Download A Limited Definition of Web Services

A Limited Definition of Web
Services
Paul Kopacz <[email protected]>
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?
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Shift towards Service Oriented Architecture
Extend what works to provide services
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Focus on a working model
Provide a definition which creates services
based on Web philosophy
Web
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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
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Fundamental property of the Web
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Resource endpoints
Expose functionality as URI
Links are immediately useful
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Show what functionality is available
Can organize functionality
Keep a history
Maintain sessions and state
Methods
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Correspond to create-retrieve-update-delete
GET
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Retrieve
Safe and idempotent
Obtain a resource
Invoke functionality with no side effects
POST
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Update
Neither safe nor idempotent
Submit data which causes changes
May return nothing (204 No Content)
Methods
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PUT
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Create
Idempotent but not safe
Use when resource requires minimal processing
DELETE
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Destroy
Idempotent but not safe
Specific URL
Headers
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Transfer non-functional requirements
Specified to be extensible
Using the body of method calls is
inconsistent
Services
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Provide functionality through the Web
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No separation between functionality and
interface
Programming against a website worked
badly
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Screen-scraping
Form inspection
No hints
Services
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Next step in offering services online
World Wide Web for Programs
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Make the Web easy for programs to access
Extend Web programming to provide
interfaces to functionality
The Definition
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An extension of Web programming to expose
an application interface through HTTP
Obey the guidelines of HTTP
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Sometimes ignored in Web applications
Providing an API
Strict and consistent
XML?
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Web works with resources of all formats
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Use whatever is necessary
Does not specify HTML
Cannot make it necessary
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Probably a good choice
Differences
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Yet another distributed systems technology
Layering on top of exiting functionality
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No extra infrastructure required
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Like RPC?
Stub repositories, registries
Document transfer instead of function calls
REST is very applicable
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Requiring no state on server unnecessary
Adoption
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Why use it?
Specific
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Technological adoption
General
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Business adoption
Technological Perspective
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Web for Programmers
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Programmers want to provide service interface
Make their job easier
Choice of technology left to engineers
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Appealing from their perspective
80/20 Rule
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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
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Web programming without presentation
Gall's Law
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“A complex system that works is invariably
found to have evolved from a simple system
that worked”
Allows simplest first approach
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Only add complexity for current iteration
No need to invent standards until there is a
problem to solve
The Browser is Easy
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Making valid Web resources allows use
through browser
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Scaffolding
Debugging
Inspecting state
Business Perspective
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Increase flexibility and reach of provided
services
Use from other applications
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Integrate into systems
Use from different platforms
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Different clients
Business to Customer
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Depends on business model
Publisher model
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Subscription model
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Programmatically filter out ads
The more choice the better
Balance may need to be struck
Eliminate lock-in
Business to Business
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Less obvious benefits
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B2B communication technologies more mature
(EDI)
Budget for integration
Technological roadblocks unlikely
Complex non-functional requirements
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Complex transactions, time limits
Other technologies are more mature (MOMs)
Conclusion
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Use Web technologies in the service oriented
model
Other choices exist but
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The Web works
Extending to provide service is easy
New problems may arise from increased
complexity
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Rely on what has worked so successfully