\section{Standard Module \sectcode{SocketServer}} \label{module-SocketServer} \stmodindex{SocketServer} The \code{SocketServer} module simplifies the task of writing network servers. There are four basic server classes: \code{TCPServer} uses the Internet TCP protocol, which provides for continuous streams of data between the client and server. \code{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 \code{UnixStreamServer} and \code{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 \emph{UNIX Network Programming} or Ralph Davis's \emph{Win32 Network Programming}. These four classes process requests \dfn{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 \code{ForkingMixIn} and \code{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 \code{BaseRequestHandler} class and overriding its \code{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 \code{handle_request()} or \code{serve_forever()} method of the server object to process one or many requests. Server classes have the same external methods and attributes, no matter what network protocol they use: \setindexsubitem{(SocketServer protocol)} %XXX should data and methods be intermingled, or separate? % how should the distinction between class and instance variables be % drawn? \begin{funcdesc}{fileno}{} Return an integer file descriptor for the socket on which the server is listening. This function is most commonly passed to \code{select.select()}, to allow monitoring multiple servers in the same process. \end{funcdesc} \begin{funcdesc}{handle_request}{} Process a single request. This function calls the following methods in order: \code{get_request()}, \code{verify_request()}, and \code{process_request()}. If the user-provided \code{handle()} method of the handler class raises an exception, the server's \code{handle_error()} method will be called. \end{funcdesc} \begin{funcdesc}{serve_forever}{} Handle an infinite number of requests. This simply calls \code{handle_request()} inside an infinite loop. \end{funcdesc} \begin{datadesc}{address_family} The family of protocols to which the server's socket belongs. \code{socket.AF_INET} and \code{socket.AF_UNIX} are two possible values. \end{datadesc} \begin{datadesc}{RequestHandlerClass} The user-provided request handler class; an instance of this class is created for each request. \end{datadesc} \begin{datadesc}{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: \code{('127.0.0.1', 80)}, for example. \end{datadesc} \begin{datadesc}{socket} The socket object on which the server will listen for incoming requests. \end{datadesc} % XXX should class variables be covered before instance variables, or % vice versa? The server classes support the following class variables: \begin{datadesc}{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 \code{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. \end{datadesc} \begin{datadesc}{socket_type} The type of socket used by the server; \code{socket.SOCK_STREAM} and \code{socket.SOCK_DGRAM} are two possible values. \end{datadesc} There are various server methods that can be overridden by subclasses of base server classes like \code{TCPServer}; these methods aren't useful to external users of the server object. % should the default implementations of these be documented, or should % it be assumed that the user will look at SocketServer.py? \begin{funcdesc}{finish_request}{} Actually processes the request by instantiating \code{RequestHandlerClass} and calling its \code{handle()} method. \end{funcdesc} \begin{funcdesc}{get_request}{} Must accept a request from the socket, and return a 2-tuple containing the \emph{new} socket object to be used to communicate with the client, and the client's address. \end{funcdesc} \begin{funcdesc}{handle_error}{request\, client_address} This function is called if the \code{RequestHandlerClass}'s \code{handle} method raises an exception. The default action is to print the traceback to standard output and continue handling further requests. \end{funcdesc} \begin{funcdesc}{process_request}{request\, client_address} Calls \code{finish_request()} to create an instance of the \code{RequestHandlerClass}. If desired, this function can create a new process or thread to handle the request; the \code{ForkingMixIn} and \code{ThreadingMixIn} classes do this. \end{funcdesc} % Is there any point in documenting the following two functions? % What would the purpose of overriding them be: initializing server % instance variables, adding new network families? \begin{funcdesc}{server_activate}{} Called by the server's constructor to activate the server. May be overridden. \end{funcdesc} \begin{funcdesc}{server_bind}{} Called by the server's constructor to bind the socket to the desired address. May be overridden. \end{funcdesc} \begin{funcdesc}{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. \end{funcdesc} The request handler class must define a new \code{handle} method, and can override any of the following methods. A new instance is created for each request. \begin{funcdesc}{finish}{} Called after the \code{handle} method to perform any clean-up actions required. The default implementation does nothing. If \code{setup()} or \code{handle()} raise an exception, this function will not be called. \end{funcdesc} \begin{funcdesc}{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 \code{self.request}; the client address as \code{self.client_request}; and the server instance as \code{self.server}, in case it needs access to per-server information. The type of \code{self.request} is different for datagram or stream services. For stream services, \code{self.request} is a socket object; for datagram services, \code{self.request} is a string. However, this can be hidden by using the mix-in request handler classes \code{StreamRequestHandler} or \code{DatagramRequestHandler}, which override the \code{setup} and \code{finish} methods, and provides \code{self.rfile} and \code{self.wfile} attributes. \code{self.rfile} and \code{self.wfile} can be read or written, respectively, to get the request data or return data to the client. \end{funcdesc} \begin{funcdesc}{setup}{} Called before the \code{handle} method to perform any initialization actions required. The default implementation does nothing. \end{funcdesc}