cpython/Doc/library/asyncore.rst

:mod:`asyncore` --- Asynchronous socket handler
===============================================

.. module:: asyncore
   :synopsis: A base class for developing asynchronous socket  handling services.
.. moduleauthor:: Sam Rushing <rushing@nightmare.com>
.. sectionauthor:: Christopher Petrilli <petrilli@amber.org>
.. sectionauthor:: Steve Holden <sholden@holdenweb.com>


This module provides the basic infrastructure for writing asynchronous  socket
service clients and servers.

.. % Heavily adapted from original documentation by Sam Rushing.

There are only two ways to have a program on a single processor do  "more than
one thing at a time." Multi-threaded programming is the  simplest and most
popular way to do it, but there is another very  different technique, that lets
you have nearly all the advantages of  multi-threading, without actually using
multiple threads.  It's really  only practical if your program is largely I/O
bound.  If your program  is processor bound, then pre-emptive scheduled threads
are probably what  you really need. Network servers are rarely processor bound,
however.

If your operating system supports the :cfunc:`select` system call in its I/O
library (and nearly all do), then you can use it to juggle multiple
communication channels at once; doing other work while your I/O is taking place
in the "background."  Although this strategy can seem strange and complex,
especially at first, it is in many ways easier to understand and control than
multi-threaded programming.  The :mod:`asyncore` module solves many of the
difficult problems for you, making the task of building sophisticated
high-performance network servers and clients a snap. For "conversational"
applications and protocols the companion :mod:`asynchat` module is invaluable.

The basic idea behind both modules is to create one or more network *channels*,
instances of class :class:`asyncore.dispatcher` and
:class:`asynchat.async_chat`. Creating the channels adds them to a global map,
used by the :func:`loop` function if you do not provide it with your own *map*.

Once the initial channel(s) is(are) created, calling the :func:`loop` function
activates channel service, which continues until the last channel (including any
that have been added to the map during asynchronous service) is closed.


.. function:: loop([timeout[, use_poll[, map[,count]]]])

   Enter a polling loop that terminates after count passes or all open channels
   have been closed.  All arguments are optional.  The *count* parameter defaults
   to None, resulting in the loop terminating only when all channels have been
   closed.  The *timeout* argument sets the timeout parameter for the appropriate
   :func:`select` or :func:`poll` call, measured in seconds; the default is 30
   seconds. The *use_poll* parameter, if true, indicates that :func:`poll` should
   be used in preference to :func:`select` (the default is ``False``).

   The *map* parameter is a dictionary whose items are the channels to watch.  As
   channels are closed they are deleted from their map.  If *map* is omitted, a
   global map is used. Channels (instances of :class:`asyncore.dispatcher`,
   :class:`asynchat.async_chat` and subclasses thereof) can freely be mixed in the
   map.


.. class:: dispatcher()

   The :class:`dispatcher` class is a thin wrapper around a low-level socket
   object. To make it more useful, it has a few methods for event-handling  which
   are called from the asynchronous loop.   Otherwise, it can be treated as a
   normal non-blocking socket object.

   Two class attributes can be modified, to improve performance, or possibly even
   to conserve memory.


   .. data:: ac_in_buffer_size

      The asynchronous input buffer size (default ``4096``).


   .. data:: ac_out_buffer_size

      The asynchronous output buffer size (default ``4096``).

   The firing of low-level events at certain times or in certain connection states
   tells the asynchronous loop that certain higher-level events have taken place.
   For example, if we have asked for a socket to connect to another host, we know
   that the connection has been made when the socket becomes writable for the first
   time (at this point you know that you may write to it with the expectation of
   success). The implied higher-level events are:

   +----------------------+----------------------------------------+
   | Event                | Description                            |
   +======================+========================================+
   | ``handle_connect()`` | Implied by the first write event       |
   +----------------------+----------------------------------------+
   | ``handle_close()``   | Implied by a read event with no data   |
   |                      | available                              |
   +----------------------+----------------------------------------+
   | ``handle_accept()``  | Implied by a read event on a listening |
   |                      | socket                                 |
   +----------------------+----------------------------------------+

   During asynchronous processing, each mapped channel's :meth:`readable` and
   :meth:`writable` methods are used to determine whether the channel's socket
   should be added to the list of channels :cfunc:`select`\ ed or :cfunc:`poll`\ ed
   for read and write events.

