New page: Why using asyncio?

Author: vstinner

index.rst

@@ -8,6 +8,7 @@ Chapter 1: First steps with asyncio
 .. toctree::
    :maxdepth: 2
 
+   why_asyncio.rst
    getting_started.rst
    hello_world.rst
    hello_clock.rst

why_asyncio.rst

@@ -0,0 +1,38 @@
+++++++++++++++++++
+Why using asyncio?
+++++++++++++++++++
+
+Why asynchronous programming?
+=============================
+
+asyncio is a library to write asynchronous applications. It is the most
+efficient way to implement a network server having to handle many concurrent
+users.
+
+
+But gevent and eventlet just work!
+==================================
+
+Or *Why should I bother with all these extra annoying async and await
+keywords?*.
+
+In short, asyncio adopted a radically different solution for race conditions.
+
+Parallel computing using threads is hard because of race conditions. Gevent and
+eventlet have a similar issue using "green" (lightweight) threads.
+
+Code written with asyncio is less error-prone: by just looking at the code, it
+is possible to identify which parts of the code are under our controls and
+where the event loop takes over the control flow and is able to run other tasks
+when our task is waiting for something.
+
+gevent and eventlet are designed to hide the asynchronous programming. For
+non-expert, and sometimes even for experts, it is really hard to guess where
+the event loop is allowed to suspend the task and run other tasks in
+background. It is even worse. A modification in a third party library can
+change the behaviour of our code, introduce a new point where the task is
+suspended.
+
+For an example, see the "Ca(sh|che Coherent) Money" section of the `Unyielding
+<https://glyph.twistedmatrix.com/2014/02/unyielding.html>` article (by Glyph,
+February, 2014).