20.17. "SocketServer" — A framework for network servers
*******************************************************

Note: The "SocketServer" module has been renamed to "socketserver"
  in Python 3.  The *2to3* tool will automatically adapt imports when
  converting your sources to Python 3.

**Source code:** Lib/SocketServer.py

======================================================================

The "SocketServer" module simplifies the task of writing network
servers.

There are four basic concrete server classes:

class SocketServer.TCPServer(server_address, RequestHandlerClass, bind_and_activate=True)

   This uses the Internet TCP protocol, which provides for continuous
   streams of data between the client and server. If
   *bind_and_activate* is true, the constructor automatically attempts
   to invoke "server_bind()" and "server_activate()".  The other
   parameters are passed to the "BaseServer" base class.

class SocketServer.UDPServer(server_address, RequestHandlerClass, bind_and_activate=True)

   This uses datagrams, which are discrete packets of information that
   may arrive out of order or be lost while in transit.  The
   parameters are the same as for "TCPServer".

class SocketServer.UnixStreamServer(server_address, RequestHandlerClass, bind_and_activate=True)
class SocketServer.UnixDatagramServer(server_address, RequestHandlerClass, bind_and_activate=True)

   These more infrequently used classes are similar to the TCP and UDP
   classes, but use Unix domain sockets; they’re not available on non-
   Unix platforms.  The parameters are the same as for "TCPServer".

These four classes process requests *synchronously*; each request must
be completed before the next request can be started.  This isn’t
suitable if each request takes a long time to complete, because it
requires a lot of computation, or because it returns a lot of data
which the client is slow to process.  The solution is to create a
separate process or thread to handle each request; the "ForkingMixIn"
and "ThreadingMixIn" mix-in classes can be used to support
asynchronous behaviour.

Creating a server requires several steps.  First, you must create a
request handler class by subclassing the "BaseRequestHandler" class
and overriding its "handle()" method; this method will process
incoming requests.  Second, you must instantiate one of the server
classes, passing it the server’s address and the request handler
class.  Then call the "handle_request()" or "serve_forever()" method
of the server object to process one or many requests.  Finally, call
"server_close()" to close the socket.

When inheriting from "ThreadingMixIn" for threaded connection
behavior, you should explicitly declare how you want your threads to
behave on an abrupt shutdown. The "ThreadingMixIn" class defines an
attribute *daemon_threads*, which indicates whether or not the server
should wait for thread termination. You should set the flag explicitly
if you would like threads to behave autonomously; the default is
"False", meaning that Python will not exit until all threads created
by "ThreadingMixIn" have exited.

Server classes have the same external methods and attributes, no
matter what network protocol they use.


20.17.1. Server Creation Notes
==============================

There are five classes in an inheritance diagram, four of which
represent synchronous servers of four types:

   +------------+
   | BaseServer |
   +------------+
         |
         v
   +-----------+        +------------------+
   | TCPServer |------->| UnixStreamServer |
   +-----------+        +------------------+
         |
         v
   +-----------+        +--------------------+
   | UDPServer |------->| UnixDatagramServer |
   +-----------+        +--------------------+

Note that "UnixDatagramServer" derives from "UDPServer", not from
"UnixStreamServer" — the only difference between an IP and a Unix
stream server is the address family, which is simply repeated in both
Unix server classes.

class SocketServer.ForkingMixIn
class SocketServer.ThreadingMixIn

   Forking and threading versions of each type of server can be
   created using these mix-in classes.  For instance,
   "ThreadingUDPServer" is created as follows:

      class ThreadingUDPServer(ThreadingMixIn, UDPServer):
          pass

   The mix-in class comes first, since it overrides a method defined
   in "UDPServer".  Setting the various attributes also changes the
   behavior of the underlying server mechanism.

   "ForkingMixIn" and the Forking classes mentioned below are only
   available on POSIX platforms that support "fork()".

class SocketServer.ForkingTCPServer
class SocketServer.ForkingUDPServer
class SocketServer.ThreadingTCPServer
class SocketServer.ThreadingUDPServer

   These classes are pre-defined using the mix-in classes.

To implement a service, you must derive a class from
"BaseRequestHandler" and redefine its "handle()" method. You can then
run various versions of the service by combining one of the server
classes with your request handler class.  The request handler class
must be different for datagram or stream services.  This can be hidden
by using the handler subclasses "StreamRequestHandler" or
"DatagramRequestHandler".

Of course, you still have to use your head!  For instance, it makes no
sense to use a forking server if the service contains state in memory
that can be modified by different requests, since the modifications in
the child process would never reach the initial state kept in the
parent process and passed to each child.  In this case, you can use a
threading server, but you will probably have to use locks to protect
the integrity of the shared data.

On the other hand, if you are building an HTTP server where all data
is stored externally (for instance, in the file system), a synchronous
class will essentially render the service “deaf” while one request is
being handled – which may be for a very long time if a client is slow
to receive all the data it has requested.  Here a threading or forking
server is appropriate.

In some cases, it may be appropriate to process part of a request
synchronously, but to finish processing in a forked child depending on
the request data.  This can be implemented by using a synchronous
server and doing an explicit fork in the request handler class
"handle()" method.

Another approach to handling multiple simultaneous requests in an
environment that supports neither threads nor "fork()" (or where these
are too expensive or inappropriate for the service) is to maintain an
explicit table of partially finished requests and to use "select()" to
decide which request to work on next (or whether to handle a new
incoming request).  This is particularly important for stream services
where each client can potentially be connected for a long time (if
threads or subprocesses cannot be used). See "asyncore" for another
way to manage this.


20.17.2. Server Objects
=======================

class SocketServer.BaseServer(server_address, RequestHandlerClass)

   This is the superclass of all Server objects in the module.  It
   defines the interface, given below, but does not implement most of
   the methods, which is done in subclasses.  The two parameters are
   stored in the respective "server_address" and "RequestHandlerClass"
   attributes.

   fileno()

      Return an integer file descriptor for the socket on which the
      server is listening.  This function is most commonly passed to
      "select.select()", to allow monitoring multiple servers in the
      same process.

   handle_request()

      Process a single request.  This function calls the following
      methods in order: "get_request()", "verify_request()", and
      "process_request()".  If the user-provided "handle()" method of
      the handler class raises an exception, the server’s
      "handle_error()" method will be called.  If no request is
      received within "timeout" seconds, "handle_timeout()" will be
      called and "handle_request()" will return.

   serve_forever(poll_interval=0.5)

      Handle requests until an explicit "shutdown()" request.  Poll
      for shutdown every *poll_interval* seconds. Ignores the
      "timeout" attribute. If you need to do periodic tasks, do them
      in another thread.

   shutdown()

      Tell the "serve_forever()" loop to stop and wait until it does.

      New in version 2.6.

   server_close()

      Clean up the server. May be overridden.

      New in version 2.6.

   address_family

      The family of protocols to which the server’s socket belongs.
      Common examples are "socket.AF_INET" and "socket.AF_UNIX".

   RequestHandlerClass

      The user-provided request handler class; an instance of this
      class is created for each request.

   server_address

      The address on which the server is listening.  The format of
      addresses varies depending on the protocol family; see the
      documentation for the "socket" module for details.  For Internet
      protocols, this is a tuple containing a string giving the
      address, and an integer port number: "('127.0.0.1', 80)", for
      example.

   socket

      The socket object on which the server will listen for incoming
      requests.

   The server classes support the following class variables:

   allow_reuse_address

      Whether the server will allow the reuse of an address. This
      defaults to "False", and can be set in subclasses to change the
      policy.

   request_queue_size

      The size of the request queue.  If it takes a long time to
      process a single request, any requests that arrive while the
      server is busy are placed into a queue, up to
      "request_queue_size" requests.  Once the queue is full, further
      requests from clients will get a “Connection denied” error.  The
      default value is usually 5, but this can be overridden by
      subclasses.

   socket_type

      The type of socket used by the server; "socket.SOCK_STREAM" and
      "socket.SOCK_DGRAM" are two common values.

   timeout

      Timeout duration, measured in seconds, or "None" if no timeout
      is desired.  If "handle_request()" receives no incoming requests
      within the timeout period, the "handle_timeout()" method is
      called.

