Doc/lib/libsocksvr.tex

   1 \section{\module{SocketServer} ---
   2          A framework for network servers.}
   3 \declaremodule{standard}{SocketServer}
   4
   5 \modulesynopsis{A framework for network servers.}
   6
   7
   8 The \module{SocketServer} module simplifies the task of writing network
   9 servers.
  10
  11 There are four basic server classes: \class{TCPServer} uses the
  12 Internet TCP protocol, which provides for continuous streams of data
  13 between the client and server.  \class{UDPServer} uses datagrams, which
  14 are discrete packets of information that may arrive out of order or be
  15 lost while in transit.  The more infrequently used
  16 \class{UnixStreamServer} and \class{UnixDatagramServer} classes are
  17 similar, but use \UNIX{} domain sockets; they're not available on
  18 non-\UNIX{} platforms.  For more details on network programming, consult
  19 a book such as W. Richard Steven's \citetitle{UNIX Network Programming}
  20 or Ralph Davis's \citetitle{Win32 Network Programming}.
  21
  22 These four classes process requests \dfn{synchronously}; each request
  23 must be completed before the next request can be started.  This isn't
  24 suitable if each request takes a long time to complete, because it
  25 requires a lot of computation, or because it returns a lot of data
  26 which the client is slow to process.  The solution is to create a
  27 separate process or thread to handle each request; the
  28 \class{ForkingMixIn} and \class{ThreadingMixIn} mix-in classes can be
  29 used to support asynchronous behaviour.
  30
  31 Creating a server requires several steps.  First, you must create a
  32 request handler class by subclassing the \class{BaseRequestHandler}
  33 class and overriding its \method{handle()} method; this method will
  34 process incoming requests.  Second, you must instantiate one of the
  35 server classes, passing it the server's address and the request
  36 handler class.  Finally, call the \method{handle_request()} or
  37 \method{serve_forever()} method of the server object to process one or
  38 many requests.
  39
  40 Server classes have the same external methods and attributes, no
  41 matter what network protocol they use:
  42
  43 \setindexsubitem{(SocketServer protocol)}
  44
  45 %XXX should data and methods be intermingled, or separate?
  46 % how should the distinction between class and instance variables be
  47 % drawn?
  48
  49 \begin{funcdesc}{fileno}{}
  50 Return an integer file descriptor for the socket on which the server
  51 is listening.  This function is most commonly passed to
  52 \function{select.select()}, to allow monitoring multiple servers in the
  53 same process.
  54 \end{funcdesc}
  55
  56 \begin{funcdesc}{handle_request}{}
  57 Process a single request.  This function calls the following methods
  58 in order: \method{get_request()}, \method{verify_request()}, and
  59 \method{process_request()}.  If the user-provided \method{handle()}
  60 method of the handler class raises an exception, the server's
  61 \method{handle_error()} method will be called.
  62 \end{funcdesc}
  63
  64 \begin{funcdesc}{serve_forever}{}
  65 Handle an infinite number of requests.  This simply calls
  66 \method{handle_request()} inside an infinite loop.
  67 \end{funcdesc}
  68
  69 \begin{datadesc}{address_family}
  70 The family of protocols to which the server's socket belongs.
  71 \constant{socket.AF_INET} and \constant{socket.AF_UNIX} are two
  72 possible values.
  73 \end{datadesc}
  74
  75 \begin{datadesc}{RequestHandlerClass}
  76 The user-provided request handler class; an instance of this class is
  77 created for each request.
  78 \end{datadesc}
  79
  80 \begin{datadesc}{server_address}
  81 The address on which the server is listening.  The format of addresses
  82 varies depending on the protocol family; see the documentation for the
  83 socket module for details.  For Internet protocols, this is a tuple
  84 containing a string giving the address, and an integer port number:
  85 \code{('127.0.0.1', 80)}, for example.
  86 \end{datadesc}
  87
  88 \begin{datadesc}{socket}
  89 The socket object on which the server will listen for incoming requests.
  90 \end{datadesc}
  91
  92 % XXX should class variables be covered before instance variables, or
  93 % vice versa?
