279 lines
11 KiB
ReStructuredText
279 lines
11 KiB
ReStructuredText
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:mod:`asyncore` --- Asynchronous socket handler
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===============================================
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.. module:: asyncore
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:synopsis: A base class for developing asynchronous socket handling
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services.
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.. moduleauthor:: Sam Rushing <rushing@nightmare.com>
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.. sectionauthor:: Christopher Petrilli <petrilli@amber.org>
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.. sectionauthor:: Steve Holden <sholden@holdenweb.com>
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.. heavily adapted from original documentation by Sam Rushing
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This module provides the basic infrastructure for writing asynchronous socket
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service clients and servers.
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There are only two ways to have a program on a single processor do "more than
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one thing at a time." Multi-threaded programming is the simplest and most
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popular way to do it, but there is another very different technique, that lets
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you have nearly all the advantages of multi-threading, without actually using
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multiple threads. It's really only practical if your program is largely I/O
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bound. If your program is processor bound, then pre-emptive scheduled threads
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are probably what you really need. Network servers are rarely processor
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bound, however.
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If your operating system supports the :cfunc:`select` system call in its I/O
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library (and nearly all do), then you can use it to juggle multiple
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communication channels at once; doing other work while your I/O is taking
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place in the "background." Although this strategy can seem strange and
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complex, especially at first, it is in many ways easier to understand and
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control than multi-threaded programming. The :mod:`asyncore` module solves
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many of the difficult problems for you, making the task of building
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sophisticated high-performance network servers and clients a snap. For
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"conversational" applications and protocols the companion :mod:`asynchat`
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module is invaluable.
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The basic idea behind both modules is to create one or more network
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*channels*, instances of class :class:`asyncore.dispatcher` and
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:class:`asynchat.async_chat`. Creating the channels adds them to a global
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map, used by the :func:`loop` function if you do not provide it with your own
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*map*.
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Once the initial channel(s) is(are) created, calling the :func:`loop` function
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activates channel service, which continues until the last channel (including
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any that have been added to the map during asynchronous service) is closed.
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.. function:: loop([timeout[, use_poll[, map[,count]]]])
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Enter a polling loop that terminates after count passes or all open
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channels have been closed. All arguments are optional. The *count*
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parameter defaults to None, resulting in the loop terminating only when all
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channels have been closed. The *timeout* argument sets the timeout
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parameter for the appropriate :func:`select` or :func:`poll` call, measured
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in seconds; the default is 30 seconds. The *use_poll* parameter, if true,
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indicates that :func:`poll` should be used in preference to :func:`select`
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(the default is ``False``).
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The *map* parameter is a dictionary whose items are the channels to watch.
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As channels are closed they are deleted from their map. If *map* is
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omitted, a global map is used. Channels (instances of
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:class:`asyncore.dispatcher`, :class:`asynchat.async_chat` and subclasses
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thereof) can freely be mixed in the map.
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.. class:: dispatcher()
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The :class:`dispatcher` class is a thin wrapper around a low-level socket
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object. To make it more useful, it has a few methods for event-handling
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which are called from the asynchronous loop. Otherwise, it can be treated
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as a normal non-blocking socket object.
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The firing of low-level events at certain times or in certain connection
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states tells the asynchronous loop that certain higher-level events have
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taken place. For example, if we have asked for a socket to connect to
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another host, we know that the connection has been made when the socket
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becomes writable for the first time (at this point you know that you may
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write to it with the expectation of success). The implied higher-level
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events are:
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+----------------------+----------------------------------------+
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| Event | Description |
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+======================+========================================+
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| ``handle_connect()`` | Implied by the first read or write |
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| | event |
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+----------------------+----------------------------------------+
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| ``handle_close()`` | Implied by a read event with no data |
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| | available |
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+----------------------+----------------------------------------+
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| ``handle_accept()`` | Implied by a read event on a listening |
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| | socket |
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+----------------------+----------------------------------------+
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During asynchronous processing, each mapped channel's :meth:`readable` and
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:meth:`writable` methods are used to determine whether the channel's socket
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should be added to the list of channels :cfunc:`select`\ ed or
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:cfunc:`poll`\ ed for read and write events.
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Thus, the set of channel events is larger than the basic socket events. The
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full set of methods that can be overridden in your subclass follows:
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.. method:: handle_read()
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Called when the asynchronous loop detects that a :meth:`read` call on the
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channel's socket will succeed.
