\section{\module{SocketServer} ---         A framework for network servers} \declaremodule{standard}{SocketServer}\modulesynopsis{A framework for network servers.}  The \module{SocketServer} module simplifies the task of writing networkservers. There are four basic server classes: \class{TCPServer} uses theInternet TCP protocol, which provides for continuous streams of databetween the client and server.  \class{UDPServer} uses datagrams, whichare discrete packets of information that may arrive out of order or belost while in transit.  The more infrequently used\class{UnixStreamServer} and \class{UnixDatagramServer} classes aresimilar, but use \UNIX{} domain sockets; they're not available onnon-\UNIX{} platforms.  For more details on network programming, consulta book such as W. Richard Steven's \citetitle{UNIX Network Programming}or Ralph Davis's \citetitle{Win32 Network Programming}. These four classes process requests \dfn{synchronously}; each requestmust be completed before the next request can be started.  This isn'tsuitable if each request takes a long time to complete, because itrequires a lot of computation, or because it returns a lot of datawhich the client is slow to process.  The solution is to create aseparate process or thread to handle each request; the\class{ForkingMixIn} and \class{ThreadingMixIn} mix-in classes can beused to support asynchronous behaviour. Creating a server requires several steps.  First, you must create arequest handler class by subclassing the \class{BaseRequestHandler}class and overriding its \method{handle()} method; this method willprocess incoming requests.  Second, you must instantiate one of theserver classes, passing it the server's address and the requesthandler class.  Finally, call the \method{handle_request()} or\method{serve_forever()} method of the server object to process one ormany requests. When inheriting from \class{ThreadingMixIn} for threaded connectionbehavior, you should explicitly declare how you want your threadsto behave on an abrupt shutdown. The \class{ThreadingMixIn} classdefines an attribute \var{daemon_threads}, which indicates whetheror not the server should wait for thread termination. You shouldset the flag explicitly if you would like threads to behaveautonomously; the default is \constant{False}, meaning that Pythonwill not exit until all threads created by \class{ThreadingMixIn} haveexited. Server classes have the same external methods and attributes, nomatter what network protocol they use: \setindexsubitem{(SocketServer protocol)} \subsection{Server Creation Notes} There are five classes in an inheritance diagram, four of which representsynchronous servers of four types: \begin{verbatim}        +------------+        | BaseServer |        +------------+              |              v        +-----------+        +------------------+        | TCPServer |------->| UnixStreamServer |        +-----------+        +------------------+              |              v        +-----------+        +--------------------+        | UDPServer |------->| UnixDatagramServer |        +-----------+        +--------------------+\end{verbatim} Note that \class{UnixDatagramServer} derives from \class{UDPServer}, notfrom \class{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 usingthe \class{ForkingMixIn} and \class{ThreadingMixIn} mix-in classes.  Forinstance, a threading UDP server class is created as follows: \begin{verbatim}    class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass\end{verbatim} The mix-in class must come first, since it overrides a method defined in\class{UDPServer}.  Setting the various member variables also changes thebehavior of the underlying server mechanism. To implement a service, you must derive a class from\class{BaseRequestHandler} and redefine its \method{handle()} method.  Youcan then run various versions of the service by combining one of the serverclasses with your request handler class.  The request handler class must bedifferent for datagram or stream services.  This can be hidden by using thehandler subclasses \class{StreamRequestHandler} or \class{DatagramRequestHandler}. Of course, you still have to use your head!  For instance, it makes no senseto use a forking server if the service contains state in memory that can bemodified by different requests, since the modifications in the child processwould never reach the initial state kept in the parent process and passed toeach child.  In this case, you can use a threading server, but you willprobably 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 isstored externally (for instance, in the file system), a synchronous classwill essentially render the service "deaf" while one request is beinghandled -- which may be for a very long time if a client is slow to receiveall the data it has requested.  Here a threading or forking server isappropriate. In some cases, it may be appropriate to process part of a requestsynchronously, but to finish processing in a forked child depending on therequest data.  This can be implemented by using a synchronous server anddoing an explicit fork in the request handler class \method{handle()}method. Another approach to handling multiple simultaneous requests in anenvironment that supports neither threads nor \function{fork()} (or wherethese are too expensive or inappropriate for the service) is to maintain anexplicit table of partially finished requests and to use \function{select()}to decide which request to work on next (or whether to handle a new incomingrequest).  