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 When inheriting from \class{ThreadingMixIn} for threaded connection
  41 behavior, you should explicitly declare how you want your threads
  42 to behave on an abrupt shutdown. The \class{ThreadingMixIn} class
  43 defines an attribute \var{daemon_threads}, which indicates whether
  44 or not the server should wait for thread termination. You should
  45 set the flag explicitly if you would like threads to behave
  46 autonomously; the default is \constant{False}, meaning that Python
  47 will not exit until all threads created by \class{ThreadingMixIn} have
  48 exited.
  49
  50 Server classes have the same external methods and attributes, no
  51 matter what network protocol they use:
  52
  53 \setindexsubitem{(SocketServer protocol)}
  54
  55 %XXX should data and methods be intermingled, or separate?
  56 % how should the distinction between class and instance variables be
  57 % drawn?
  58
  59 \begin{funcdesc}{fileno}{}
  60 Return an integer file descriptor for the socket on which the server
  61 is listening.  This function is most commonly passed to
  62 \function{select.select()}, to allow monitoring multiple servers in the
  63 same process.
  64 \end{funcdesc}
  65
  66 \begin{funcdesc}{handle_request}{}
  67 Process a single request.  This function calls the following methods
  68 in order: \method{get_request()}, \method{verify_request()}, and
  69 \method{process_request()}.  If the user-provided \method{handle()}
  70 method of the handler class raises an exception, the server's
  71 \method{handle_error()} method will be called.
  72 \end{funcdesc}
  73
  74 \begin{funcdesc}{serve_forever}{}
  75 Handle an infinite number of requests.  This simply calls
  76 \method{handle_request()} inside an infinite loop.
  77 \end{funcdesc}
  78
  79 \begin{datadesc}{address_family}
  80 The family of protocols to which the server's socket belongs.
  81 \constant{socket.AF_INET} and \constant{socket.AF_UNIX} are two
  82 possible values.
  83 \end{datadesc}
  84
  85 \begin{datadesc}{RequestHandlerClass}
  86 The user-provided request handler class; an instance of this class is
  87 created for each request.
  88 \end{datadesc}
  89
  90 \begin{datadesc}{server_address}
  91 The address on which the server is listening.  The format of addresses
  92 varies depending on the protocol family; see the documentation for the
  93 socket module for details.  For Internet protocols, this is a tuple
  94 containing a string giving the address, and an integer port number:
  95 \code{('127.0.0.1', 80)}, for example.
  96 \end{datadesc}
  97
  98 \begin{datadesc}{socket}
  99 The socket object on which the server will listen for incoming requests.
 100 \end{datadesc}
 101
 102 % XXX should class variables be covered before instance variables, or
 103 % vice versa?
 104
 105 The server classes support the following class variables:
 106
 107 \begin{datadesc}{allow_reuse_address}
 108 Whether the server will allow the reuse of an address. This defaults
 109 to \constant{False}, and can be set in subclasses to change the policy.
 110 \end{datadesc}
 111
 112 \begin{datadesc}{request_queue_size}
 113 The size of the request queue.  If it takes a long time to process a
 114 single request, any requests that arrive while the server is busy are
 115 placed into a queue, up to \member{request_queue_size} requests.  Once
 116 the queue is full, further requests from clients will get a
 117 ``Connection denied'' error.  The default value is usually 5, but this
 118 can be overridden by subclasses.
 119 \end{datadesc}
 120
 121 \begin{datadesc}{socket_type}
 122 The type of socket used by the server; \constant{socket.SOCK_STREAM}
 123 and \constant{socket.SOCK_DGRAM} are two possible values.
 124 \end{datadesc}
 125
 126 There are various server methods that can be overridden by subclasses
 127 of base server classes like \class{TCPServer}; these methods aren't
 128 useful to external users of the server object.
 129
 130 % should the default implementations of these be documented, or should
 131 % it be assumed that the user will look at SocketServer.py?
 132
 133 \begin{funcdesc}{finish_request}{}
 134 Actually processes the request by instantiating
 135 \member{RequestHandlerClass} and calling its \method{handle()} method.
 136 \end{funcdesc}
 137
 138 \begin{funcdesc}{get_request}{}
 139 Must accept a request from the socket, and return a 2-tuple containing
 140 the \emph{new} socket object to be used to communicate with the
 141 client, and the client's address.
 142 \end{funcdesc}
 143
 144 \begin{funcdesc}{handle_error}{request, client_address}
 145 This function is called if the \member{RequestHandlerClass}'s
 146 \method{handle()} method raises an exception.  The default action is
 147 to print the traceback to standard output and continue handling
 148 further requests.
 149 \end{funcdesc}
 150
 151 \begin{funcdesc}{process_request}{request, client_address}
 152 Calls \method{finish_request()} to create an instance of the
 153 \member{RequestHandlerClass}.  If desired, this function can create a
 154 new process or thread to handle the request; the \class{ForkingMixIn}
 155 and \class{ThreadingMixIn} classes do this.
 156 \end{funcdesc}
 157
 158 % Is there any point in documenting the following two functions?
 159 % What would the purpose of overriding them be: initializing server
 160 % instance variables, adding new network families?
 161
 162 \begin{funcdesc}{server_activate}{}
 163 Called by the server's constructor to activate the server.
 164 May be overridden.
 165 \end{funcdesc}
 166
 167 \begin{funcdesc}{server_bind}{}
 168 Called by the server's constructor to bind the socket to the desired
 169 address.  May be overridden.
 170 \end{funcdesc}
 171
 172 \begin{funcdesc}{verify_request}{request, client_address}
 173 Must return a Boolean value; if the value is \constant{True}, the request will be
 174 processed, and if it's \constant{False}, the request will be denied.
 175 This function can be overridden to implement access controls for a server.
 176 The default implementation always returns \constant{True}.
 177 \end{funcdesc}
 178
 179 The request handler class must define a new \method{handle()} method,
 180 and can override any of the following methods.  A new instance is
 181 created for each request.
 182
 183 \begin{funcdesc}{finish}{}
 184 Called after the \method{handle()} method to perform any clean-up
 185 actions required.  The default implementation does nothing.  If
 186 \method{setup()} or \method{handle()} raise an exception, this
 187 function will not be called.
 188 \end{funcdesc}
 189
 190 \begin{funcdesc}{handle}{}
 191 This function must do all the work required to service a request.
 192 Several instance attributes are available to it; the request is
 193 available as \member{self.request}; the client address as
 194 \member{self.client_address}; and the server instance as
 195 \member{self.server}, in case it needs access to per-server
 196 information.
 197
 198 The type of \member{self.request} is different for datagram or stream
 199 services.  For stream services, \member{self.request} is a socket
 200 object; for datagram services, \member{self.request} is a string.
 201 However, this can be hidden by using the mix-in request handler
 202 classes
 203 \class{StreamRequestHandler} or \class{DatagramRequestHandler}, which
 204 override the \method{setup()} and \method{finish()} methods, and
 205 provides \member{self.rfile} and \member{self.wfile} attributes.
 206 \member{self.rfile} and \member{self.wfile} can be read or written,
 207 respectively, to get the request data or return data to the client.
 208 \end{funcdesc}
 209
 210 \begin{funcdesc}{setup}{}
 211 Called before the \method{handle()} method to perform any
 212 initialization actions required.  The default implementation does
 213 nothing.
 214 \end{funcdesc}