17.6. asyncore — 异步socket处理器

源码： Lib/asyncore.py

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该模块提供用于编写异步套接字服务客户端与服务端的基础构件。

只有两种方法让单个处理器上的程序“同一时间完成不止一件事”。 多线程编程是最简单和最流行的方法，但是还有另一种非常不同的技术，它可以让你拥有多线程的几乎所有优点，而无需实际使用多线程。 它仅仅在你的程序主要受 I/O 限制时有用，那么。 如果你的程序受处理器限制，那么先发制人的预定线程可能就是你真正需要的。 但是，网络服务器很少受处理器限制。

如果你的操作系统在其 I/O 库中支持 select() 系统调用（几乎所有操作系统），那么你可以使用它来同时处理多个通信通道；在 I/O 正在“后台”时进行其他工作。 虽然这种策略看起来很奇怪和复杂，特别是起初，它在很多方面比多线程编程更容易理解和控制。 asyncore 模块为您解决了许多难题，使得构建复杂的高性能网络服务器和客户端的任务变得轻而易举。 对于“会话”应用程序和协议，伴侣 asynchat 模块是非常宝贵的。

这两个模块背后的基本思想是创建一个或多个网络 通道 ，类的实例 asyncore.dispatcher 和 asynchat.async_chat 。 创建通道会将它们添加到全局映射中，如果你不为它提供自己的 映射 ，则由 loop() 函数使用。

一旦创建了初始通道，调用 loop() 函数将激活通道服务，该服务将一直持续到最后一个通道（包括在异步服务期间已添加到映射中的任何通道）关闭。

asyncore.loop([timeout[, use_poll[, map[, count]]]])

进入一个轮询循环，其在循环计数超出或所有打开的通道关闭后终止。 所有参数都是可选的。 count 形参默认为 None ，导致循环仅在所有通道关闭时终止。 timeout 形参为适当的 select() 或 poll() 调用设置超时参数，以秒为单位; 默认值为30秒。 use_poll 形参，如果为 True ，则表示 poll() 应优先使用 select`（默认为``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.

class asyncore.dispatcher

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:

Event	描述
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 select()ed or 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:

handle_read()

Called when the asynchronous loop detects that a read() call on the channel’s socket will succeed.

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:]

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.

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.

handle_close()

Called when the socket is closed.

handle_error()

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

handle_accept()

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

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.

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.

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 socket documentation for information on creating sockets.

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.

send(data)

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

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.

Note that recv() may raise socket.error with EAGAIN or EWOULDBLOCK, even though select.select() or select.poll() has reported the socket ready for reading.

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).

bind(address)

Bind the socket to address. The socket must not already be bound. (The format of address depends on the address family — refer to the socket documentation for more information.) To mark the socket as re-usable (setting the SO_REUSEADDR option), call the dispatcher object’s set_reuse_addr() method.

accept()

Accept a connection. The socket must be bound to an address and listening for connections. The return value can be either None or 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. When None is returned it means the connection didn’t take place, in which case the server should just ignore this event and keep listening for further incoming connections.

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.

class asyncore.dispatcher_with_send

A dispatcher subclass which adds simple buffered output capability, useful for simple clients. For more sophisticated usage use asynchat.async_chat.

class asyncore.file_dispatcher

A file_dispatcher takes a file descriptor or file object along with an optional map argument and wraps it for use with the poll() or loop() functions. If provided a file object or anything with a fileno() method, that method will be called and passed to the file_wrapper constructor. Availability: UNIX.

class asyncore.file_wrapper

A file_wrapper takes an integer file descriptor and calls os.dup() to duplicate the handle so that the original handle may be closed independently of the file_wrapper. This class implements sufficient methods to emulate a socket for use by the file_dispatcher class. Availability: UNIX.

17.6.1. asyncore Example basic HTTP client

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

import asyncore, socket

class HTTPClient(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:]


client = HTTPClient('www.python.org', '/')
asyncore.loop()

17.6.2. asyncore Example basic echo server

Here is a basic echo server that uses the dispatcher class to accept connections and dispatches the incoming connections to a handler:

import asyncore
import socket

class EchoHandler(asyncore.dispatcher_with_send):

    def handle_read(self):
        data = self.recv(8192)
        if data:
            self.send(data)

class EchoServer(asyncore.dispatcher):

    def __init__(self, host, port):
        asyncore.dispatcher.__init__(self)
        self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
        self.set_reuse_addr()
        self.bind((host, port))
        self.listen(5)

    def handle_accept(self):
        pair = self.accept()
        if pair is not None:
            sock, addr = pair
            print 'Incoming connection from %s' % repr(addr)
            handler = EchoHandler(sock)

server = EchoServer('localhost', 8080)
asyncore.loop()