update async docs

docs/async-await.rst

@@ -0,0 +1,81 @@
+.. _async_await:
+
+Using ``async`` and ``await``
+=============================
+
+.. versionadded:: 2.0
+
+Routes, error handlers, before request, after request, and teardown
+functions can all be coroutine functions if Flask is installed with the
+``async`` extra (``pip install flask[async]``). This allows views to be
+defined with ``async def`` and use ``await``.
+
+.. code-block:: python
+
+    @app.route("/get-data")
+    async def get_data():
+        data = await async_db_query(...)
+        return jsonify(data)
+
+
+Performance
+-----------
+
+Async functions require an event loop to run. Flask, as a WSGI
+application, uses one worker to handle one request/response cycle.
+When a request comes in to an async view, Flask will start an event loop
+in a thread, run the view function there, then return the result.
+
+Each request still ties up one worker, even for async views. The upside
+is that you can run async code within a view, for example to make
+multiple concurrent database queries, HTTP requests to an external API,
+etc. However, the number of requests your application can handle at one
+time will remain the same.
+
+**Async is not inherently faster than sync code.** Async is beneficial
+when performing concurrent IO-bound tasks, but will probably not improve
+CPU-bound tasks. Traditional Flask views will still be appropriate for
+most use cases, but Flask's async support enables writing and using
+code that wasn't possible natively before.
+
+
+When to use Quart instead
+-------------------------
+
+Flask's async support is less performant than async-first frameworks due
+to the way it is implemented. If you have a mainly async codebase it
+would make sense to consider `Quart`_. Quart is a reimplementation of
+Flask based on the `ASGI`_ standard instead of WSGI. This allows it to
+handle many concurrent requests, long running requests, and websockets
+without requiring individual 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.
+
+.. _Quart: https://gitlab.com/pgjones/quart
+.. _ASGI: https://asgi.readthedocs.io/en/latest/
+
+
+Extensions
+----------
+
+Existing Flask extensions only expect views to be synchronous. If they
+provide decorators to add functionality to views, those will probably
+not work with async views because they will not await the function or be
+awaitable. Other functions they provide will not be awaitable either and
+will probably be blocking if called within an async view.
+
+Check the changelog of the extension you want to use to see if they've
+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.

docs/async_await.rst

@@ -1,46 +0,0 @@
-.. _async_await:
-
-Using async and await
-=====================
-
-.. versionadded:: 2.0
-
-Routes, error handlers, before request, after request, and teardown
-functions can all be coroutine functions if Flask is installed with
-the ``async`` extra (``pip install flask[async]``). This allows code
-such as,
-
-.. code-block:: python
-
-    @app.route("/")
-    async def index():
-        return await ...
-
-including the usage of any asyncio based libraries.
-
-
-When to use Quart instead
--------------------------
-
-Flask's ``async/await`` support is less performant than async first
-frameworks due to the way it is implemented. Therefore if you have a
-mainly async codebase it would make sense to consider `Quart
-<https://gitlab.com/pgjones/quart>`_. Quart is a reimplementation of
-the Flask using ``async/await`` based on the ASGI standard (Flask is
-based on the WSGI standard).
-
-
-Decorators
-----------
-
-Decorators designed for Flask, such as those in Flask extensions are
-unlikely to work. This is because the decorator will not await the
-coroutine function nor will they themselves be awaitable.
-
-
-Other event loops
------------------
-
-At the moment Flask only supports asyncio - the
-:meth:`flask.Flask.ensure_sync` should be overridden to support
-alternative event loops.

docs/design.rst

@@ -171,16 +171,23 @@ Also see the :doc:`/becomingbig` section of the documentation for some
 inspiration for larger applications based on Flask.
 
 
-Async-await and ASGI support
+Async/await and ASGI support
 ----------------------------
 
-Flask supports ``async`` coroutines for view functions, and certain
-others by executing the coroutine on a seperate thread instead of
-utilising an event loop on the main thread as an async first (ASGI)
-frameworks would. This is necessary for Flask to remain backwards
-compatibility with extensions and code built before ``async`` was
-introduced into Python. This compromise introduces a performance cost
-compared with the ASGI frameworks, due to the overhead of the threads.
+Flask supports ``async`` coroutines for view functions by executing the
+coroutine on a separate thread instead of using an event loop on the
+main thread as an async-first (ASGI) framework would. This is necessary
+for Flask to remain backwards compatible with extensions and code built
+before ``async`` was introduced into Python. This compromise introduces
+a performance cost compared with the ASGI frameworks, due to the
+overhead of the threads.
+
+Due to how tied to WSGI Flask's code is, it's not clear if it's possible
+to make the ``Flask`` class support ASGI and WSGI at the same time. Work
+is currently being done in Werkzeug to work with ASGI, which may
+eventually enable support in Flask as well.
+
+See :doc:`/async-await` for more discussion.
 
 
 What Flask is, What Flask is Not

docs/index.rst

@@ -59,7 +59,7 @@ instructions for web development with Flask.
    patterns/index
    deploying/index
    becomingbig
-   async_await
+   async-await
 
 
 API Reference

src/flask/app.py

@@ -1520,12 +1520,13 @@ class Flask(Scaffold):
         return False
 
     def ensure_sync(self, func):
-        """Ensure that the returned function is sync and calls the async func.
+        """Ensure that the function is synchronous for WSGI workers.
+        Plain ``def`` functions are returned as-is. ``async def``
+        functions are wrapped to run and wait for the response.
+
+        Override this method to change how the app runs async views.
 
         .. versionadded:: 2.0
-
-        Override if you wish to change how asynchronous functions are
-        run.
         """
         if iscoroutinefunction(func):
             return run_async(func)

src/flask/blueprints.py

@@ -475,8 +475,8 @@ class Blueprint(Scaffold):
         """Ensure the function is synchronous.
 
         Override if you would like custom async to sync behaviour in
-        this blueprint. Otherwise :meth:`~flask.Flask..ensure_sync` is
-        used.
+        this blueprint. Otherwise the app's
+        :meth:`~flask.Flask.ensure_sync` is used.
 
         .. versionadded:: 2.0
         """

src/flask/helpers.py

@@ -742,9 +742,10 @@ def run_async(func):
             "Install Flask with the 'async' extra in order to use async views."
         )
 
+    # Check that Werkzeug isn't using its fallback ContextVar class.
     if ContextVar.__module__ == "werkzeug.local":
         raise RuntimeError(
-            "async cannot be used with this combination of Python & Greenlet versions"
+            "Async cannot be used with this combination of Python & Greenlet versions."
         )
 
     @wraps(func)
@@ -763,9 +764,9 @@ def run_async(func):
         async def inner(*a, **k):
             """This restores the context before awaiting the func.
 
-            This is required as the func must be awaited within the
-            context. Simply calling func (as per the
-            copy_current_xxx_context functions) doesn't work as the
+            This is required as the function must be awaited within the
+            context. Only calling ``func`` (as per the
+            ``copy_current_xxx_context`` functions) doesn't work as the
             with block will close before the coroutine is awaited.
             """
             if ctx is not None: