# 18.18 `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.

When inheriting from `ThreadingMixIn` for threaded connection behavior, you should explicitly declare how you want your threads to behave on an abrupt shutdown. The `ThreadingMixIn` class defines an attribute daemon_threads, which indicates whether or not the server should wait for thread termination. You should set the flag explicitly if you would like threads to behave autonomously; the default is `False`, meaning that Python will not exit until all threads created by `ThreadingMixIn` have exited.

Server classes have the same external methods and attributes, no matter what network protocol they use: