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An introduction to xml-rpc
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by
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Adrian Likins


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What is xml-rpc

A simple http based RPC protocol that happens
to use XML as the encoding method.

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	Emphasis on simple and RPC.	
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	For 99% of useage, the xml is transparent.
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	RPC is Remote Procedure Call

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60,000 Feet


XML-RPC is a simple, relaitvely lightweight
mechanism for sending information to another
server about which and with what you want to
run some function, and then getting the 
results back in a constitent format.

It does this via well known standards, and
as simply as possible.

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A Metaphor, for those that dont get it

Think of it as a standarized way of talking
to a web app. Like running a CGI, but with a
well understood input, and output format
that is easy to parse.


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Thats it?

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	Yes, mostly. I said simple right?
%pause
	Simple enough to actually be really useful

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How it works
(client->server)

Application level usage
Client and server libaray API's
The encoding of the RPC
The transport of the RPC
the unencoding of the RPC

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The Parts
(server->client)

The running of the RPC
The Encoding of the results
The transport of the results
the unencoding of the results
Library Layer
Application Layer

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App And libs

Application level usage
	heavily dependent on the libary used

Library Layer
	Varies greatly from language to language, \
	and even implmentation to implementation.

XML-RPC is _NOT_ an API specification.
It is a protocol specification.
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The Encoding of the RPC

Dont worry about the XML part. Forget it exists.

The spec allows you to encode:
                methodName
                params

Params are arguments to the function call.
        They can be of types:
                int
                boolean
                string
                double
                dateTime.iso8601
                arbitrary blob of stuff <base64>
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More data types


Can I do structs?
	Yes

Any of the above data types can be used for values \
in either structs or arrays.

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Structs

each element in a struct has a name and value

Generally maps to hashes/dictionays/associative arrays \
in languages

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Arrays

Arrays are lists of values. The values can be \
different types, but have no names associated with them.

Generally maps to list or arrays

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XML

If you must know how XML fits in...

All of the above types are encoded as various bits \
of XML

See the XML-RPC spec for details.

But you really dont need to know that.

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Transport

Now that you have this blob of data, how do you get it to the \
other machine?

For xml-rpc, the transport is HTTP or HTTPS

In particular, http POST request to the server.

The response is a standard http response, with \
mime type of text/xml
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Unencoding

The server now has the information (method name, and params)

The server side unencodes it into a method name, a set of params.

Then it invokes this method. Ie, demarshalling. 

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The Response

Server gets results of method invocation, now it has to send \
it back to the server.

It will then encode these, and returns them as part of the \
http response body.

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NOTE: This is done as the response to the intial request, \
not as a seperate http request. This is important. More \
important that that xml bit you wont stop thinking about.
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Client receives repsonse

Client receives the response, unencodes it, returns it to \
the library/app level.

The app does whatever it wants. 
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Availability

XML-RPC implementations are available for:

python
perl
c
c++
php
java
tcl
asp
com
applescript
dylan/ common lisp / rebol 
other languages no one users 
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What do you use it for?

As a method to allow two or more processes on \
seperate machines to communicate via the network.

Those cases where you think "I really need to ask \
that machine something, and use it on this machine" are \
probabaly good canidates for XML-RPC.

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Who uses it?

The Red Hat Network uses xml-rpc extensively, including:

up2date
rhn_register
rhn_check

We also use it internally to facilitate communications \
RHN apps written in python, and web apps written in perl.

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Why?

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It's easy.
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It's cross platform.
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It's cross language.
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It's network oriented.
%pause
It's relatively lightweight.
%pause
It's easy.

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What it isnt


low latency
A bulk data transport
Asyncrounous
Complicated and sophisticated
		Writing and xml-rpc implementation is probabaly \
        easier than just using CORBA

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Case Study
The Red Hat Updater

	rpm
	grpm
	up2date v1
	rpmfind/gnorpm
	up2date v2

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Example

#/usr/bin/python
import os,sys
import xmlrpclib
from xmlrpclib import *

server = Server('http://192.68.0.1/RPC2')
print server.mp3.playlist()


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	Ingo Molnar is still smarter than you
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Simple

	Remember, I told you it was simple. 

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Server code example

	<insert stuff peter wrote here>


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Soap

Soap is the result of Microsoft and IBM looking at xml-rpc and saying, "thats a good idea, but it needs a few things."

Core technology of .NET

Which is really annoyinig, because it is actually a pretty
nice protocol. 

SOAP is more complicated than XML-RPC, but less than CORBA

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XML Protocol

XML Protocol is the result of the W3C looking at SOAP and \
saying "thats a good idea, but it needs a few things."

Even more complicated.

High "standards" bloat.

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Other RPC protocols

Why to not use SOAP/CORBA/DCE RPC/RMI/etc

Because they are complicated and I r dum

The Primary advantage of XML-RPC is it's simplicity

It's so simple and stupid, it's actually pretty useful.

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URLS

http://www.xml-rpc.com

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