Doc/lib/libsocksvr.tex

   1 \section{\module{SocketServer} ---
   2          A framework for network servers}
   3
   4 \declaremodule{standard}{SocketServer}
   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}{allow_reuse_address}
  98 Whether the server will allow the reuse of an address. This defaults
  99 to true, and can be set in subclasses to change the policy.
 100 \end{datadesc}
 101
 102 \begin{datadesc}{request_queue_size}
 103 The size of the request queue.  If it takes a long time to process a
 104 single request, any requests that arrive while the server is busy are
 105 placed into a queue, up to \member{request_queue_size} requests.  Once
 106 the queue is full, further requests from clients will get a
 107 ``Connection denied'' error.  The default value is usually 5, but this
 108 can be overridden by subclasses.
 109 \end{datadesc}
 110
 111 \begin{datadesc}{socket_type}
 112 The type of socket used by the server; \constant{socket.SOCK_STREAM}
 113 and \constant{socket.SOCK_DGRAM} are two possible values.
 114 \end{datadesc}
 115
 116 There are various server methods that can be overridden by subclasses
 117 of base server classes like \class{TCPServer}; these methods aren't
 118 useful to external users of the server object.
 119
 120 % should the default implementations of these be documented, or should
 121 % it be assumed that the user will look at SocketServer.py?
 122
 123 \begin{funcdesc}{finish_request}{}
 124 Actually processes the request by instantiating
 125 \member{RequestHandlerClass} and calling its \method{handle()} method.
 126 \end{funcdesc}
 127
 128 \begin{funcdesc}{get_request}{}
 129 Must accept a request from the socket, and return a 2-tuple containing
 130 the \emph{new} socket object to be used to communicate with the
 131 client, and the client's address.
 132 \end{funcdesc}
 133
 134 \begin{funcdesc}{handle_error}{request, client_address}
 135 This function is called if the \member{RequestHandlerClass}'s
 136 \method{handle()} method raises an exception.  The default action is
 137 to print the traceback to standard output and continue handling
 138 further requests.
 139 \end{funcdesc}
 140
 141 \begin{funcdesc}{process_request}{request, client_address}
 142 Calls \method{finish_request()} to create an instance of the
 143 \member{RequestHandlerClass}.  If desired, this function can create a
 144 new process or thread to handle the request; the \class{ForkingMixIn}
 145 and \class{ThreadingMixIn} classes do this.
 146 \end{funcdesc}
 147
 148 % Is there any point in documenting the following two functions?
 149 % What would the purpose of overriding them be: initializing server
 150 % instance variables, adding new network families?
 151
 152 \begin{funcdesc}{server_activate}{}
 153 Called by the server's constructor to activate the server.
 154 May be overridden.
 155 \end{funcdesc}
 156
 157 \begin{funcdesc}{server_bind}{}
 158 Called by the server's constructor to bind the socket to the desired
 159 address.  May be overridden.
 160 \end{funcdesc}
 161
 162 \begin{funcdesc}{verify_request}{request, client_address}
 163 Must return a Boolean value; if the value is true, the request will be
 164 processed, and if it's false, the request will be denied.
 165 This function can be overridden to implement access controls for a server.
 166 The default implementation always return true.
 167 \end{funcdesc}
 168
 169 The request handler class must define a new \method{handle()} method,
 170 and can override any of the following methods.  A new instance is
 171 created for each request.
 172
 173 \begin{funcdesc}{finish}{}
 174 Called after the \method{handle()} method to perform any clean-up
 175 actions required.  The default implementation does nothing.  If
 176 \method{setup()} or \method{handle()} raise an exception, this
 177 function will not be called.
 178 \end{funcdesc}
 179
 180 \begin{funcdesc}{handle}{}
 181 This function must do all the work required to service a request.
 182 Several instance attributes are available to it; the request is
 183 available as \member{self.request}; the client address as
 184 \member{self.client_address}; and the server instance as
 185 \member{self.server}, in case it needs access to per-server
 186 information.
 187
 188 The type of \member{self.request} is different for datagram or stream
 189 services.  For stream services, \member{self.request} is a socket
 190 object; for datagram services, \member{self.request} is a string.
 191 However, this can be hidden by using the mix-in request handler
 192 classes
 193 \class{StreamRequestHandler} or \class{DatagramRequestHandler}, which
 194 override the \method{setup()} and \method{finish()} methods, and
 195 provides \member{self.rfile} and \member{self.wfile} attributes.
 196 \member{self.rfile} and \member{self.wfile} can be read or written,
 197 respectively, to get the request data or return data to the client.
 198 \end{funcdesc}
 199
 200 \begin{funcdesc}{setup}{}
 201 Called before the \method{handle()} method to perform any
 202 initialization actions required.  The default implementation does
 203 nothing.
 204 \end{funcdesc}