   There are various server methods that can be overridden by
   subclasses of base server classes like "TCPServer"; these methods
   aren’t useful to external users of the server object.

   finish_request(request, client_address)

      Actually processes the request by instantiating
      "RequestHandlerClass" and calling its "handle()" method.

   get_request()

      Must accept a request from the socket, and return a 2-tuple
      containing the *new* socket object to be used to communicate
      with the client, and the client’s address.

   handle_error(request, client_address)

      This function is called if the "handle()" method of a
      "RequestHandlerClass" instance raises an exception.  The default
      action is to print the traceback to standard output and continue
      handling further requests.

   handle_timeout()

      This function is called when the "timeout" attribute has been
      set to a value other than "None" and the timeout period has
      passed with no requests being received.  The default action for
      forking servers is to collect the status of any child processes
      that have exited, while in threading servers this method does
      nothing.

   process_request(request, client_address)

      Calls "finish_request()" to create an instance of the
      "RequestHandlerClass".  If desired, this function can create a
      new process or thread to handle the request; the "ForkingMixIn"
      and "ThreadingMixIn" classes do this.

   server_activate()

      Called by the server’s constructor to activate the server.  The
      default behavior for a TCP server just invokes "listen()" on the
      server’s socket. May be overridden.

   server_bind()

      Called by the server’s constructor to bind the socket to the
      desired address. May be overridden.

   verify_request(request, client_address)

      Must return a Boolean value; if the value is "True", the request
      will be processed, and if it’s "False", the request will be
      denied. This function can be overridden to implement access
      controls for a server. The default implementation always returns
      "True".


20.17.3. Request Handler Objects
================================

class SocketServer.BaseRequestHandler

   This is the superclass of all request handler objects.  It defines
   the interface, given below.  A concrete request handler subclass
   must define a new "handle()" method, and can override any of the
   other methods.  A new instance of the subclass is created for each
   request.

   setup()

      Called before the "handle()" method to perform any
      initialization actions required.  The default implementation
      does nothing.

   handle()

      This function must do all the work required to service a
      request.  The default implementation does nothing.  Several
      instance attributes are available to it; the request is
      available as "self.request"; the client address as
      "self.client_address"; and the server instance as "self.server",
      in case it needs access to per-server information.

      The type of "self.request" is different for datagram or stream
      services.  For stream services, "self.request" is a socket
      object; for datagram services, "self.request" is a pair of
      string and socket.

   finish()

      Called after the "handle()" method to perform any clean-up
      actions required.  The default implementation does nothing.  If
      "setup()" raises an exception, this function will not be called.

class SocketServer.StreamRequestHandler
class SocketServer.DatagramRequestHandler

   These "BaseRequestHandler" subclasses override the "setup()" and
   "finish()" methods, and provide "self.rfile" and "self.wfile"
   attributes. The "self.rfile" and "self.wfile" attributes can be
   read or written, respectively, to get the request data or return
   data to the client.


20.17.4. Examples
=================


20.17.4.1. "SocketServer.TCPServer" Example
-------------------------------------------

This is the server side:

   import SocketServer

   class MyTCPHandler(SocketServer.BaseRequestHandler):
       """
       The request handler class for our server.

       It is instantiated once per connection to the server, and must
       override the handle() method to implement communication to the
       client.
       """

       def handle(self):
           # self.request is the TCP socket connected to the client
           self.data = self.request.recv(1024).strip()
           print "{} wrote:".format(self.client_address[0])
           print self.data
           # just send back the same data, but upper-cased
           self.request.sendall(self.data.upper())

   if __name__ == "__main__":
       HOST, PORT = "localhost", 9999

       # Create the server, binding to localhost on port 9999
       server = SocketServer.TCPServer((HOST, PORT), MyTCPHandler)

       # Activate the server; this will keep running until you
       # interrupt the program with Ctrl-C
       server.serve_forever()

An alternative request handler class that makes use of streams (file-
like objects that simplify communication by providing the standard
file interface):

   class MyTCPHandler(SocketServer.StreamRequestHandler):

       def handle(self):
           # self.rfile is a file-like object created by the handler;
           # we can now use e.g. readline() instead of raw recv() calls
           self.data = self.rfile.readline().strip()
           print "{} wrote:".format(self.client_address[0])
           print self.data
           # Likewise, self.wfile is a file-like object used to write back
           # to the client
           self.wfile.write(self.data.upper())

The difference is that the "readline()" call in the second handler
will call "recv()" multiple times until it encounters a newline
character, while the single "recv()" call in the first handler will
just return what has been sent from the client in one "sendall()"
call.

