Beginning Python - From Novice to Professional

CHAPTER 27 ■ PROJECT 8: FILE SHARING WITH XML-RPC 501
Useful Tools
In this project, you’ll use quite a few standard library modules. I won’t describe them all in
detail; refer to Chapter 10 and the Python Library Reference for additional details.
The main modules you’ll be using are xmlrpclib and its close friend SimpleXMLRPCServer.
The xmlrpclib module is discussed in Chapter 15, and SimpleXMLRPCServer is explored in more
detail in this chapter.
■Caution Make sure you have an updated version of xmlrpclib and SimpleXMLRPCServer to avoid
some security problems present in older versions. See the caution in the “XML-RPC” section of Chapter 15.
For the interface to the file sharing program, you’ll be using a module from the standard
library, called cmd (already mentioned in Chapter 24); to get some (very limited) parallelism,
you’ll use the threading module, and to extract the components of a URL, you’ll use the
urlparse module. All of these modules are explained later in the chapter.
Other modules you might want to brush up on are random, string, time, and os.path.
Preparations
First of all, you must make sure you have the proper libraries (that you can import xmlrpclib
and SimpleXMLRPCServer). Make sure you’ve got an updated version of SimpleXMLRPCServer.
You don’t strictly have to be connected to a network to use the software in this project, but
it will make things more interesting. If you have access to two (or more) separate machines that
are connected, you can run the software on each of these machines and have them communicate
with each other. For testing purposes, it is also possible to run multiple file sharing nodes
on the same machine.
First Implementation
Before you can write a first prototype of the Node class (a single node or peer in the system), you
have to learn a bit about how the SimpleXMLRPCServer class works. It is instantiated with a tuple
of the form (servername, port). The server name is the name of the machine on which the
server will run (you can use an empty string here to indicate localhost, the machine where
you’re actually executing the program). The port number can be any port you have access to,
typically 1024 and above.
After you have instantiated the server, you may register an instance that implements its
“remote methods,” with the register_instance method. Alternatively, you can register individual
functions with the register_function method. When you’re ready to run the server (so
that it can respond to requests from outside), you call its method serve_forever. You can easily
try this out. Start two interactive Python interpreters. In the first one, enter the code that appears
after the caution.