
-.. _eventlet: https://eventlet.net/

docs/deploying/index.rst

@@ -36,7 +36,6 @@ discusses platforms that can manage this for you.
     mod_wsgi
     uwsgi
     gevent
-    eventlet
     asgi
 
 WSGI servers have HTTP servers built-in. However, a dedicated HTTP

docs/deploying/uwsgi.rst

@@ -119,15 +119,16 @@ IP address in your browser.
 Async with gevent
 -----------------
 
-The default sync worker is appropriate for many use cases. If you need
-asynchronous support, uWSGI provides a `gevent`_ worker. This is not the
-same as Python's ``async/await``, or the ASGI server spec. You must
-actually use gevent in your own code to see any benefit to using the
-worker.
+The default sync worker is appropriate for most use cases. If you need numerous,
+long running, concurrent connections, uWSGI provides an asynchronous worker
+using `gevent`_. This is not the same as Python's ``async/await``, or the ASGI
+server spec. See :doc:`/gevent` for more information about enabling it in your
+application.
 
-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.
+.. _gevent: https://www.gevent.org/
+
+When using gevent, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is
+required.
 
 .. code-block:: text
 
@@ -140,6 +141,3 @@ PyPy>=7.3.7 is required.
     spawned uWSGI worker 1 (pid: x, cores: 100)
     spawned uWSGI http 1 (pid: x)
     *** running gevent loop engine [addr:x] ***
-
-
-.. _gevent: https://www.gevent.org/

docs/gevent.rst

@@ -0,0 +1,125 @@
+Async with Gevent
+=================
+
+`Gevent`_ patches Python's standard library to run within special async workers
+called `greenlets`_. Gevent has existed since long before Python's native
+asyncio was available, and Flask has always worked with it.
+
+.. _gevent: https://www.gevent.org
+.. _greenlets: https://greenlet.readthedocs.io
+
+Gevent is a reliable way to handle numerous, long lived, concurrent connections,
+and to achieve similar capabilities to ASGI and asyncio. This works without
+needing to write ``async def`` or ``await`` anywhere, but relies on gevent and
+greenlet's low level manipulation of the Python interpreter.
+
+Deciding whether you should use gevent with Flask, or `Quart`_, or something
+else, is ultimately up to understanding the specific needs of your project.
+
+.. _quart: https://quart.palletsprojects.com
+
+
+Enabling gevent
+---------------
+
+You need to apply gevent's patching as early as possible in your code. This
+enables gevent's underlying event loop and converts many Python internals to run
+inside it. Add the following at the top of your project's module or top
+``__init__.py``:
+
+.. code-block:: python
+
+    import gevent.monkey
+    gevent.monkey.patch_all()
+
+When deploying in production, use :doc:`/deploying/gunicorn` or
+:doc:`/deploying/uwsgi` with a gevent worker, as described on those pages.
+
+To run concurrent tasks within your own code, such as views, use
+|gevent.spawn|_:
+
+.. |gevent.spawn| replace:: ``gevent.spawn()``
+.. _gevent.spawn: https://www.gevent.org/api/gevent.html#gevent.spawn
+
+.. code-block:: python
+
+    @app.post("/send")
+    def send_email():
+        gevent.spawn(email.send, to="example@example.example", text="example")
+        return "Email is being sent."
+
+If you need to access :data:`request` or other Flask context globals within the
+spawned function, decorate the function with :func:`.stream_with_context` or
+:func:`.copy_current_request_context`. Prefer passing the exact data you need
+when spawning the function, rather than using the decorators.
+
+.. note::
+
+    When using gevent, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7
+    is required.
+
+
+.. _gevent-asyncio:
+
+Combining with ``async``/``await``
+----------------------------------
+
+Gevent's patching does not interact well with Flask's built-in asyncio support.
+If you want to use Gevent and asyncio in the same app, you'll need to override
+:meth:`flask.Flask.async_to_sync` to run async functions inside gevent.
+
+.. code-block:: python
+
+    import gevent.monkey
+    gevent.monkey.patch_all()
+
+    import asyncio
+    from flask import Flask, request
+
+    loop = asyncio.EventLoop()
+    gevent.spawn(loop.run_forever)
+
+    class GeventFlask(Flask):
+        def async_to_sync(self, func):
+            def run(*args, **kwargs):
+                coro = func(*args, **kwargs)
+                future = asyncio.run_coroutine_threadsafe(coro, loop)
+                return future.result()
+
+            return run
+
+    app = GeventFlask(__name__)
+
+    @app.get("/")
+    async def greet():
+        await asyncio.sleep(1)
+        return f"Hello, {request.args.get("name", "World")}!"
+
+This starts an asyncio event loop in a gevent worker. Async functions are
+scheduled on that event loop. This may still have limitations, and may need to
+be modified further when using other asyncio implementations.
+
+
+libuv
+~~~~~
+
+`libuv`_ is another event loop implementation that `gevent supports`_. There's
+also a project called `uvloop`_ that enables libuv in asyncio. If you want to
+use libuv, use gevent's support, not uvloop. It may be possible to further
+modify the ``async_to_sync`` code from the previous section to work with uvloop,
+but that's not currently known.
+
+.. _libuv: https://libuv.org/
+.. _gevent supports: https://www.gevent.org/loop_impls.html
+.. _uvloop: https://uvloop.readthedocs.io/
+
+To enable gevent's libuv support, add the following at the *very* top of your
+code, before ``gevent.monkey.patch_all()``:
+
+.. code-block:: python
+
+    import gevent
+    gevent.config.loop = "libuv"
+
+    import gevent.monkey
+    gevent.monkey.patch_all()

docs/index.rst

@@ -61,6 +61,7 @@ community-maintained extensions to add even more functionality.
    patterns/index
    web-security
    deploying/index
+   gevent
    async-await
 
 

docs/installation.rst

@@ -51,9 +51,8 @@ use them if you install them.
 greenlet
 ~~~~~~~~
 
-You may choose to use gevent or eventlet with your application. In this
-case, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is
-required.
+You may choose to use :doc:`/gevent` with your application. In this case,
+greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is required.
 
 These are not minimum supported versions, they only indicate the first
 versions that added necessary features. You should use the latest