/Doc/library/asyncore.rst
ReStructuredText | 278 lines | 187 code | 91 blank | 0 comment | 0 complexity | 95651df0f8b9f3af175d8135b51a1364 MD5 | raw file
1 2:mod:`asyncore` --- Asynchronous socket handler 3=============================================== 4 5.. module:: asyncore 6 :synopsis: A base class for developing asynchronous socket handling 7 services. 8.. moduleauthor:: Sam Rushing <rushing@nightmare.com> 9.. sectionauthor:: Christopher Petrilli <petrilli@amber.org> 10.. sectionauthor:: Steve Holden <sholden@holdenweb.com> 11.. heavily adapted from original documentation by Sam Rushing 12 13 14This module provides the basic infrastructure for writing asynchronous socket 15service clients and servers. 16 17There are only two ways to have a program on a single processor do "more than 18one thing at a time." Multi-threaded programming is the simplest and most 19popular way to do it, but there is another very different technique, that lets 20you have nearly all the advantages of multi-threading, without actually using 21multiple threads. It's really only practical if your program is largely I/O 22bound. If your program is processor bound, then pre-emptive scheduled threads 23are probably what you really need. Network servers are rarely processor 24bound, however. 25 26If your operating system supports the :cfunc:`select` system call in its I/O 27library (and nearly all do), then you can use it to juggle multiple 28communication channels at once; doing other work while your I/O is taking 29place in the "background." Although this strategy can seem strange and 30complex, especially at first, it is in many ways easier to understand and 31control than multi-threaded programming. The :mod:`asyncore` module solves 32many of the difficult problems for you, making the task of building 33sophisticated high-performance network servers and clients a snap. For 34"conversational" applications and protocols the companion :mod:`asynchat` 35module is invaluable. 36 37The basic idea behind both modules is to create one or more network 38*channels*, instances of class :class:`asyncore.dispatcher` and 39:class:`asynchat.async_chat`. Creating the channels adds them to a global 40map, used by the :func:`loop` function if you do not provide it with your own 41*map*. 42 43Once the initial channel(s) is(are) created, calling the :func:`loop` function 44activates channel service, which continues until the last channel (including 45any that have been added to the map during asynchronous service) is closed. 46 47 48.. function:: loop([timeout[, use_poll[, map[,count]]]]) 49 50 Enter a polling loop that terminates after count passes or all open 51 channels have been closed. All arguments are optional. The *count* 52 parameter defaults to None, resulting in the loop terminating only when all 53 channels have been closed. The *timeout* argument sets the timeout 54 parameter for the appropriate :func:`select` or :func:`poll` call, measured 55 in seconds; the default is 30 seconds. The *use_poll* parameter, if true, 56 indicates that :func:`poll` should be used in preference to :func:`select` 57 (the default is ``False``). 58 59 The *map* parameter is a dictionary whose items are the channels to watch. 60 As channels are closed they are deleted from their map. If *map* is 61 omitted, a global map is used. Channels (instances of 62 :class:`asyncore.dispatcher`, :class:`asynchat.async_chat` and subclasses 63 thereof) can freely be mixed in the map. 64 65 66.. class:: dispatcher() 67 68 The :class:`dispatcher` class is a thin wrapper around a low-level socket 69 object. To make it more useful, it has a few methods for event-handling 70 which are called from the asynchronous loop. Otherwise, it can be treated 71 as a normal non-blocking socket object. 72 73 The firing of low-level events at certain times or in certain connection 74 states tells the asynchronous loop that certain higher-level events have 75 taken place. For example, if we have asked for a socket to connect to 76 another host, we know that the connection has been made when the socket 77 becomes writable for the first time (at this point you know that you may 78 write to it with the expectation of success). The implied higher-level 79 events are: 80 81 +----------------------+----------------------------------------+ 82 | Event | Description | 83 +======================+========================================+ 84 | ``handle_connect()`` | Implied by the first read or write | 85 | | event | 86 +----------------------+----------------------------------------+ 87 | ``handle_close()`` | Implied by a read event with no data | 88 | | available | 89 +----------------------+----------------------------------------+ 90 | ``handle_accept()`` | Implied by a read event on a listening | 91 | | socket | 92 +----------------------+----------------------------------------+ 93 94 During asynchronous processing, each mapped channel's :meth:`readable` and 95 :meth:`writable` methods are used to determine whether the channel's socket 96 should be added to the list of channels :cfunc:`select`\ ed or 97 :cfunc:`poll`\ ed for read and write events. 98 99 Thus, the set of channel events is larger than the basic socket events. The 100 full set of methods that can be overridden in your subclass follows: 101 102 103 .. method:: handle_read() 104 105 Called when the asynchronous loop detects that a :meth:`read` call on the 106 channel's socket will succeed. 