11.15 `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:

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.

serve_forever( ): Handle an infinite number of requests. This simply calls handle_request() inside an infinite loop.

address_family: The family of protocols to which the server's socket belongs. socket.AF_INET and socket.AF_UNIX are two possible values.

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 possible values.

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

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. 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 return true.

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.

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.

handle( ): This function must do all the work required to service a request. 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 string. However, this can be hidden by using the mix-in request handler classes StreamRequestHandler or DatagramRequestHandler, which override the setup() and finish() methods, and provides 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.

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

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11.15 SocketServer -- A framework for network servers

11.15 `SocketServer` -- A framework for network servers