  94
  95 The server classes support the following class variables:
  96
  97 \begin{datadesc}{request_queue_size}
  98 The size of the request queue.  If it takes a long time to process a
  99 single request, any requests that arrive while the server is busy are
 100 placed into a queue, up to \member{request_queue_size} requests.  Once
 101 the queue is full, further requests from clients will get a
 102 ``Connection denied'' error.  The default value is usually 5, but this
 103 can be overridden by subclasses.
 104 \end{datadesc}
 105
 106 \begin{datadesc}{socket_type}
 107 The type of socket used by the server; \constant{socket.SOCK_STREAM}
 108 and \constant{socket.SOCK_DGRAM} are two possible values.
 109 \end{datadesc}
 110
 111 There are various server methods that can be overridden by subclasses
 112 of base server classes like \class{TCPServer}; these methods aren't
 113 useful to external users of the server object.
 114
 115 % should the default implementations of these be documented, or should
 116 % it be assumed that the user will look at SocketServer.py?
 117
 118 \begin{funcdesc}{finish_request}{}
 119 Actually processes the request by instantiating
 120 \member{RequestHandlerClass} and calling its \method{handle()} method.
 121 \end{funcdesc}
 122
 123 \begin{funcdesc}{get_request}{}
 124 Must accept a request from the socket, and return a 2-tuple containing
 125 the \emph{new} socket object to be used to communicate with the
 126 client, and the client's address.
 127 \end{funcdesc}
 128
 129 \begin{funcdesc}{handle_error}{request, client_address}
 130 This function is called if the \member{RequestHandlerClass}'s
 131 \method{handle()} method raises an exception.  The default action is
 132 to print the traceback to standard output and continue handling
 133 further requests.
 134 \end{funcdesc}
 135
 136 \begin{funcdesc}{process_request}{request, client_address}
 137 Calls \method{finish_request()} to create an instance of the
 138 \member{RequestHandlerClass}.  If desired, this function can create a
 139 new process or thread to handle the request; the \class{ForkingMixIn}
 140 and \class{ThreadingMixIn} classes do this.
 141 \end{funcdesc}
 142
 143 % Is there any point in documenting the following two functions?
 144 % What would the purpose of overriding them be: initializing server
 145 % instance variables, adding new network families?
 146
 147 \begin{funcdesc}{server_activate}{}
 148 Called by the server's constructor to activate the server.
 149 May be overridden.
 150 \end{funcdesc}
 151
 152 \begin{funcdesc}{server_bind}{}
 153 Called by the server's constructor to bind the socket to the desired
 154 address.  May be overridden.
 155 \end{funcdesc}
 156
 157 \begin{funcdesc}{verify_request}{request, client_address}
 158 Must return a Boolean value; if the value is true, the request will be
 159 processed, and if it's false, the request will be denied.
 160 This function can be overridden to implement access controls for a server.
 161 The default implementation always return true.
 162 \end{funcdesc}
 163
 164 The request handler class must define a new \method{handle()} method,
 165 and can override any of the following methods.  A new instance is
 166 created for each request.
 167
 168 \begin{funcdesc}{finish}{}
 169 Called after the \method{handle()} method to perform any clean-up
 170 actions required.  The default implementation does nothing.  If
 171 \method{setup()} or \method{handle()} raise an exception, this
 172 function will not be called.
 173 \end{funcdesc}
 174
 175 \begin{funcdesc}{handle}{}
 176 This function must do all the work required to service a request.
 177 Several instance attributes are available to it; the request is
 178 available as \member{self.request}; the client address as
 179 \member{self.client_address}; and the server instance as
 180 \member{self.server}, in case it needs access to per-server
 181 information.
 182
 183 The type of \member{self.request} is different for datagram or stream
 184 services.  For stream services, \member{self.request} is a socket
 185 object; for datagram services, \member{self.request} is a string.
 186 However, this can be hidden by using the mix-in request handler
 187 classes
 188 \class{StreamRequestHandler} or \class{DatagramRequestHandler}, which
 189 override the \method{setup()} and \method{finish()} methods, and
 190 provides \member{self.rfile} and \member{self.wfile} attributes.
 191 \member{self.rfile} and \member{self.wfile} can be read or written,
 192 respectively, to get the request data or return data to the client.
 193 \end{funcdesc}
 194
 195 \begin{funcdesc}{setup}{}
 196 Called before the \method{handle()} method to perform any
 197 initialization actions required.  The default implementation does
 198 nothing.
 199 \end{funcdesc}