This module provides the basic infrastructure for writing asynchronous socket service clients and servers.
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 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 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 asynchat module is invaluable.
The basic idea behind both modules is to create one or more network channels, instances of class asyncore.dispatcher and asynchat.async_chat. Creating the channels adds them to a global map, used by the loop() function if you do not provide it with your own map.
Once the initial channel(s) is(are) created, calling the 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.
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 select() or poll() call, measured in seconds; the default is 30 seconds. The use_poll parameter, if true, indicates that poll() should be used in preference to 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 asyncore.dispatcher, asynchat.async_chat and subclasses thereof) can freely be mixed in the map.
The 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.
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:
|handle_connect()||Implied by the first read or 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 readable() and writable() methods are used to determine whether the channel’s socket should be added to the list of channels selected or polled 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:
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:]
In addition, each channel delegates or extends many of the socket methods. Most of these are nearly identical to their socket partners.
Here is a very basic HTTP client that uses the 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()