.. _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 updated. The event loop caches the current time at the start of the event loop tick in order to reduce the number of time-related system calls. #. 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*. #. Due timers are run. All active timers scheduled for a time before the loop's concept of *now* get their callbacks called. #. 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. #. Special case in case the loop was run with ``UV_RUN_ONCE``, as it implies forward progress. It's possible that no I/O callbacks were fired after blocking for I/O, but some time has passed so there might be timers which are due, those timers get their callbacks called. #. 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, checkout `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.