XMLRPC a lightweight data communication protocol

1
XML-RPCa lightweight data communication protocol

• Jing Deng
• Computer Science Department
• University of Colorado at Boulder
• 26 September 2001

2
Overview

• Introduction
• XML-RPC protocol
• How it works
• Data format
• How to use XML-RPC (Java example)
• References

3
Introduction

• What is XML-RPC
• Remote process calling protocol with XML format
• What can it do
• allow software running on disparate operating
  systems, running in different environments to
  make procedure calls over the Internet.
• History
• Created by UserLand Software in April 1998, for
  its Frontier software communication

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Fundamentals

• XML
• A very powerful meta-language for description
  data. XML-RPC just uses a little bit of XML
• RPC
• A mechanism which allows a program running on one
  computer to execute a function that is actually
  running on another computer
• HTTP
• Most data on Internet are transferred under HTTP
  protocol
• Not very efficient, but easy to use
• Make XML-RPC more convenient to be used in web
  applications

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Definition

• XML-RPC is a remote procedure calling protocol
  that works over the Internet.
• XML-RPC is composed by an HTTP-POST request and a
  HTTP reply.
• The body of the request and the value returned
  from server is formatted by XML.

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XML-RPC Overview

• XML-RPC call is conducted between two parties
• A client to send RPC request
• A server to process RPC request and send back the
  return value to the client
• Servers address is in a standard URL
• http//example.org8080/rpcserver/
• The data is composed by a HTTP header and a XML
  body

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XML-RPC Overview (Cont.)

• A XML-RPC request and a XML-RPC response
• Both of them are composed by a HTTP header and a
  XML body
• Request should indicate a method name and its
  parameters
• Response should contain a function return value
  and indicates if it is a successful return or a
  error message
• There is a data type set for parameters and
  return value

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How does XML-RPC work
client
Web server
Client program
procedure
XML-RPC listener
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Request Example

• POST /RPC2 HTTP/1.0
• User-Agent Frontier/5.1.2
• Host xml.colorado.edu
• Content-Type text/xml
• Content-length 181

• lt?xml version1.0?gt
• ltmethodCallgt
• ltmethodNamegtexamples.getStateNamelt/methodNamegt
• ltparamsgt ltparamgt
• ltvaluegtlti4gt41lt/i4gtlt/valuegt
• lt/paramgtlt/paramsgt
• lt/methodCallgt

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Request Format

• A single ltmethodCallgt structure, which contains
• A single ltmethodNamegt. It contains the calling
  methods name
• A single ltparamsgt, which can contain any number
  of
• ltparamgts. Each ltparamgt has a
• ltvaluegt

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Data Type Scalar ltvaluegts

• lti4gt or ltintgt four-byte signed integer
  -12
• ltbooleangt 0 (false) or 1 (true)
  1
• ltstringgt ASCII string
  Hi!
• ltdoublegt double-precision
  3.1415
• ltdateTime.iso8601gt date/time
  19980717T140855
• ltbase64gt base64-encoded binary
  eW91IGNhbid
• (default type is string)

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Data Type (Cont.) - ltstructgt

• ltstructgt contains ltmembergts, each ltmembergt
  contains a ltnamegt and a ltvaluegt
• ltstructgt
• ltmembergt
• ltnamegtlowerBoundlt/namegt ltvaluegtlti4gt18lt/i4gtlt
  /valuegt
• lt/membergt
• ltmembergt
• ltnamegtupperBoundlt/namegt ltvaluegtlti4gt122lt/i4gt
  lt/valuegt
• lt/membergt
• lt/structgt

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Data Type (Cont.) - ltarraygt

• An ltarraygt contains a single ltdatagt element,
  which can contain any number of ltvaluegts
• ltarraygt
• ltdatagt
• ltvaluegtlti4gt12lt/i4gtlt/valuegt
• ltvaluegtltstringgtEgyptlt/ltstringgtlt/valuegt
• ltvaluegtltbooleangt0lt/booleangtlt/valuegt
• ltvaluegtlti4gt-31lt/i4gtlt/valuegt
• lt/datagt
• lt/arraygt

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Response Example

• HTTP/1.1 200 OK
• Connection close
• Content-Length 158
• Content-Type text/xml
• Date Wed, 26 Sep 2001 101028 GMT
• Server UserLand Frontier/5.1.2-WinNT
• lt?xml version1.0?gt
• ltmethodResponsegt
• ltparamsgt ltparamgt
• ltvaluegtltstringgtSouth Dakotalt/stringgtlt/valuegt
• lt/paramgtlt/paramsgt
• lt/methodResponsegt

