XML and Web Services

1
XML and Web Services

• An Introduction

2
Client/Server Recap

• Web services are a client/server technology
• Usually implemented on top of HTTP
• This makes it easy to deploy and maintain web
  services

3
Remote Procedure Calls (RPC)

• Popularized by Sun
• C/C development (now other languages)
• Allows remote clients to run functions on server
• Server publishes specification of available
  functions
• See also
• Remote Method Invocation (RMI)
• Common Object Request Broker Architecture (CORBA)

4
RPC Example

• Stock market application
• Client wants to inquire on stock price
• Server has a RPC function
• float stock_price(stockName)
• Client calls server function and server returns
  the current stock price

5
RPC Issues

• What if the function is changed to include a time
  parameter?
• Client and server must recompile
• What if different computers store floating point
  numbers differently?
• Need to translate to a common representation

6
XML

• eXtensible Markup Language
• Subset of SGML (Standard Generalized Markup
  Language)
• Well-formed
• Self describing
• Platform independent
• Widely supported in modern programming languages
  (Java, C, Python, Perl, PHP, JavaScript)

7
HTML Example

• Tags enclosed in lt and gt
• End tags enclosed in lt/ and gt
• Attributes take the form namevalue
• Notice the ltbrgt tag does not have a matching end
  tag

lttablegtlttrgt lttd width"100"gt ltp
align"center"gt ltbgtltspangtComS
nbsp486Xlt/spangt ltspangtlt/spangtlt/bgt
ltspangt ltbrgtSpring 2006 lt/spangt
lt/tdgt lt/trgtlt/tablegt
8
XML vs. HTML

• As shown, XML and HTML are incompatible
• HTML is not well-formed
• XML requires that all tags have a matching end
  tag
• To solve this problem a XML compatible version of
  HTML was developed XHTML

9
XHTML Example

• The ltbrgt tag now has a matching lt/brgt tag
• This snippet is now well-formed XHTML
• The difference is minor, but illustrates a
  critical XML property

lttablegtlttrgt lttd width"100"gt ltp
align"center"gt ltbgtltspangtComS
nbsp486Xlt/spangt ltspangtlt/spangtlt/bgt
ltspangt ltbrgtlt/brgt Spring 2006 lt/spangt
lt/tdgt lt/trgtlt/tablegt
10
Document Type Definition (DTD)

• Each XML schema has a DTD (or a XSD, not covered
  today)
• DTDs can be published or included in an XML
  document
• This is the key to XMLs self-describing property
• Anyone can create a DTD thus defining a new type
  of XML

11
XHTML DTD1 Example

• ELEMENT and ATTRIBUTE tags define the tags and
  properties of the HTML
• Ironically, DTDs are not well-formed
• XSD solves this

lt!ELEMENT table (tbodytr))gt lt!ELEMENT
tbody (tr)gt lt!ELEMENT tr (thtd)gt lt!ELEMENT th
Flowgt lt!ELEMENT td Flowgt lt!ATTLIST table
attrs summary Text IMPLIED
width Length IMPLIED cellspacing
Length IMPLIED gt
1. http//www.w3.org/TR/xhtml1/DTD/xhtml1-strict.d
td
12
So far

• We have discussed
• Client/server
• RPC
• XML
• How does this relate to web services?
• First we need to answer what is a web service?

13
Web Services

• From Wikipedia
• According to the W3C a Web service is a software
  system designed to support interoperable
  machine-to-machine interaction over a network. It
  has an interface that is described in a
  machine-processable format such as WSDL. Other
  systems interact with the Web service in a manner
  prescribed by its interface using messages, which
  may be enclosed in a SOAP envelope, or follow a
  REST approach. These messages are typically
  conveyed using HTTP, and normally comprise XML in
  conjunction with other Web-related standards.

14
Web Services (2)

• interoperable machine-to-machine interaction
  over a network
• This is no different from the alternatives (RPC,
  etc)
• an interface that is described in a
  machine-processable format
• This is important in that it does not restrict
  web services in terms of software, unlike RPC, etc

15
Web Services (3)

• systems interact with the Web service in a
  manner prescribed by its interface using
  messages
• The interface is published and also
  self-describing
• messages are typically conveyed using HTTP, and
  normally comprise XML
• These two points really differentiate web
  services from RPC, etc

16
Miscellaneous Jargon

• WSDL Web Service Definition language
• Defines the interface
• Developer tools can auto-generate code templates
  from this (Eclipse extensions, Visual Studio)
• SOAP Simple Object Access Protocol
• XML protocol for messaging in web services
• Becoming widely supported
• Supports complex operations, data types, binary
  attachments, etc

17
Web Service Example

• Google API
• http//www.google.com/apis/
• Allows Google search to be incorporated in
  programs
• Without web services this becomes a very hard
  task
• Includes WSDL and coding examples for Java and
  .Net
• Free but must sign up for the service
• Only so many searches per day

18
Summary

• Web services are about exchanging structured data
  between remote hosts in a platform independent
  manner
• Wide range of applications
• Semantic web
• Further study
• Look into SOAP, WSDL
• Tinker with Eclipses web service extensions