A Limited Definition of Web Services

Paul Kopacz <pkopacz@uoguelph.ca>CIS*6650.01 Service Oriented Architecture

Instructor: Qusay H. Mahmoud, Ph.D.February 15, 2007

A Limited Definition of Web Services - Inspiration

“The World Wide Web uses relatively simple technologies with sufficient scalability, efficiency and utility that they have resulted in a remarkable

information space of interrelated resources, growing across languages, cultures, and media” -

W3C's Architecture of the World Wide Web

http://www.w3.org/TR/webarch/

A Limited Definition of Web Services - Why?

● Shift towards Service Oriented Architecture● Extend what works to provide services

Focus on a working model● Provide a definition which creates services

based on Web philosophy

Web

● Must use “Web” properly● Not just a transport mechanism● HTTP has well understood interface● Must work within the interface to be

considered Web● “Web Services based on SOAP and WSDL

are Web in name only” - Nick Gall

http://www.w3.org/2007/01/wos-papers/gall

Resources

● Fundamental property of the Web Resource endpoints Expose functionality as URI

● Links are immediately useful Show what functionality is available Can organize functionality Keep a history Maintain sessions and state

Methods

● Correspond to create-retrieve-update-delete● GET

Retrieve Safe and idempotent Obtain a resource Invoke functionality with no side effects

● POST Update Neither safe nor idempotent Submit data which causes changes May return nothing (204 No Content)

Methods

● PUT Create Idempotent but not safe Use when resource requires minimal processing

● DELETE Destroy Idempotent but not safe Specific URL

Headers

● Transfer non-functional requirements● Specified to be extensible● Using the body of method calls is

inconsistent

Services

● Provide functionality through the Web No separation between functionality and

interface● Programming against a website worked

badly Screen-scraping Form inspection No hints

Services

● Next step in offering services online● World Wide Web for Programs

Make the Web easy for programs to access● Extend Web programming to provide

interfaces to functionality

The Definition

● An extension of Web programming to expose an application interface through HTTP

● Obey the guidelines of HTTP Sometimes ignored in Web applications Providing an API Strict and consistent

XML?

● Web works with resources of all formats Use whatever is necessary Does not specify HTML

● Cannot make it necessary Probably a good choice

Differences

● Yet another distributed systems technology● Layering on top of exiting functionality

Like RPC?● No extra infrastructure required

Stub repositories, registries● Document transfer instead of function calls● REST is very applicable

Requiring no state on server unnecessary

Adoption

● Why use it?● Specific

Technological adoption● General

Business adoption

Technological Perspective

● Web for Programmers Programmers want to provide service interface Make their job easier

● Choice of technology left to engineers Appealing from their perspective

80/20 Rule

● A technology has a high likelihood of success if 80% of the benefit can be realized with 20% of the implementation work

● Shortest path to exposing existing functionality

● Brand new functionality Web programming without presentation

Gall's Law

● “A complex system that works is invariably found to have evolved from a simple system that worked”

● Allows simplest first approach Only add complexity for current iteration

● No need to invent standards until there is a problem to solve

The Browser is Easy

● Making valid Web resources allows use through browser Scaffolding Debugging Inspecting state

Business Perspective

● Increase flexibility and reach of provided services

● Use from other applications Integrate into systems

● Use from different platforms Different clients

Business to Customer

● Depends on business model● Publisher model

Programmatically filter out ads● Subscription model

The more choice the better● Balance may need to be struck● Eliminate lock-in

Business to Business

● Less obvious benefits B2B communication technologies more mature

(EDI) Budget for integration Technological roadblocks unlikely

● Complex non-functional requirements Complex transactions, time limits Other technologies are more mature (MOMs)

Conclusion

● Use Web technologies in the service oriented model

● Other choices exist but The Web works Extending to provide service is easy

● New problems may arise from increased complexity Rely on what has worked so successfully