五、Mix-In混合类
昨天介绍了BaseServer和BaseRequestHandler两个基类，它们只用与派生，所以贴了它们派生的子类代码。
今天介绍两个混合类，ForkingMix-In 和 ThreadingMix-In，两者分别实现了核心的进程化和线程化的功能，如前面简介中所提，作为混合类，它们与服务器类一并使用以提供一些异步特性，Mix-in 这个类必须首先实现，因为它重写了定义UDPServer的方法。注意，它们不会被直接实例化。
5.1 ForkingMixIn
该类针对每一个监听到来的新的连接请求都会fork一个新的子进程，在子进程中完成请求处理。这里需要注意，由于windows对fork支持的不完整，所以无法在windows环境下运行该模块的forkingMixIn类，只能在linux或Mac环境中测试运行。
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class ForkingMixIn: timeout = 300 active_children = None max_children = 40</span>
首先定义了超时时间为300，最大的子进程数量为40个。
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<span style="white-space:pre"> </span>def collect_children(self): """Internal routine to wait for children that have exited.""" if self.active_children is None: return # If we're above the max number of children, wait and reap them until # we go back below threshold. Note that we use waitpid(-1) below to be # able to collect children in size(<defunct children>) syscalls instead # of size(<children>): the downside is that this might reap children # which we didn't spawn, which is why we only resort to this when we're # above max_children. while len(self.active_children) >= self.max_children: try: pid, _ = os.waitpid(-1, 0) self.active_children.discard(pid) except OSError as e: if e.errno == errno.ECHILD: # we don't have any children, we're done self.active_children.clear() elif e.errno != errno.EINTR: break # Now reap all defunct children. for pid in self.active_children.copy(): try: pid, _ = os.waitpid(pid, os.WNOHANG) # if the child hasn't exited yet, pid will be 0 and ignored by # discard() below self.active_children.discard(pid) except OSError as e: if e.errno == errno.ECHILD: # someone else reaped it self.active_children.discard(pid)</span><span style="font-size:18px;"></span>
collect_children()方法用于判断当前的子进程数是否超过阈值，以保证程序的稳定运行。如果当前的fork的子进程数超过阈值40，我们把主进程阻塞住，使os.waitpid(-1)，直到有子进程处理完成请求并且断开连接，等待总体的正在运行的子进程数降到阈值以下；与此同时，该方法也会通过分配出去的pid遍历所有fork的子进程，查看它们是否在正常工作，如果发现僵死进程或者不存在的子进程，主进程则会调用discard()方法将子进程占用的资源回收，以便分配给其他新到来的请求。
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<span style="white-space:pre"> </span>def process_request(self, request, client_address): """Fork a new subprocess to process the request.""" self.collect_children() pid = os.fork() if pid: # Parent process if self.active_children is None: self.active_children = set() self.active_children.add(pid) self.close_request(request) #close handle in parent process return else: # Child process. # This must never return, hence os._exit()! try: self.finish_request(request, client_address) self.shutdown_request(request) os._exit(0) except: try: self.handle_error(request, client_address) self.shutdown_request(request) finally: os._exit(1)</span>
process_request()方法为主进程监听连接请求，一旦发现了连接请求，首先调用上面的collect_children()方法，查看set()这个资源池中的子进程数是否达到阈值，如果没有，则为新到来的请求fork一个子进程，分配一个pid放入set()资源池中，然后在子进程中处理到来的请求，注意，子进程中不能有return值，只能用os._exit()退出子进程。为了便于理解，在centos上写了个简单的server和client，当有多个连接请求时，我们 ps -ef | grep sock ，发现server端有多个子进程。如下图：
很明显，主进程fork了三个子进程处理连接请求，而我也恰好开了三个TCP的连接。此时，我们把一个客户端断开，再ps -ef | grep sock ，如下图：
此时，我们发现了僵死进程（19385）<defunct>，然而当我把断开的客户端重新启动时，就恢复了3个活跃的子进程，并且进程号增加，说明父进程在新的连接请求到来时清理了set()资源池，把僵死的子进程干掉，并为新的请求分配了新的pid，放入set()资源池中。
5.2 ThreadingMixIn
该类会针对每一个新到来的连接请求分配一个新的线程来处理，这里面有用到python的thread和threading模块，可以回忆之前的这篇文章 ：python--多线程 （中间备考隔了太长时间，自己都忘得差不多了，尴尬）
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class ThreadingMixIn: """Mix-in class to handle each request in a new thread.""" # Decides how threads will act upon termination of the # main process daemon_threads = False</span>
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def process_request_thread(self, request, client_address): """Same as in BaseServer but as a thread. In addition, exception handling is done here. """ try: self.finish_request(request, client_address) self.shutdown_request(request) except: self.handle_error(request, client_address) self.shutdown_request(request)</span>
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def process_request(self, request, client_address): """Start a new thread to process the request.""" t = threading.Thread(target = self.process_request_thread, args = (request, client_address)) t.daemon = self.daemon_threads t.start()</span>
六、应用举例
以上提到的所有的类，我们最好不要直接实例化，简介里面提到过，我们最好使用它们的混合来使我们的server更加简单强大，我们需要做的就是按照我们的需求重写Handle方法，然后调用以下四种方法之一就可以了：
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class ForkingUDPServer(ForkingMixIn, UDPServer): passclass ForkingTCPServer(ForkingMixIn, TCPServer): passclass ThreadingUDPServer(ThreadingMixIn, UDPServer): passclass ThreadingTCPServer(ThreadingMixIn, TCPServer): pass</span>
下面是服务器端调用ThreadingTCPServer类实例化的一个简单例子。供日后参考：
server端：
# -*- coding:utf-8 -*-
__author__ = 'webber'
import SocketServer
'''
多线程并发实现TCP连接
'''
class MyTCPHandler(SocketServer.BaseRequestHandler):
def handle(self):
while True:
try:
#self.request is the TCP socket connected to the client
self.data = self.request.recv(1024).strip()
print "{} wrote:".format(self.client_address)
#client_address[0]:客户端IP
#client_address[1]:客户端端口号
print self.data
#just send back the same data,but upper-cased
self.request.sendall(self.data.upper())
except Exception:
print 'A client has left!!!'
break
if __name__ == "__main__":
HOST,PORT = "localhost",9999
# 把类实例化,把自己写的类绑定到ThreadingTCPServer上
server = SocketServer.ForkingTCPServer((HOST, PORT), MyTCPHandler)
# Activate the server; this will keep running until you
# interrupt the program with Ctrl-C
'''
Handle one request at a time until shutdown.
Polls for shutdown every poll_interval seconds. Ignores
self.timeout. If you need to do periodic tasks, do them in
another thread.
'''
print 'waiting for connection........ '
server.serve_forever()
客户端：
# -*- coding:utf-8 -*-
__author__ = 'webber'
import socket
HOST = 'localhost'
PORT = 9999
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
try:
while True:
msg = raw_input(">>>:").strip()
if len(msg) is None:
continue
s.sendall(msg)
data = s.recv(1024)
print "Received: ",data
s.close()
except socket.error,e:
print "server disconnected!!!", e