2 :mod:`SocketServer` --- A framework for network servers
3 =======================================================
5 .. module:: SocketServer
6 :synopsis: A framework for network servers.
10 The :mod:`SocketServer` module has been renamed to :mod:`socketserver` in
11 Python 3.0. The :term:`2to3` tool will automatically adapt imports when
12 converting your sources to 3.0.
15 The :mod:`SocketServer` module simplifies the task of writing network servers.
17 There are four basic server classes: :class:`TCPServer` uses the Internet TCP
18 protocol, which provides for continuous streams of data between the client and
19 server. :class:`UDPServer` uses datagrams, which are discrete packets of
20 information that may arrive out of order or be lost while in transit. The more
21 infrequently used :class:`UnixStreamServer` and :class:`UnixDatagramServer`
22 classes are similar, but use Unix domain sockets; they're not available on
23 non-Unix platforms. For more details on network programming, consult a book
25 W. Richard Steven's UNIX Network Programming or Ralph Davis's Win32 Network
28 These four classes process requests :dfn:`synchronously`; each request must be
29 completed before the next request can be started. This isn't suitable if each
30 request takes a long time to complete, because it requires a lot of computation,
31 or because it returns a lot of data which the client is slow to process. The
32 solution is to create a separate process or thread to handle each request; the
33 :class:`ForkingMixIn` and :class:`ThreadingMixIn` mix-in classes can be used to
34 support asynchronous behaviour.
36 Creating a server requires several steps. First, you must create a request
37 handler class by subclassing the :class:`BaseRequestHandler` class and
38 overriding its :meth:`handle` method; this method will process incoming
39 requests. Second, you must instantiate one of the server classes, passing it
40 the server's address and the request handler class. Finally, call the
41 :meth:`handle_request` or :meth:`serve_forever` method of the server object to
42 process one or many requests.
44 When inheriting from :class:`ThreadingMixIn` for threaded connection behavior,
45 you should explicitly declare how you want your threads to behave on an abrupt
46 shutdown. The :class:`ThreadingMixIn` class defines an attribute
47 *daemon_threads*, which indicates whether or not the server should wait for
48 thread termination. You should set the flag explicitly if you would like threads
49 to behave autonomously; the default is :const:`False`, meaning that Python will
50 not exit until all threads created by :class:`ThreadingMixIn` have exited.
52 Server classes have the same external methods and attributes, no matter what
53 network protocol they use.
59 There are five classes in an inheritance diagram, four of which represent
60 synchronous servers of four types::
67 +-----------+ +------------------+
68 | TCPServer |------->| UnixStreamServer |
69 +-----------+ +------------------+
72 +-----------+ +--------------------+
73 | UDPServer |------->| UnixDatagramServer |
74 +-----------+ +--------------------+
76 Note that :class:`UnixDatagramServer` derives from :class:`UDPServer`, not from
77 :class:`UnixStreamServer` --- the only difference between an IP and a Unix
78 stream server is the address family, which is simply repeated in both Unix
81 Forking and threading versions of each type of server can be created using the
82 :class:`ForkingMixIn` and :class:`ThreadingMixIn` mix-in classes. For instance,
83 a threading UDP server class is created as follows::
85 class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass
87 The mix-in class must come first, since it overrides a method defined in
88 :class:`UDPServer`. Setting the various member variables also changes the
89 behavior of the underlying server mechanism.
91 To implement a service, you must derive a class from :class:`BaseRequestHandler`
92 and redefine its :meth:`handle` method. You can then run various versions of
93 the service by combining one of the server classes with your request handler
94 class. The request handler class must be different for datagram or stream
95 services. This can be hidden by using the handler subclasses
96 :class:`StreamRequestHandler` or :class:`DatagramRequestHandler`.
98 Of course, you still have to use your head! For instance, it makes no sense to
99 use a forking server if the service contains state in memory that can be
100 modified by different requests, since the modifications in the child process
101 would never reach the initial state kept in the parent process and passed to
102 each child. In this case, you can use a threading server, but you will probably
103 have to use locks to protect the integrity of the shared data.
105 On the other hand, if you are building an HTTP server where all data is stored
106 externally (for instance, in the file system), a synchronous class will
107 essentially render the service "deaf" while one request is being handled --
108 which may be for a very long time if a client is slow to receive all the data it
109 has requested. Here a threading or forking server is appropriate.
111 In some cases, it may be appropriate to process part of a request synchronously,
112 but to finish processing in a forked child depending on the request data. This
113 can be implemented by using a synchronous server and doing an explicit fork in
114 the request handler class :meth:`handle` method.
116 Another approach to handling multiple simultaneous requests in an environment
117 that supports neither threads nor :func:`fork` (or where these are too expensive
118 or inappropriate for the service) is to maintain an explicit table of partially
119 finished requests and to use :func:`select` to decide which request to work on
120 next (or whether to handle a new incoming request). This is particularly
121 important for stream services where each client can potentially be connected for
122 a long time (if threads or subprocesses cannot be used). See :mod:`asyncore` for
123 another way to manage this.
125 .. XXX should data and methods be intermingled, or separate?
126 how should the distinction between class and instance variables be drawn?
132 .. class:: BaseServer
134 This is the superclass of all Server objects in the module. It defines the
135 interface, given below, but does not implement most of the methods, which is
139 .. method:: BaseServer.fileno()
141 Return an integer file descriptor for the socket on which the server is
142 listening. This function is most commonly passed to :func:`select.select`, to
143 allow monitoring multiple servers in the same process.
146 .. method:: BaseServer.handle_request()
148 Process a single request. This function calls the following methods in
149 order: :meth:`get_request`, :meth:`verify_request`, and
150 :meth:`process_request`. If the user-provided :meth:`handle` method of the
151 handler class raises an exception, the server's :meth:`handle_error` method
152 will be called. If no request is received within :attr:`self.timeout`
153 seconds, :meth:`handle_timeout` will be called and :meth:`handle_request`
157 .. method:: BaseServer.serve_forever(poll_interval=0.5)
159 Handle requests until an explicit :meth:`shutdown` request. Polls for
160 shutdown every *poll_interval* seconds.
163 .. method:: BaseServer.shutdown()
165 Tells the :meth:`serve_forever` loop to stop and waits until it does.
167 .. versionadded:: 2.6
170 .. attribute:: BaseServer.address_family
172 The family of protocols to which the server's socket belongs.
173 Common examples are :const:`socket.AF_INET` and :const:`socket.AF_UNIX`.
176 .. attribute:: BaseServer.RequestHandlerClass
178 The user-provided request handler class; an instance of this class is created
182 .. attribute:: BaseServer.server_address
184 The address on which the server is listening. The format of addresses varies
185 depending on the protocol family; see the documentation for the socket module
186 for details. For Internet protocols, this is a tuple containing a string giving
187 the address, and an integer port number: ``('127.0.0.1', 80)``, for example.
190 .. attribute:: BaseServer.socket
192 The socket object on which the server will listen for incoming requests.
195 The server classes support the following class variables:
197 .. XXX should class variables be covered before instance variables, or vice versa?
199 .. attribute:: BaseServer.allow_reuse_address
201 Whether the server will allow the reuse of an address. This defaults to
202 :const:`False`, and can be set in subclasses to change the policy.
205 .. attribute:: BaseServer.request_queue_size
207 The size of the request queue. If it takes a long time to process a single
208 request, any requests that arrive while the server is busy are placed into a
209 queue, up to :attr:`request_queue_size` requests. Once the queue is full,
210 further requests from clients will get a "Connection denied" error. The default
211 value is usually 5, but this can be overridden by subclasses.
214 .. attribute:: BaseServer.socket_type
216 The type of socket used by the server; :const:`socket.SOCK_STREAM` and
217 :const:`socket.SOCK_DGRAM` are two common values.
220 .. attribute:: BaseServer.timeout
222 Timeout duration, measured in seconds, or :const:`None` if no timeout is
223 desired. If :meth:`handle_request` receives no incoming requests within the
224 timeout period, the :meth:`handle_timeout` method is called.
227 There are various server methods that can be overridden by subclasses of base
228 server classes like :class:`TCPServer`; these methods aren't useful to external
229 users of the server object.
231 .. XXX should the default implementations of these be documented, or should
232 it be assumed that the user will look at SocketServer.py?
234 .. method:: BaseServer.finish_request()
236 Actually processes the request by instantiating :attr:`RequestHandlerClass` and
237 calling its :meth:`handle` method.
240 .. method:: BaseServer.get_request()
242 Must accept a request from the socket, and return a 2-tuple containing the *new*
243 socket object to be used to communicate with the client, and the client's
247 .. method:: BaseServer.handle_error(request, client_address)
249 This function is called if the :attr:`RequestHandlerClass`'s :meth:`handle`
250 method raises an exception. The default action is to print the traceback to
251 standard output and continue handling further requests.
254 .. method:: BaseServer.handle_timeout()
256 This function is called when the :attr:`timeout` attribute has been set to a
257 value other than :const:`None` and the timeout period has passed with no
258 requests being received. The default action for forking servers is
259 to collect the status of any child processes that have exited, while
260 in threading servers this method does nothing.
263 .. method:: BaseServer.process_request(request, client_address)
265 Calls :meth:`finish_request` to create an instance of the
266 :attr:`RequestHandlerClass`. If desired, this function can create a new process
267 or thread to handle the request; the :class:`ForkingMixIn` and
268 :class:`ThreadingMixIn` classes do this.
271 .. Is there any point in documenting the following two functions?
272 What would the purpose of overriding them be: initializing server
273 instance variables, adding new network families?
275 .. method:: BaseServer.server_activate()
277 Called by the server's constructor to activate the server. The default behavior
278 just :meth:`listen`\ s to the server's socket. May be overridden.
281 .. method:: BaseServer.server_bind()
283 Called by the server's constructor to bind the socket to the desired address.
287 .. method:: BaseServer.verify_request(request, client_address)
289 Must return a Boolean value; if the value is :const:`True`, the request will be
290 processed, and if it's :const:`False`, the request will be denied. This function
291 can be overridden to implement access controls for a server. The default
292 implementation always returns :const:`True`.
295 RequestHandler Objects
296 ----------------------
298 The request handler class must define a new :meth:`handle` method, and can
299 override any of the following methods. A new instance is created for each
303 .. method:: RequestHandler.finish()
305 Called after the :meth:`handle` method to perform any clean-up actions
306 required. The default implementation does nothing. If :meth:`setup` or
307 :meth:`handle` raise an exception, this function will not be called.
310 .. method:: RequestHandler.handle()
312 This function must do all the work required to service a request. The
313 default implementation does nothing. Several instance attributes are
314 available to it; the request is available as :attr:`self.request`; the client
315 address as :attr:`self.client_address`; and the server instance as
316 :attr:`self.server`, in case it needs access to per-server information.
318 The type of :attr:`self.request` is different for datagram or stream
319 services. For stream services, :attr:`self.request` is a socket object; for
320 datagram services, :attr:`self.request` is a pair of string and socket.
321 However, this can be hidden by using the request handler subclasses
322 :class:`StreamRequestHandler` or :class:`DatagramRequestHandler`, which
323 override the :meth:`setup` and :meth:`finish` methods, and provide
324 :attr:`self.rfile` and :attr:`self.wfile` attributes. :attr:`self.rfile` and
325 :attr:`self.wfile` can be read or written, respectively, to get the request
326 data or return data to the client.
329 .. method:: RequestHandler.setup()
331 Called before the :meth:`handle` method to perform any initialization actions
332 required. The default implementation does nothing.
338 :class:`SocketServer.TCPServer` Example
339 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
341 This is the server side::
345 class MyTCPHandler(SocketServer.BaseRequestHandler):
347 The RequestHandler class for our server.
349 It is instantiated once per connection to the server, and must
350 override the handle() method to implement communication to the
355 # self.request is the TCP socket connected to the client
356 self.data = self.request.recv(1024).strip()
357 print "%s wrote:" % self.client_address[0]
359 # just send back the same data, but upper-cased
360 self.request.send(self.data.upper())
362 if __name__ == "__main__":
363 HOST, PORT = "localhost", 9999
365 # Create the server, binding to localhost on port 9999
366 server = SocketServer.TCPServer((HOST, PORT), MyTCPHandler)
368 # Activate the server; this will keep running until you
369 # interrupt the program with Ctrl-C
370 server.serve_forever()
372 An alternative request handler class that makes use of streams (file-like
373 objects that simplify communication by providing the standard file interface)::
375 class MyTCPHandler(SocketServer.StreamRequestHandler):
378 # self.rfile is a file-like object created by the handler;
379 # we can now use e.g. readline() instead of raw recv() calls
380 self.data = self.rfile.readline().strip()
381 print "%s wrote:" % self.client_address[0]
383 # Likewise, self.wfile is a file-like object used to write back
385 self.wfile.write(self.data.upper())
387 The difference is that the ``readline()`` call in the second handler will call
388 ``recv()`` multiple times until it encounters a newline character, while the
389 single ``recv()`` call in the first handler will just return what has been sent
390 from the client in one ``send()`` call.
393 This is the client side::
398 HOST, PORT = "localhost", 9999
399 data = " ".join(sys.argv[1:])
401 # Create a socket (SOCK_STREAM means a TCP socket)
402 sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
404 # Connect to server and send data
405 sock.connect((HOST, PORT))
406 sock.send(data + "\n")
408 # Receive data from the server and shut down
409 received = sock.recv(1024)
412 print "Sent: %s" % data
413 print "Received: %s" % received
416 The output of the example should look something like this:
420 $ python TCPServer.py
428 $ python TCPClient.py hello world with TCP
429 Sent: hello world with TCP
430 Received: HELLO WORLD WITH TCP
431 $ python TCPClient.py python is nice
433 Received: PYTHON IS NICE
436 :class:`SocketServer.UDPServer` Example
437 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
439 This is the server side::
443 class MyUDPHandler(SocketServer.BaseRequestHandler):
445 This class works similar to the TCP handler class, except that
446 self.request consists of a pair of data and client socket, and since
447 there is no connection the client address must be given explicitly
448 when sending data back via sendto().
452 data = self.request[0].strip()
453 socket = self.request[1]
454 print "%s wrote:" % self.client_address[0]
456 socket.sendto(data.upper(), self.client_address)
458 if __name__ == "__main__":
459 HOST, PORT = "localhost", 9999
460 server = SocketServer.UDPServer((HOST, PORT), MyUDPHandler)
461 server.serve_forever()
463 This is the client side::
468 HOST, PORT = "localhost"
469 data = " ".join(sys.argv[1:])
471 # SOCK_DGRAM is the socket type to use for UDP sockets
472 sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
474 # As you can see, there is no connect() call; UDP has no connections.
475 # Instead, data is directly sent to the recipient via sendto().
476 sock.sendto(data + "\n", (HOST, PORT))
477 received = sock.recv(1024)
479 print "Sent: %s" % data
480 print "Received: %s" % received
482 The output of the example should look exactly like for the TCP server example.
488 To build asynchronous handlers, use the :class:`ThreadingMixIn` and
489 :class:`ForkingMixIn` classes.
491 An example for the :class:`ThreadingMixIn` class::
497 class ThreadedTCPRequestHandler(SocketServer.BaseRequestHandler):
500 data = self.request.recv(1024)
501 cur_thread = threading.currentThread()
502 response = "%s: %s" % (cur_thread.getName(), data)
503 self.request.send(response)
505 class ThreadedTCPServer(SocketServer.ThreadingMixIn, SocketServer.TCPServer):
508 def client(ip, port, message):
509 sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
510 sock.connect((ip, port))
512 response = sock.recv(1024)
513 print "Received: %s" % response
516 if __name__ == "__main__":
517 # Port 0 means to select an arbitrary unused port
518 HOST, PORT = "localhost", 0
520 server = ThreadedTCPServer((HOST, PORT), ThreadedTCPRequestHandler)
521 ip, port = server.server_address
523 # Start a thread with the server -- that thread will then start one
524 # more thread for each request
525 server_thread = threading.Thread(target=server.serve_forever)
526 # Exit the server thread when the main thread terminates
527 server_thread.setDaemon(True)
528 server_thread.start()
529 print "Server loop running in thread:", server_thread.getName()
531 client(ip, port, "Hello World 1")
532 client(ip, port, "Hello World 2")
533 client(ip, port, "Hello World 3")
537 The output of the example should look something like this::
539 $ python ThreadedTCPServer.py
540 Server loop running in thread: Thread-1
541 Received: Thread-2: Hello World 1
542 Received: Thread-3: Hello World 2
543 Received: Thread-4: Hello World 3
546 The :class:`ForkingMixIn` class is used in the same way, except that the server
547 will spawn a new process for each request.