
``socketserver`` --- A framework for network servers
****************************************************

The ``socketserver`` module simplifies the task of writing network
servers.

There are four basic server classes: ``TCPServer`` uses the Internet
TCP protocol, which provides for continuous streams of data between
the client and server.  ``UDPServer`` uses datagrams, which are
discrete packets of information that may arrive out of order or be
lost while in transit.  The more infrequently used
``UnixStreamServer`` and ``UnixDatagramServer`` classes are similar,
but use Unix domain sockets; they're not available on non-Unix
platforms.  For more details on network programming, consult a book
such as W. Richard Steven's UNIX Network Programming or Ralph Davis's
Win32 Network Programming.

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.  Finally, call the ``handle_request()`` or ``serve_forever()``
method of the server object to process one or many requests.

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.


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.

Forking and threading versions of each type of server can be created
using the ``ForkingMixIn`` and ``ThreadingMixIn`` mix-in classes.  For
instance, a threading UDP server class is created as follows:

   class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass

The mix-in class must come first, since it overrides a method defined
in ``UDPServer``.  Setting the various member variables also changes
the behavior of the underlying server mechanism.

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.


Server Objects
==============

class class socketserver.BaseServer

   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.

BaseServer.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.

BaseServer.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 ``self.timeout`` seconds, ``handle_timeout()`` will
   be called and ``handle_request()`` will return.

BaseServer.serve_forever(poll_interval=0.5)

   Handle requests until an explicit ``shutdown()`` request.  Polls
   for shutdown every *poll_interval* seconds.

BaseServer.shutdown()

   Tells the ``serve_forever()`` loop to stop and waits until it does.

BaseServer.address_family

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

BaseServer.RequestHandlerClass

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

BaseServer.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.

BaseServer.socket

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

The server classes support the following class variables:

BaseServer.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.

BaseServer.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.

BaseServer.socket_type

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

BaseServer.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.

BaseServer.finish_request()

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

BaseServer.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.

BaseServer.handle_error(request, client_address)

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

BaseServer.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.

BaseServer.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.

BaseServer.server_activate()

   Called by the server's constructor to activate the server.  The
   default behavior just ``listen()``s to the server's socket. May be
   overridden.

BaseServer.server_bind()

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

BaseServer.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``.


RequestHandler Objects
======================

The request handler class must define a new ``handle()`` method, and
can override any of the following methods.  A new instance is created
for each request.

RequestHandler.finish()

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

RequestHandler.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. However, this can be hidden by using the request
   handler subclasses ``StreamRequestHandler`` or
   ``DatagramRequestHandler``, which override the ``setup()`` and
   ``finish()`` methods, and provide ``self.rfile`` and ``self.wfile``
   attributes.  ``self.rfile`` and ``self.wfile`` can be read or
   written, respectively, to get the request data or return data to
   the client.

RequestHandler.setup()

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


Examples
========


``socketserver.TCPServer`` Example
----------------------------------

This is the server side:

   import socketserver

   class MyTCPHandler(socketserver.BaseRequestHandler):
       """
       The RequestHandler 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("%s wrote:" % self.client_address[0])
           print(self.data)
           # just send back the same data, but upper-cased
           self.request.send(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("%s wrote:" % 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 ``send()`` 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)

   # Connect to server and send data
   sock.connect((HOST, PORT))
   sock.send(bytes(data + "\n","utf8"))

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

   print("Sent:     %s" % data)
   print("Received: %s" % received)

The output of the example should look something like this:

Server:

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

Client:

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


``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("%s wrote:" % 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(bytes(data + "\n","utf8"), (HOST, PORT))
   received = sock.recv(1024)

   print("Sent:     %s" % data)
   print("Received: %s" % received)

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


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 = bytes("%s: %s" % (cur_thread.getName(), data),'ascii')
           self.request.send(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))
       sock.send(message)
       response = sock.recv(1024)
       print("Received: %s" % response)
       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.setDaemon(True)
       server_thread.start()
       print("Server loop running in thread:", server_thread.name)

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

       server.shutdown()

The output of the example should look something like this:

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

The ``ForkingMixIn`` class is used in the same way, except that the
server will spawn a new process for each request.
