Chapter 36. Programs: Clients, Servers, the Internet and the World Wide
Web Services: The xmlrpclib Module
When we looked at HTTP in the section called “Web Servers and the HTTP protocol”, we
were interested in its original use case of serving web pages for
people. We can build on HTTP, creating an interface between software
components, something called a web service. A web
service leverages the essential request-reply nature of HTTP, but takes
the elaborate human-centric HTML web page out of the response. Instead
of sending back something for people to read, web services use XML to
send just the facts.
Web services allow us to have a multi-server architecture. A
central web server provides interaction with people. When a person's
browser makes a request, this central web server can make web service
requests of other servers to gather the information. After gathering the
information, the central server aggregates it and builds the final
HTML-based presentation, which is the reply sent to the human
Web services are an adaptation of HTTP; see the section called “About HTTP” for a summary. Web services rely on
a number of other technologies. There are several competing
alternatives, so we'll look at web services in general before looking at
the xmlrpclib module in particular.
Web Services Overview
There are a number of ways of approaching the problem of
coordinating work between clients and servers. All of these
alternatives have their advantages and disadvantages.
The XML-RPC protcol uses XML notation to make a remote
procedure call (RPC). It works by sending an HTTP request that
contains the name of the procedure to call and the arguments to
that procedure. This protocol uses HTTP "POST" requests to
provide the XML document.
There are two variations on the Simple Object Access
Protocol (SOAP): remote procedure call variation and document.
The RPC variant is basically the next generation of XML-RPC,
where an XML document encodes the name of the procedure and the
arguments. The document variant merely sends an XML document;
the document provides all the information required by the
server. This protocol is heavily supported by additional
standards like Web Services Definition Language (WSDL).
The Representational State Transfer (REST) protcol uses
the HTTP operations (POST, GET, PUT, DELETE) and Uniform
Resource Identifiers (URI) to manipulate remote objects. This
protocol is perhaps the simplest of the web services protocols;
for this reason it is very popular.
The essence of RPC is that we are calling a procedure that
resides on another, remote computer. In order to do this, the argument
values on our local computer must be marshalled and sent through the
internet to this remote service. The service must unmarshall the
argument values, evaluate the procedure, marshall the results, and
send them back to the original requester. Finally, the requester must
unmarshall the response.
We have, therefore, three separate issues that we have to
Packaging the data. This means writing the argument values
in XML notation. We'll see how the
xmlrpclib module handles this
transformation between XML and Python. The Simple Object Access
Protocol (SOAP) is an alternative to the XMLRPC approach to
sending objects from one computer to another; it is not widely
used in the Python community.
Making the client request. This means marshalling the
arguments, making the request, and unmarshalling the response.
Since this is based on HTTP, this is a kind of HTTP client, akin
to what a browser does when it makes a POST request.
Serving requests. This means unmarshalling arguments, doing
something useful, and marshalling a response. Since this is based
on HTTP, this is handling a POST request with an XML request, and
providing an XML reply.
The essential ingredient in making RPC work is to have a local
object which acts as a proxy for the remote service. This
ServerProxy appears as if it is doing the work.
In fact, it is merely marshalling arguments, transmitting the request
via HTTP and unmarshalling the response.
Web Services Client
Let's imagine that a colleague has built a web service which
provides us with an extremely good simulation of a roulette wheel.
(We'll actually build this in the next section.) Our colleague has
provided us with the following summary of this web service.
10.0.1.5. While IP address numbers are the lowest-common
denominator in naming, some people will create Domain Name
Servers (DNS) which provide interesting names instead of numeric
8008. While the basic HTTP service is defined to run on
port 80, you may have other web services which, for security
reasons, aren't available on port 80. Port numbers from 1024 and
up may be allocated for other purposes, so port numbers are
often changed as part of the configuration of a program.
/. Your HTTP handler may have different families or
collections of web services, each with a different path.
Python tuple with the outcome. The tuple includes number,
color, even/odd, high/low for the result.
To create a web services client, we can use the
xmlrpclib module to access an XML-RPC protocol
web service. We'll need to define a proxy for this service.
Example 36.2. wheelclient.py
""" Quick Demo of the spin service.
server= xmlrpclib.ServerProxy( "http://10.0.1.5:8008/" )
for i in range(10):
We import the xmlrpclib
We synthesize the interface information (protocol, host,
port and path) into a URI which identifies the web service.
This statement creates a local object that appears to have all
of the methods that are part of the remote service.
We evalulate the spin method on the remote server, and
we get back a result that we simply print. In this case, we
expect to get a tuple with number, color, even/odd and
high/low attributes of the number.
There are some limitations on what kind of structures can be
marshalled by the XML-RPC protocol. For example, Python makes a
distinction between tuple and list. The XML-RPC protocol, however, can
only create lists.
Web Services Server
A web services is usually built into a more complete web
application framework. Often the server will have a human interface as
well as a web service interface. The human interface will use HTML and
port 80. The web service interface will use XML and some other port
number, usually a number above 8000. Since web services are built in
HTTP, we can adapt our SimpleHTTPServer example
toward providing web services.
A common technique is to have a single server process that
includes a dispatcher. The dispatch method examines the path of the
request to determine which group of web services are being invoked. In
this example, we won't include any path dispatching.
The following example shows how to implement the wheel
Example 36.3. wheelservice.py
import SimpleXMLRPCServer as xmlrpc
class Wheel( object ):
def _color( self, number ):
if number in ("0", "00"): return "GREEN"
elif int(number) in self.redSet: return "RED"
else: return "BLACK"
def _even( self, number ):
if number in ("0", "00"): return "ZERO"
elif int(number) % 2 == 0: return "EVEN"
else: return "ODD"
def _high( self, number ):
if number in ("0", "00"): return "ZERO"
elif int(number) <= 18: return "LOW"
else: return "HIGH"
def __init__( self ):
self.wheel= map( str, range(0,37) ) + ["00"]
def spin( self ):
number= self.rng.choice( self.wheel )
return ( number, self._color(number), self._even(number), self._high(number) )
def spinList( self, spins=1 ):
return [ self.spin() for i in range(spins) ]
service = xmlrpc.SimpleXMLRPCServer(("", 8008))
service.register_instance( theWheel )
if __name__ == "__main__":
We import the SimpleXMLRPCServer
module. Since the name is so long, we provide an alias,
xmlrpc, to make it easier to type. We
also import the random module.
This class contains the state and methods that we will
expose as a web service. We included some private methods,
with names prefaced by _. The
SimpleXMLRPCServer class uses this
"leading _" convention to identify a private
method that isn't published on the web.
We've defined three private methods,
_high. These methods will be used as
part of responding to web service requests.
We initialize an instance of Wheel by creating a random
number generator. We also initialize the set of numbers on the
wheel. In this case, we are folding in double zero to make an
American-style wheel. A subclass could modify this
initialization to create a European wheel with only a single
The spin and
spinList methods are the public
interface to this class. The spin
method makes a random choice of the numbers on the wheel. It
then creates a tuple with the number, the color, the even/odd
and the high/low values. The spinList
method returns a list of tuples by calling
The server function creates and
runs the web server. First, it creates the
theWheel, that we'll use. Second, it
creates the server, service, using the
server's web address and port number. The empty string is a
special short-hand meaning "this host's IP address". We
register all public methods of theWheel.
The server_forever method handles
requests until we stop the Python interpreter by killing the
We use the main switch so that we can easily reuse this
Wheel class definition.
Web Servers and the HTTP protocol
Mid-Level Protocols: The urllib2
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