107 108 109 .. method:: handle_write() 110 111 Called when the asynchronous loop detects that a writable socket can be 112 written. Often this method will implement the necessary buffering for 113 performance. For example:: 114 115 def handle_write(self): 116 sent = self.send(self.buffer) 117 self.buffer = self.buffer[sent:] 118 119 120 .. method:: handle_expt() 121 122 Called when there is out of band (OOB) data for a socket connection. This 123 will almost never happen, as OOB is tenuously supported and rarely used. 124 125 126 .. method:: handle_connect() 127 128 Called when the active opener's socket actually makes a connection. Might 129 send a "welcome" banner, or initiate a protocol negotiation with the 130 remote endpoint, for example. 131 132 133 .. method:: handle_close() 134 135 Called when the socket is closed. 136 137 138 .. method:: handle_error() 139 140 Called when an exception is raised and not otherwise handled. The default 141 version prints a condensed traceback. 142 143 144 .. method:: handle_accept() 145 146 Called on listening channels (passive openers) when a connection can be 147 established with a new remote endpoint that has issued a :meth:`connect` 148 call for the local endpoint. 149 150 151 .. method:: readable() 152 153 Called each time around the asynchronous loop to determine whether a 154 channel's socket should be added to the list on which read events can 155 occur. The default method simply returns ``True``, indicating that by 156 default, all channels will be interested in read events. 157 158 159 .. method:: writable() 160 161 Called each time around the asynchronous loop to determine whether a 162 channel's socket should be added to the list on which write events can 163 occur. The default method simply returns ``True``, indicating that by 164 default, all channels will be interested in write events. 165 166 167 In addition, each channel delegates or extends many of the socket methods. 168 Most of these are nearly identical to their socket partners. 169 170 171 .. method:: create_socket(family, type) 172 173 This is identical to the creation of a normal socket, and will use the 174 same options for creation. Refer to the :mod:`socket` documentation for 175 information on creating sockets. 176 177 178 .. method:: connect(address) 179 180 As with the normal socket object, *address* is a tuple with the first 181 element the host to connect to, and the second the port number. 182 183 184 .. method:: send(data) 185 186 Send *data* to the remote end-point of the socket. 187 188 189 .. method:: recv(buffer_size) 190 191 Read at most *buffer_size* bytes from the socket's remote end-point. An 192 empty string implies that the channel has been closed from the other end. 193 194 195 .. method:: listen(backlog) 196 197 Listen for connections made to the socket. The *backlog* argument 198 specifies the maximum number of queued connections and should be at least 199 1; the maximum value is system-dependent (usually 5). 200 201 202 .. method:: bind(address) 203 204 Bind the socket to *address*. The socket must not already be bound. (The 205 format of *address* depends on the address family --- see above.) To mark 206 the socket as re-usable (setting the :const:`SO_REUSEADDR` option), call 207 the :class:`dispatcher` object's :meth:`set_reuse_addr` method. 208 209 210 .. method:: accept() 211 212 Accept a connection. The socket must be bound to an address and listening 213 for connections. The return value is a pair ``(conn, address)`` where 214 *conn* is a *new* socket object usable to send and receive data on the 215 connection, and *address* is the address bound to the socket on the other 216 end of the connection. 217 218 219 .. method:: close() 220 221 Close the socket. All future operations on the socket object will fail. 222 The remote end-point will receive no more data (after queued data is 223 flushed). Sockets are automatically closed when they are 224 garbage-collected. 225 226.. class:: file_dispatcher() 227 228 A file_dispatcher takes a file descriptor or file object along with an 229 optional map argument and wraps it for use with the :cfunc:`poll` or 230 :cfunc:`loop` functions. If provided a file object or anything with a 231 :cfunc:`fileno` method, that method will be called and passed to the 232 :class:`file_wrapper` constructor. Availability: UNIX. 233 234.. class:: file_wrapper() 235 236 A file_wrapper takes an integer file descriptor and calls :func:`os.dup` to 237 duplicate the handle so that the original handle may be closed independently 238 of the file_wrapper. This class implements sufficient methods to emulate a 239 socket for use by the :class:`file_dispatcher` class. Availability: UNIX. 240 241 242.. _asyncore-example: 243 244asyncore Example basic HTTP client 245---------------------------------- 246 247Here is a very basic HTTP client that uses the :class:`dispatcher` class to 248implement its socket handling:: 249 250 import asyncore, socket 251 252 class http_client(asyncore.dispatcher): 253 254 def __init__(self, host, path): 255 asyncore.dispatcher.__init__(self) 256 self.create_socket(socket.AF_INET, socket.SOCK_STREAM) 257 self.connect( (host, 80) ) 258 self.buffer = 'GET %s HTTP/1.0\r\n\r\n' % path 259 260 def handle_connect(self): 261 pass 262 263 def handle_close(self): 264 self.close() 265 266 def handle_read(self): 267 print self.recv(8192) 268 269 def writable(self): 270 return (len(self.buffer) > 0) 271 272 def handle_write(self): 273 sent = self.send(self.buffer) 274 self.buffer = self.buffer[sent:] 275 276 c = http_client('www.python.org', '/') 277 278 asyncore.loop()