SocketServer¶

Generic socket server classes.

This module tries to capture the various aspects of defining a server:

For socket-based servers:

• address family:

  • AF_INET{,6}: IP (Internet Protocol) sockets (default)
  • AF_UNIX: Unix domain sockets
  • others, e.g. AF_DECNET are conceivable (see <socket.h>

• socket type:

  • SOCK_STREAM (reliable stream, e.g. TCP)
  • SOCK_DGRAM (datagrams, e.g. UDP)

For request-based servers (including socket-based):

• client address verification before further looking at the request

  (This is actually a hook for any processing that needs to look

  at the request before anything else, e.g. logging)

• how to handle multiple requests:

  • synchronous (one request is handled at a time)
  • forking (each request is handled by a new process)
  • threading (each request is handled by a new thread)

The classes in this module favor the server type that is simplest to write: a synchronous TCP/IP server. This is bad class design, but save some typing. (There’s also the issue that a deep class hierarchy slows down method lookups.)

There are five classes in an inheritance diagram, four of which represent synchronous servers of four types:

BaseServer

v

+———–+ +——————+ | TCPServer |——->| UnixStreamServer | +———–+ +——————+

v

+———–+ +——————–+ | UDPServer |——->| UnixDatagramServer | +———–+ +——————–+

Note that UnixDatagramServer derives from UDPServer, not from UnixStreamServer – the only difference between an IP and a Unix stream server is the address family, which is simply repeated in both unix server classes.

Forking and threading versions of each type of server can be created using the ForkingMixIn and ThreadingMixIn mix-in classes. For instance, a threading UDP server class is created as follows:

class ThreadingUDPServer(ThreadingMixIn, UDPServer): pass

The Mix-in class must come first, since it overrides a method defined in UDPServer! Setting the various member variables also changes the behavior of the underlying server mechanism.

To implement a service, you must derive a class from BaseRequestHandler and redefine its handle() method. You can then run various versions of the service by combining one of the server classes with your request handler class.

The request handler class must be different for datagram or stream services. This can be hidden by using the request handler subclasses StreamRequestHandler or DatagramRequestHandler.

Of course, you still have to use your head!

For instance, it makes no sense to use a forking server if the service contains state in memory that can be modified by requests (since the modifications in the child process would never reach the initial state kept in the parent process and passed to each child). In this case, you can use a threading server, but you will probably have to use locks to avoid two requests that come in nearly simultaneous to apply conflicting changes to the server state.

On the other hand, if you are building e.g. an HTTP server, where all data is stored externally (e.g. in the file system), a synchronous class will essentially render the service “deaf” while one request is being handled – which may be for a very long time if a client is slow to read all the data it has requested. Here a threading or forking server is appropriate.

In some cases, it may be appropriate to process part of a request synchronously, but to finish processing in a forked child depending on the request data. This can be implemented by using a synchronous server and doing an explicit fork in the request handler class handle() method.

Another approach to handling multiple simultaneous requests in an environment that supports neither threads nor fork (or where these are too expensive or inappropriate for the service) is to maintain an explicit table of partially finished requests and to use select() to decide which request to work on next (or whether to handle a new incoming request). This is particularly important for stream services where each client can potentially be connected for a long time (if threads or subprocesses cannot be used).

Future work: - Standard classes for Sun RPC (which uses either UDP or TCP) - Standard mix-in classes to implement various authentication

and encryption schemes

• Standard framework for select-based multiplexing

XXX Open problems: - What to do with out-of-band data?

BaseServer: - split generic “request” functionality out into BaseServer class.

Copyright (C) 2000 Luke Kenneth Casson Leighton <lkcl@samba.org>

example: read entries from a SQL database (requires overriding get_request() to return a table entry from the database). entry is processed by a RequestHandlerClass.