This is the client side:

   import socket
   import sys

   HOST, PORT = "localhost", 9999
   data = " ".join(sys.argv[1:])

   # Create a socket (SOCK_STREAM means a TCP socket)
   sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)

   try:
       # Connect to server and send data
       sock.connect((HOST, PORT))
       sock.sendall(data + "\n")

       # Receive data from the server and shut down
       received = sock.recv(1024)
   finally:
       sock.close()

   print "Sent:     {}".format(data)
   print "Received: {}".format(received)

The output of the example should look something like this:

Server:

   $ python TCPServer.py
   127.0.0.1 wrote:
   hello world with TCP
   127.0.0.1 wrote:
   python is nice

Client:

   $ python TCPClient.py hello world with TCP
   Sent:     hello world with TCP
   Received: HELLO WORLD WITH TCP
   $ python TCPClient.py python is nice
   Sent:     python is nice
   Received: PYTHON IS NICE


20.17.4.2. "SocketServer.UDPServer" Example
-------------------------------------------

This is the server side:

   import SocketServer

   class MyUDPHandler(SocketServer.BaseRequestHandler):
       """
       This class works similar to the TCP handler class, except that
       self.request consists of a pair of data and client socket, and since
       there is no connection the client address must be given explicitly
       when sending data back via sendto().
       """

       def handle(self):
           data = self.request[0].strip()
           socket = self.request[1]
           print "{} wrote:".format(self.client_address[0])
           print data
           socket.sendto(data.upper(), self.client_address)

   if __name__ == "__main__":
       HOST, PORT = "localhost", 9999
       server = SocketServer.UDPServer((HOST, PORT), MyUDPHandler)
       server.serve_forever()

This is the client side:

   import socket
   import sys

   HOST, PORT = "localhost", 9999
   data = " ".join(sys.argv[1:])

   # SOCK_DGRAM is the socket type to use for UDP sockets
   sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

   # As you can see, there is no connect() call; UDP has no connections.
   # Instead, data is directly sent to the recipient via sendto().
   sock.sendto(data + "\n", (HOST, PORT))
   received = sock.recv(1024)

   print "Sent:     {}".format(data)
   print "Received: {}".format(received)

The output of the example should look exactly like for the TCP server
example.


20.17.4.3. Asynchronous Mixins
------------------------------

To build asynchronous handlers, use the "ThreadingMixIn" and
"ForkingMixIn" classes.

An example for the "ThreadingMixIn" class:

   import socket
   import threading
   import SocketServer

   class ThreadedTCPRequestHandler(SocketServer.BaseRequestHandler):

       def handle(self):
           data = self.request.recv(1024)
           cur_thread = threading.current_thread()
           response = "{}: {}".format(cur_thread.name, data)
           self.request.sendall(response)

   class ThreadedTCPServer(SocketServer.ThreadingMixIn, SocketServer.TCPServer):
       pass

   def client(ip, port, message):
       sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
       sock.connect((ip, port))
       try:
           sock.sendall(message)
           response = sock.recv(1024)
           print "Received: {}".format(response)
       finally:
           sock.close()

   if __name__ == "__main__":
       # Port 0 means to select an arbitrary unused port
       HOST, PORT = "localhost", 0

       server = ThreadedTCPServer((HOST, PORT), ThreadedTCPRequestHandler)
       ip, port = server.server_address

       # Start a thread with the server -- that thread will then start one
       # more thread for each request
       server_thread = threading.Thread(target=server.serve_forever)
       # Exit the server thread when the main thread terminates
       server_thread.daemon = True
       server_thread.start()
       print "Server loop running in thread:", server_thread.name

       client(ip, port, "Hello World 1")
       client(ip, port, "Hello World 2")
       client(ip, port, "Hello World 3")

       server.shutdown()
       server.server_close()

The output of the example should look something like this:

   $ python ThreadedTCPServer.py
   Server loop running in thread: Thread-1
   Received: Thread-2: Hello World 1
   Received: Thread-3: Hello World 2
   Received: Thread-4: Hello World 3

The "ForkingMixIn" class is used in the same way, except that the
server will spawn a new process for each request. Available only on
POSIX platforms that support "fork()".