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Response Format

• One ltmethodResponsegt, it may contains either
• A single ltparamsgt -- a successful procedure
  return
• It contains a single ltparamgt which contains a
  single ltvaluegt
• A ltfaultgt -- a failure procedure return
• It contains a ltvaluegt which is a ltstructgt that
  contains two members ltfaultCodegt and ltintgt,
  ltfaultStringgt and ltstringgt

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Example - XML-RPC in Java

• Many XML-RPC applications encapsulate the details
  of XML-RPC protocol
• They provides an interface for users to send
  remote procedure call, and retrieve return value
• A XML-RPC Java library helma.xmlrpc

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Helma.xmlprc

• A free XML-RPC library
• http//xmlrpc.helma.org/
• Contains source code, documentations, and
  examples
• Has a jar file as a library xmlrpc.jar, which
  contains a OpenXML parser as well
• Easy to install

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XML-RPC Client

• Send XML-RPC to server, and get the return value
• Interface (encapsulate most protocol details
  HTTP, XML)
• XmlRpcClient class
• XmlRpcClient(String URL)
• public Object execute(String procedure_name,
• Vector params)

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XML-RPC Server

• Process XML-RPC request, call the procedure that
  request wants to call, and send back the return
  value
• Two kinds of server
• A mini web server, only process XML-RPC
• WebServer class
• (See its interface later)
• XML-RPC listener which cooperates with a web
  server
• XmlRpcServer class
• Input is a XML stream, and output is a XML stream
  also
• execute() method to process RPC

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XML-RPC Handler

• Contains the procedures client wants to call
• We need to register handler to server
• Automatic registration
• Explicit registration
• Implement XmlRpcHanlder interface
• Use Xml.RpcHandler.execute() to process input
  parameter
• Interface
• WebServer.addHandler(String handler_name, Object
  handler)

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Example - Handler

• public class AreaHandler
• public Double rectArea(double length, double
  width)
• return new Double(length width)
• public Double circleArea(double radious)
• double value (radious radious Math.PI)
• return new Double (value)

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Example - Server

• import java.io.IOException
• import helma.xmlrpc.WebServer
• import helma.xmlrpc.XmlRpc
• public class AreaServer
• public static void main(String args)
• try
• WebServer server new WebServer(Integer.p
  arseInt(args0))
• server.addHandler("area", new
  AreaHandler())
• catch (IOException e)
• System.out.println("Could not start
  server " e.getMessage())

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Example - Client

• import java.io.IOException
• import java.util.Vector
• import helma.xmlrpc.XmlRpc
• import helma.xmlrpc.XmlRpcClient
• import helma.xmlrpc.XmlRpcException
• public class AreaClient
• public static void main(String args)
• try
• XmlRpcClient client
• new XmlRpcClient("http//localhost889
  9")
• // continue.

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Example Client (Cont.)

• // continue from last slide..
• Vector params new Vector()
• params.addElement(new Double(args0))
• Object result client.execute("area.circl
  eArea", params)
• System.out.println(result.toString())
• catch (IOException e)
• System.out.println("IO Exception "
  e.getMessage())
• catch (XmlRpcException e)
• System.out.println("Exception within
  XML-RPC " e.getMessage())

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Discussions

• Characters of XML-RPC
• Platform independent because it is a protocol
• Very simple, very easy to implement, very ease to
  use
• Too simple for some applications
• Implemented with many languages Java, C/C,
  Perl, Python, etc
• Other important issues I didnt mention
• Security
• Apply XML-RPC in web applications

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Discussions (Cont.)

• Comparisons
• Differences between XML-RPC and web page
• Differences between XML-RPC and RMI, Corba
• Differences between XML-RPC and SOAP
• Beyond XML-RPC
• SOAP, UDDI, WSDL, BXXP

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References

• XML-RPC documents
• http//www.xmlrpc.com
• http//www.xmlrpc.com/spec
• http//xmlrpc-c.sourceforge.net/xmlrpc-howto/xmlr
  pc-howto.html
• XML-RPC in Java
• http//xmlrpc.helma.org
• XML-RPC in C and C
• http//xmlrpc-c.sourceforge.net/sample-code.php

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References (Cont.)

• Book
• Simon St.Laurent, Joe Johnston, Edd Dumbill,
  programming web services with XML-RPC, OReilly,
  2001
• Others
• http//www.w3.org/2000/xp/
• http//www.google.com

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Thanks!