# 11.15 `SocketServer` -- A framework for network servers

The `SocketServer` module simplifies the task of writing network servers.

There are four basic server classes: `TCPServer` uses the Internet TCP protocol, which provides for continuous streams of data between the client and server. `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 `UnixStreamServer` and `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 UNIX Network Programming or Ralph Davis's Win32 Network Programming.

These four classes process requests 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 `ForkingMixIn` and `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 `BaseRequestHandler` class and overriding its `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 `handle_request()` or `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:

`fileno`()
  Return an integer file descriptor for the socket on which the server is listening. This function is most commonly passed to `select.select()`, to allow monitoring multiple servers in the same process.

`handle_request`()
  Process a single request. This function calls the following methods in order: `get_request()`, `verify_request()`, and `process_request()`. If the user-provided `handle()` method of the handler class raises an exception, the server's `handle_error()` method will be called.

`serve_forever`()
  Handle an infinite number of requests. This simply calls `handle_request()` inside an infinite loop.

`address_family`
  The family of protocols to which the server's socket belongs. `socket.AF_INET` and `socket.AF_UNIX` are two possible values.

`RequestHandlerClass`
  The user-provided request handler class; an instance of this class is created for each request.

`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: `('127.0.0.1', 80)`, for example.

`socket`
  The socket object on which the server will listen for incoming requests.

The server classes support the following class variables:

`allow_reuse_address`
  Whether the server will allow the reuse of an address. This defaults to `False`, and can be set in subclasses to change the policy.

`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 `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.

`socket_type`
  The type of socket used by the server; `socket.SOCK_STREAM` and `socket.SOCK_DGRAM` are two possible values.

There are various server methods that can be overridden by subclasses of base server classes like `TCPServer`; these methods aren't useful to external users of the server object.

`finish_request`()
  Actually processes the request by instantiating `RequestHandlerClass` and calling its `handle()` method.

`get_request`()
  Must accept a request from the socket, and return a 2-tuple containing the new socket object to be used to communicate with the client, and the client's address.

`handle_error`(request, client_address)
  This function is called if the `RequestHandlerClass`'s `handle()` method raises an exception. The default action is to print the traceback to standard output and continue handling further requests.

`process_request`(request, client_address)
  Calls `finish_request()` to create an instance of the `RequestHandlerClass`. If desired, this function can create a new process or thread to handle the request; the `ForkingMixIn` and `ThreadingMixIn` classes do this.

`server_activate`()
  Called by the server's constructor to activate the server. May be overridden.

`server_bind`()
  Called by the server's constructor to bind the socket to the desired address. May be overridden.

`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.

The request handler class must define a new `handle()` method, and can override any of the following methods. A new instance is created for each request.

`finish`()
  Called after the `handle()` method to perform any clean-up actions required. The default implementation does nothing. If `setup()` or `handle()` raise an exception, this function will not be called.

`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 `self.request`; the client address as `self.client_address`; and the server instance as `self.server`, in case it needs access to per-server information.

  The type of `self.request` is different for datagram or stream services. For stream services, `self.request` is a socket object; for datagram services, `self.request` is a string. However, this can be hidden by using the mix-in request handler classes `StreamRequestHandler` or `DatagramRequestHandler`, which override the `setup()` and `finish()` methods, and provides `self.rfile` and `self.wfile` attributes. `self.rfile` and `self.wfile` can be read or written, respectively, to get the request data or return data to the client.

`setup`()
  Called before the `handle()` method to perform any initialization actions required. The default implementation does nothing.