Thus, the set of channel events is larger than the basic socket events. The full
set of methods that can be overridden in your subclass follows:


.. method:: dispatcher.handle_read()

   Called when the asynchronous loop detects that a :meth:`read` call on the
   channel's socket will succeed.


.. method:: dispatcher.handle_write()

   Called when the asynchronous loop detects that a writable socket can be written.
   Often this method will implement the necessary buffering for  performance.  For
   example::

      def handle_write(self):
          sent = self.send(self.buffer)
          self.buffer = self.buffer[sent:]


.. method:: dispatcher.handle_expt()

   Called when there is out of band (OOB) data for a socket  connection.  This will
   almost never happen, as OOB is  tenuously supported and rarely used.


.. method:: dispatcher.handle_connect()

   Called when the active opener's socket actually makes a connection. Might send a
   "welcome" banner, or initiate a protocol negotiation with the remote endpoint,
   for example.


.. method:: dispatcher.handle_close()

   Called when the socket is closed.


.. method:: dispatcher.handle_error()

   Called when an exception is raised and not otherwise handled.  The default
   version prints a condensed traceback.


.. method:: dispatcher.handle_accept()

   Called on listening channels (passive openers) when a   connection can be
   established with a new remote endpoint that has issued a :meth:`connect` call
   for the local endpoint.


.. method:: dispatcher.readable()

   Called each time around the asynchronous loop to determine whether a channel's
   socket should be added to the list on which read events can occur.  The default
   method simply returns ``True``,  indicating that by default, all channels will
   be interested in read events.


.. method:: dispatcher.writable()

   Called each time around the asynchronous loop to determine whether a channel's
   socket should be added to the list on which write events can occur.  The default
   method simply returns ``True``,  indicating that by default, all channels will
   be interested in write events.

In addition, each channel delegates or extends many of the socket methods. Most
of these are nearly identical to their socket partners.


.. method:: dispatcher.create_socket(family, type)

   This is identical to the creation of a normal socket, and  will use the same
   options for creation.  Refer to the :mod:`socket` documentation for information
   on creating sockets.


.. method:: dispatcher.connect(address)

   As with the normal socket object, *address* is a  tuple with the first element
   the host to connect to, and the  second the port number.


.. method:: dispatcher.send(data)

   Send *data* to the remote end-point of the socket.


.. method:: dispatcher.recv(buffer_size)

   Read at most *buffer_size* bytes from the socket's remote end-point. An empty
   string implies that the channel has been closed from the other end.


.. method:: dispatcher.listen(backlog)

   Listen for connections made to the socket.  The *backlog* argument specifies the
   maximum number of queued connections and should be at least 1; the maximum value
   is system-dependent (usually 5).


.. method:: dispatcher.bind(address)

   Bind the socket to *address*.  The socket must not already be bound.  (The
   format of *address* depends on the address family --- see above.)  To mark the
   socket as re-usable (setting the :const:`SO_REUSEADDR` option), call the
   :class:`dispatcher` object's :meth:`set_reuse_addr` method.


.. method:: dispatcher.accept()

   Accept a connection.  The socket must be bound to an address and listening for
   connections.  The return value is a pair ``(conn, address)`` where *conn* is a
   *new* socket object usable to send and receive data on the connection, and
   *address* is the address bound to the socket on the other end of the connection.


.. method:: dispatcher.close()

   Close the socket.  All future operations on the socket object will fail.  The
   remote end-point will receive no more data (after queued data is flushed).
   Sockets are automatically closed when they are garbage-collected.


.. _asyncore-example:

asyncore Example basic HTTP client
----------------------------------

Here is a very basic HTTP client that uses the :class:`dispatcher` class to
implement its socket handling::

   import asyncore, socket

   class http_client(asyncore.dispatcher):

       def __init__(self, host, path):
           asyncore.dispatcher.__init__(self)
           self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
           self.connect( (host, 80) )
           self.buffer = 'GET %s HTTP/1.0\r\n\r\n' % path

       def handle_connect(self):
           pass

       def handle_close(self):
           self.close()

       def handle_read(self):
           print self.recv(8192)

       def writable(self):
           return (len(self.buffer) > 0)

       def handle_write(self):
           sent = self.send(self.buffer)
           self.buffer = self.buffer[sent:]

   c = http_client('www.python.org', '/')

   asyncore.loop()