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.. method:: handle_write()
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Called when the asynchronous loop detects that a writable socket can be
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written. Often this method will implement the necessary buffering for
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performance. For example::
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def handle_write(self):
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sent = self.send(self.buffer)
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self.buffer = self.buffer[sent:]
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.. method:: handle_expt()
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Called when there is out of band (OOB) data for a socket connection. This
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will almost never happen, as OOB is tenuously supported and rarely used.
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.. method:: handle_connect()
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Called when the active opener's socket actually makes a connection. Might
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send a "welcome" banner, or initiate a protocol negotiation with the
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remote endpoint, for example.
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.. method:: handle_close()
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Called when the socket is closed.
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.. method:: handle_error()
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Called when an exception is raised and not otherwise handled. The default
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version prints a condensed traceback.
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.. method:: handle_accept()
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Called on listening channels (passive openers) when a connection can be
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established with a new remote endpoint that has issued a :meth:`connect`
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call for the local endpoint.
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.. method:: readable()
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Called each time around the asynchronous loop to determine whether a
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channel's socket should be added to the list on which read events can
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occur. The default method simply returns ``True``, indicating that by
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default, all channels will be interested in read events.
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.. method:: writable()
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Called each time around the asynchronous loop to determine whether a
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channel's socket should be added to the list on which write events can
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occur. The default method simply returns ``True``, indicating that by
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default, all channels will be interested in write events.
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In addition, each channel delegates or extends many of the socket methods.
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Most of these are nearly identical to their socket partners.
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.. method:: create_socket(family, type)
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This is identical to the creation of a normal socket, and will use the
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same options for creation. Refer to the :mod:`socket` documentation for
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information on creating sockets.
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.. method:: connect(address)
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As with the normal socket object, *address* is a tuple with the first
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element the host to connect to, and the second the port number.
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.. method:: send(data)
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Send *data* to the remote end-point of the socket.
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.. method:: recv(buffer_size)
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Read at most *buffer_size* bytes from the socket's remote end-point. An
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empty string implies that the channel has been closed from the other end.
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.. method:: listen(backlog)
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Listen for connections made to the socket. The *backlog* argument
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specifies the maximum number of queued connections and should be at least
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1; the maximum value is system-dependent (usually 5).
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.. method:: bind(address)
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Bind the socket to *address*. The socket must not already be bound. (The
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format of *address* depends on the address family --- see above.) To mark
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the socket as re-usable (setting the :const:`SO_REUSEADDR` option), call
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the :class:`dispatcher` object's :meth:`set_reuse_addr` method.
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.. method:: accept()
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Accept a connection. The socket must be bound to an address and listening
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for connections. The return value is a pair ``(conn, address)`` where
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*conn* is a *new* socket object usable to send and receive data on the
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connection, and *address* is the address bound to the socket on the other
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end of the connection.
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.. method:: close()
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Close the socket. All future operations on the socket object will fail.
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The remote end-point will receive no more data (after queued data is
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flushed). Sockets are automatically closed when they are
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garbage-collected.
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.. class:: file_dispatcher()
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A file_dispatcher takes a file descriptor or file object along with an
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optional map argument and wraps it for use with the :cfunc:`poll` or
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:cfunc:`loop` functions. If provided a file object or anything with a
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:cfunc:`fileno` method, that method will be called and passed to the
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:class:`file_wrapper` constructor. Availability: UNIX.
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.. class:: file_wrapper()
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A file_wrapper takes an integer file descriptor and calls :func:`os.dup` to
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duplicate the handle so that the original handle may be closed independently
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of the file_wrapper. This class implements sufficient methods to emulate a
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socket for use by the :class:`file_dispatcher` class. Availability: UNIX.
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.. _asyncore-example:
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asyncore Example basic HTTP client
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----------------------------------
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Here is a very basic HTTP client that uses the :class:`dispatcher` class to
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implement its socket handling::
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import asyncore, socket
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class http_client(asyncore.dispatcher):
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def __init__(self, host, path):
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asyncore.dispatcher.__init__(self)
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self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
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self.connect( (host, 80) )
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self.buffer = 'GET %s HTTP/1.0\r\n\r\n' % path
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def handle_connect(self):
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pass
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def handle_close(self):
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self.close()
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def handle_read(self):
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print self.recv(8192)
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def writable(self):
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return (len(self.buffer) > 0)
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def handle_write(self):
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sent = self.send(self.buffer)
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self.buffer = self.buffer[sent:]
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c = http_client('www.python.org', '/')
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asyncore.loop()
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