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+
+.. _design:
+
+Design overview
+===============
+
+libuv is cross-platform support library which was originally written for `Node.js`_. It's designed
+around the event-driven asynchronous I/O model.
+
+.. _Node.js: https://nodejs.org
+
+The library provides much more than a simple abstraction over different I/O polling mechanisms:
+'handles' and 'streams' provide a high level abstraction for sockets and other entities;
+cross-platform file I/O and threading functionality is also provided, amongst other things.
+
+Here is a diagram illustrating the different parts that compose libuv and what subsystem they
+relate to:
+
+.. image:: static/architecture.png
+    :scale: 75%
+    :align: center
+
+
+Handles and requests
+^^^^^^^^^^^^^^^^^^^^
+
+libuv provides users with 2 abstractions to work with, in combination with the event loop:
+handles and requests.
+
+Handles represent long-lived objects capable of performing certain operations while active. Some examples:
+
+- A prepare handle gets its callback called once every loop iteration when active.
+- A TCP server handle that gets its connection callback called every time there is a new connection.
+
+Requests represent (typically) short-lived operations. These operations can be performed over a
+handle: write requests are used to write data on a handle; or standalone: getaddrinfo requests
+don't need a handle they run directly on the loop.
+
+
+The I/O loop
+^^^^^^^^^^^^
+
+The I/O (or event) loop is the central part of libuv. It establishes the content for all I/O
+operations, and it's meant to be tied to a single thread. One can run multiple event loops
+as long as each runs in a different thread. The libuv event loop (or any other API involving
+the loop or handles, for that matter) **is not thread-safe** except where stated otherwise.
+
+The event loop follows the rather usual single threaded asynchronous I/O approach: all (network)
+I/O is performed on non-blocking sockets which are polled using the best mechanism available
+on the given platform: epoll on Linux, kqueue on OSX and other BSDs, event ports on SunOS and IOCP
+on Windows. As part of a loop iteration the loop will block waiting for I/O activity on sockets
+which have been added to the poller and callbacks will be fired indicating socket conditions
+(readable, writable hangup) so handles can read, write or perform the desired I/O operation.
+
+In order to better understand how the event loop operates, the following diagram illustrates all
+stages of a loop iteration:
+
+.. image:: static/loop_iteration.png
+    :scale: 75%
+    :align: center
+
+
+#. The loop concept of 'now' is initially set.
+
+#. Due timers are run if the loop was run with ``UV_RUN_DEFAULT``. All active timers scheduled
+   for a time before the loop's concept of *now* get their callbacks called.
+
+#. If the loop is *alive*  an iteration is started, otherwise the loop will exit immediately. So,
+   when is a loop considered to be *alive*? If a loop has active and ref'd handles, active
+   requests or closing handles it's considered to be *alive*.
+
+#. Pending callbacks are called. All I/O callbacks are called right after polling for I/O, for the
+   most part. There are cases, however, in which calling such a callback is deferred for the next
+   loop iteration. If the previous iteration deferred any I/O callback it will be run at this point.
+
+#. Idle handle callbacks are called. Despite the unfortunate name, idle handles are run on every
+   loop iteration, if they are active.
+
+#. Prepare handle callbacks are called. Prepare handles get their callbacks called right before
+   the loop will block for I/O.
+
+#. Poll timeout is calculated. Before blocking for I/O the loop calculates for how long it should
+   block. These are the rules when calculating the timeout:
+
+        * If the loop was run with the ``UV_RUN_NOWAIT`` flag, the timeout is 0.
+        * If the loop is going to be stopped (:c:func:`uv_stop` was called), the timeout is 0.
+        * If there are no active handles or requests, the timeout is 0.
+        * If there are any idle handles active, the timeout is 0.
+        * If there are any handles pending to be closed, the timeout is 0.
+        * If none of the above cases matches, the timeout of the closest timer is taken, or
+          if there are no active timers, infinity.
+
+#. The loop blocks for I/O. At this point the loop will block for I/O for the duration calculated
+   in the previous step. All I/O related handles that were monitoring a given file descriptor
+   for a read or write operation get their callbacks called at this point.
+
+#. Check handle callbacks are called. Check handles get their callbacks called right after the
+   loop has blocked for I/O. Check handles are essentially the counterpart of prepare handles.
+
+#. Close callbacks are called. If a handle was closed by calling :c:func:`uv_close` it will
+   get the close callback called.
+
+#. The loop concept of 'now' is updated.
+
+#. Due timers are run. Note that 'now' is not updated again until the next loop iteration.
+   So if a timer became due while other timers were being processed, it won't be run until
+   the following event loop iteration.
+
+#. Iteration ends. If the loop was run with ``UV_RUN_NOWAIT`` or ``UV_RUN_ONCE`` modes the
+   iteration ends and :c:func:`uv_run` will return. If the loop was run with ``UV_RUN_DEFAULT``
+   it will continue from the start if it's still *alive*, otherwise it will also end.
+
+
+.. important::
+    libuv uses a thread pool to make asynchronous file I/O operations possible, but
+    network I/O is **always** performed in a single thread, each loop's thread.
+
+.. note::
+    While the polling mechanism is different, libuv makes the execution model consistent
+    across Unix systems and Windows.
+
+
+File I/O
+^^^^^^^^
+
+Unlike network I/O, there are no platform-specific file I/O primitives libuv could rely on,
+so the current approach is to run blocking file I/O operations in a thread pool.
+
+For a thorough explanation of the cross-platform file I/O landscape, check out
+`this post <https://blog.libtorrent.org/2012/10/asynchronous-disk-io/>`_.
+
+libuv currently uses a global thread pool on which all loops can queue work. 3 types of
+operations are currently run on this pool:
+
+    * File system operations
+    * DNS functions (getaddrinfo and getnameinfo)
+    * User specified code via :c:func:`uv_queue_work`
+
+.. warning::
+    See the :c:ref:`threadpool` section for more details, but keep in mind the thread pool size
+    is quite limited.