This is particularly important for stream services where eachclient can potentially be connected for a long time (if threads orsubprocesses cannot be used). %XXX should data and methods be intermingled, or separate?% how should the distinction between class and instance variables be% drawn? \subsection{Server Objects} \begin{funcdesc}{fileno}{}Return an integer file descriptor for the socket on which the serveris listening.  This function is most commonly passed to\function{select.select()}, to allow monitoring multiple servers in thesame process.\end{funcdesc} \begin{funcdesc}{handle_request}{}Process a single request.  This function calls the following methodsin order: \method{get_request()}, \method{verify_request()}, and\method{process_request()}.  If the user-provided \method{handle()}method of the handler class raises an exception, the server's\method{handle_error()} method will be called.\end{funcdesc} \begin{funcdesc}{serve_forever}{}Handle an infinite number of requests.  This simply calls\method{handle_request()} inside an infinite loop.\end{funcdesc} \begin{datadesc}{address_family}The family of protocols to which the server's socket belongs.\constant{socket.AF_INET} and \constant{socket.AF_UNIX} are twopossible values.\end{datadesc} \begin{datadesc}{RequestHandlerClass}The user-provided request handler class; an instance of this class iscreated for each request.\end{datadesc} \begin{datadesc}{server_address}The address on which the server is listening.  The format of addressesvaries depending on the protocol family; see the documentation for thesocket module for details.  For Internet protocols, this is a tuplecontaining 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}{allow_reuse_address}Whether the server will allow the reuse of an address. This defaultsto \constant{False}, and can be set in subclasses to change the policy.\end{datadesc} \begin{datadesc}{request_queue_size}The size of the request queue.  If it takes a long time to process asingle request, any requests that arrive while the server is busy areplaced into a queue, up to \member{request_queue_size} requests.  Oncethe queue is full, further requests from clients will get a``Connection denied'' error.  The default value is usually 5, but thiscan be overridden by subclasses.\end{datadesc} \begin{datadesc}{socket_type}The type of socket used by the server; \constant{socket.SOCK_STREAM}and \constant{socket.SOCK_DGRAM} are two possible values.\end{datadesc} There are various server methods that can be overridden by subclassesof base server classes like \class{TCPServer}; these methods aren'tuseful 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\member{RequestHandlerClass} and calling its \method{handle()} method.\end{funcdesc} \begin{funcdesc}{get_request}{}Must accept a request from the socket, and return a 2-tuple containingthe \emph{new} socket object to be used to communicate with theclient, and the client's address.\end{funcdesc} \begin{funcdesc}{handle_error}{request, client_address}This function is called if the \member{RequestHandlerClass}'s\method{handle()} method raises an exception.  The default action isto print the traceback to standard output and continue handlingfurther requests.\end{funcdesc} \begin{funcdesc}{process_request}{request, client_address}Calls \method{finish_request()} to create an instance of the\member{RequestHandlerClass}.  If desired, this function can create anew process or thread to handle the request; the \class{ForkingMixIn}and \class{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.  The defaultbehavior just \method{listen}s to the server's socket.May be overridden.\end{funcdesc} \begin{funcdesc}{server_bind}{}Called by the server's constructor to bind the socket to the desiredaddress.  May be overridden.\end{funcdesc} \begin{funcdesc}{verify_request}{request, client_address}Must return a Boolean value; if the value is \constant{True}, the request will beprocessed, and if it's \constant{False}, the request will be denied.This function can be overridden to implement access controls for a server.The default implementation always returns \constant{True}.\end{funcdesc} \subsection{RequestHandler Objects} The request handler class must define a new \method{handle()} method,and can override any of the following methods.  A new instance iscreated for each request. \begin{funcdesc}{finish}{}Called after the \method{handle()} method to perform any clean-upactions required.  The default implementation does nothing.  If\method{setup()} or \method{handle()} raise an exception, thisfunction will not be called.\end{funcdesc} \begin{funcdesc}{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 isavailable as \member{self.request}; the client address as\member{self.client_address}; and the server instance as\member{self.server}, in case it needs access to per-serverinformation. The type of \member{self.request} is different for datagram or streamservices.  For stream services, \member{self.request} is a socketobject; for datagram services, \member{self.request} is a string.However, this can be hidden by using the  request handler subclasses\class{StreamRequestHandler} or \class{DatagramRequestHandler}, whichoverride the \method{setup()} and \method{finish()} methods, andprovide \member{self.rfile} and \member{self.wfile} attributes.\member{self.rfile} and \member{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 \method{handle()} method to perform anyinitialization actions required.  The default implementation doesnothing.\end{funcdesc}