python Asyncore.dispatcher 理解

1、Asyncore是python的标准库。Asyncore.dispatcher 是这个库中的一个socket的框架，为socket添加了一些通用的回调方法，比如：

def listen(self, num):

    def bind(self, addr):

    def connect(self, address):

    def accept(self):

    def send(self, data):

    def recv(self, buffer_size):

    def close(self):

  def handle_read(self):
        self.log_info('unhandled read event', 'warning')

    def handle_write(self):
        self.log_info('unhandled write event', 'warning')

    def handle_connect(self):
        self.log_info('unhandled connect event', 'warning')

    def handle_accept(self):
        self.log_info('unhandled accept event', 'warning')

    def handle_close(self):
        self.log_info('unhandled close event', 'warning')
        self.close()

2、Asyncore.dispatcher 对socket进行了一次封装之后，这个socket就可以添加到一个全局的字典中，Asyncore模块中的loop函数会对这些socket文件描述符进行监视，具体可能是select或者poll方法实现，windows上面是poll实现的。通过I/O多路复用实现同时处理多个请求，避免使用多线程或者多进程的方式。下面中loop中传入的map就是socket-dispatcher的对象集合，当发现对应的socket有可读可写的请求时，调用对应的handle_read 和handle_write方法。

def poll(timeout=0.0, map=None):
    if map is None:
        map = socket_map
    if map:
        r = []; w = []; e = []
        for fd, obj in map.items():
            is_r = obj.readable()
            is_w = obj.writable()
            if is_r:
                r.append(fd)
            # accepting sockets should not be writable
            if is_w and not obj.accepting:
                w.append(fd)
            if is_r or is_w:
                e.append(fd)
        if [] == r == w == e:
            time.sleep(timeout)
            return

        try:
            r, w, e = select.select(r, w, e, timeout)
        except select.error, err:
            if err.args[0] != EINTR:
                raise
            else:
                return

        for fd in r:
            obj = map.get(fd)
            if obj is None:
                continue
            read(obj)

        for fd in w:
            obj = map.get(fd)
            if obj is None:
                continue
            write(obj)

        for fd in e:
            obj = map.get(fd)
            if obj is None:
                continue
            _exception(obj)

def poll2(timeout=0.0, map=None):
    # Use the poll() support added to the select module in Python 2.0
    if map is None:
        map = socket_map
    if timeout is not None:
        # timeout is in milliseconds
        timeout = int(timeout*1000)
    pollster = select.poll()
    if map:
        for fd, obj in map.items():
            flags = 0
            if obj.readable():
                flags |= select.POLLIN | select.POLLPRI
            # accepting sockets should not be writable
            if obj.writable() and not obj.accepting:
                flags |= select.POLLOUT
            if flags:
                # Only check for exceptions if object was either readable
                # or writable.
                flags |= select.POLLERR | select.POLLHUP | select.POLLNVAL
                pollster.register(fd, flags)
        try:
            r = pollster.poll(timeout)
        except select.error, err:
            if err.args[0] != EINTR:
                raise
            r = []
        for fd, flags in r:
            obj = map.get(fd)
            if obj is None:
                continue
            readwrite(obj, flags)

poll3 = poll2                           # Alias for backward compatibility

def loop(timeout=30.0, use_poll=False, map=None, count=None):
    if map is None:
        map = socket_map

    if use_poll and hasattr(select, 'poll'):
        poll_fun = poll2
    else:
        poll_fun = poll

    if count is None:
        while map:
            poll_fun(timeout, map)

    else:
        while map and count > 0:
            poll_fun(timeout, map)
            count = count – 1

3、如何实现一个异步的服务器，官方给出了一个比较好的代码：

import logging
import asyncore
import socket

logging.basicConfig(level=logging.DEBUG, format="%(created)-15s %(msecs)d %(levelname)8s %(thread)d %(name)s %(message)s")
log                     = logging.getLogger(__name__)

BACKLOG                 = 5
SIZE                    = 1024

class EchoHandler(asyncore.dispatcher):

    def __init__(self, conn_sock, client_address, server):
        self.server             = server
        self.client_address     = client_address
        self.buffer             = ""

        # We dont have anything to write, to start with
        self.is_writable        = False

        # Create ourselves, but with an already provided socket
        asyncore.dispatcher.__init__(self, conn_sock)
        log.debug("created handler; waiting for loop")

    def readable(self):
        return True     # We are always happy to read

    def writable(self):
        return self.is_writable # But we might not have
                                # anything to send all the time

    def handle_read(self):
        log.debug("handle_read")
        data = self.recv(SIZE)
        log.debug("after recv")
        if data:
            log.debug("got data")
            self.buffer += data
            self.is_writable = True  # sth to send back now
        else:
            log.debug("got null data")

    def handle_write(self):
        log.debug("handle_write")
        if self.buffer:
            sent = self.send(self.buffer)
            log.debug("sent data")
            self.buffer = self.buffer[sent:]
        else:
            log.debug("nothing to send")
        if len(self.buffer) == 0:
            self.is_writable = False

    # Will this ever get called?  Does loop() call
    # handle_close() if we called close, to start with?
    def handle_close(self):
        log.debug("handle_close")
        log.info("conn_closed: client_address=%s:%s" % \
                     (self.client_address[0],
                      self.client_address[1]))
        self.close()
        #pass

class EchoServer(asyncore.dispatcher):

    allow_reuse_address         = False
    request_queue_size          = 5
    address_family              = socket.AF_INET
    socket_type                 = socket.SOCK_STREAM

    def __init__(self, address, handlerClass=EchoHandler):
        self.address            = address
        self.handlerClass       = handlerClass

        asyncore.dispatcher.__init__(self)
        self.create_socket(self.address_family,
                               self.socket_type)

        if self.allow_reuse_address:
            self.set_reuse_addr()

        self.server_bind()
        self.server_activate()

    def server_bind(self):
        self.bind(self.address)
        log.debug("bind: address=%s:%s" % (self.address[0], self.address[1]))

    def server_activate(self):
        self.listen(self.request_queue_size)
        log.debug("listen: backlog=%d" % self.request_queue_size)

    def fileno(self):
        return self.socket.fileno()

    def serve_forever(self):
        asyncore.loop()

    # TODO: try to implement handle_request()

    # Internal use
    def handle_accept(self):
        (conn_sock, client_address) = self.accept()
        if self.verify_request(conn_sock, client_address):
            self.process_request(conn_sock, client_address)

    def verify_request(self, conn_sock, client_address):
        return True

    def process_request(self, conn_sock, client_address):
        log.info("conn_made: client_address=%s:%s" % \
                     (client_address[0],
                      client_address[1]))
        self.handlerClass(conn_sock, client_address, self)

    def handle_close(self):
        self.close()

使用示例：

interface = "0.0.0.0"
port = 8080
server = asyncore_echo_server.EchoServer((interface, port))
server.serve_forever()

代码中创建了两个dispatcher的子类，一个是server专门用于接受客户端的请求；一个是EchoHandler，用于收到请求的时候处理客户端的每一个请求。

3、Asyncore.dispatcher 子类中的server和普通的Handler有什么区别？

区别只有一个地方，dispatcher初始化的时候，server会先创建一个socket，binding本地的地址和端口，然后listening，在listening的过程中会执行下列代码

def listen(self, num):
        self.accepting = True
        if os.name == 'nt' and num > 5:
            num = 5
        return self.socket.listen(num)

当self.accepting = True 之后，客户端所有的请求会当成accept处理，也就是收到一个请求，得到一个socket。如果这个时候不把这个socket传给handler，那么这个socket因为没有人保存会立即释放，导致socket关闭。