Web Services

Web Services

1. Recapa) Objects and Resourceb) Service Orientation

2. Backgrounda) Timeline

3. Back to Data a) Semantic Webb) Web Servicesc) Semantic Web

Remember Object Orientation andResource Orientation?

• In Object Oriented distributed systems an application has the concept of a remote object that receives messages.– An object has lifetime (you can create it and destroy it).– It encapsulated State and operations on that state.– Examples: Jini, CORBA, DCOM, Legion

• In Resource Oriented systems, applications support the concept of resources– They are accessible via different protocols.– They have state, but no operations on that state.– Operations are defined by the access protocol.– The example: the Web

• Web Services are used to create Service Oriented Architectures– These are service oriented– What is Service Orientation?

Service Orientation• Service Orientation is an attempt to make distributed systems

more loosely coupled• In Object Orientation an object maintains data, and operations

for accessing/manipulating that data.– State and operations are combined into single entity - think

BankAccount.java• In Service Orientation state and the operations on it are

separated– A service is an interface to something with state, but we

don’t know and don’t care what that thing is. – All we see is the service interface which defines activities.– The Service interface provides operations on the state but

does not have state in itself.

Benefits of Service Orientation• A service does not have state, so I don’t have to worry about lifetime

issues• The service interface is typically defined as a contract

– The contract describes the messages received and sent by the service.

– Each message contains everything the service needs to know to perform its task - nothing is implied through ‘shared knowledge’.

– So I don’t have to ‘know’ what sort of thing it is, or what classes it inherits from like I do with objects. Instead, I can interpret the contract at runtime and communicate with the service immediately.

• Service interfaces are usually coarse-grained• The lack of state in services and the fact that messages contain all

necessary information means many interactions can be idempotent– They have no side-effects if they are repeated. - I send a message,

wait… get no response, so send it again without worrying that I have induced an inconsistent state at the server side.

• Easy to swap out/replicate services if they have the same interface

data

object

Objects, Resources and Services

operations

service

data

operations

data

resource

operations

Access protocol

representation

What’s Important?• Objects and Services define their own operations

(interfaces)• Resources do not.• What does that mean?

• Services and Resources are not ‘instances’• What does that mean?

Know this Man?

Tim Berners-LeeInventor of the WWW

Timeline• 1991 – first website (CERN HTTPd)http://info.cern.ch/hypertext/WWW/TheProject.html

• 1998– XML becomes a W3C recommendation– WDDX (Web Distributed Data eXchange)– XML-RPC MS, Don Box, Dave Winer, et al– SOAP MS, Don Box – Dave Winer, et al

• 1999– Resource Description Framework (RDF) v1

becomes a W3C recommendation• 2001

– Dot-com bubble bursts

• 2002– Tim Berners Lee: “As we look forward, we are tempted to distinguish

between the multimedia world of information targeted for human perception, and the well-defined world of data which machines handle....”

– Tim Berners Lee: “The Web technology must allow information intended for a human to be effectively presented, and also allow machine processable data to be conveyed. Only then can we start to use computers as tools again”

• 2003– SOAP becomes a W3C recommendation

• 2004– Web 2.0 term coined– RDF v2 published

• 2001 – 2007– SOAP vs REST wars

Machine-to-Machine Web

Data exported as Web pages

machine-processable information

Web Services• Companies needed standard mechanisms to

be able to

• Publish• Advertise • Discover• And a security model

links to applications, rather than web pagesNo good for business or security scraping web

pages

Hence XML-RPC -> SOAP

Exchanging Data

This focus on representing and exchanging data led to two main development thrusts:

• The Semantic Web: an extension of the current World Wide Web (Resource Oriented) in which information is given well-defined meaning, better enabling computers and people to work in cooperation. – focused on the representation of data– seeks to create a machine processable Web. – led by W3C + large number of researchers and industrial partners.– based on the Resource Description Framework (RDF)

• A Web Service: a software system designed to support interoperable machine-to-machine interaction over a network (Service Oriented)– They provide the definitions (and infrastructure) to allow applications

to remotely exchange XML messages with each other.

Semantic Web & Web Services

Complement each other:• Web Services met immediate

technology needs• Semantic Web is still growing

Semantic Web

• Based on RDF– subject, object, predicate– classification of resources and properties is

called an ontology– example: FOAF

• Dave knows Sally– Allows you to create graphs based on the

knows predicate which model more information than is present in an individual person

FOAF visualized

Timothy M. O'Brien Aug. 21, 2005

OWL• Web Ontology Language (OWL)

– extends RDF• Supports reasoning over resources

– via classes, class hierarchies– axioms – restrictions on class properties

and their relationships with other classes– A cat is an animal; Felix is a cat; Therefore

Felix is an animal– also (disjoint) constraints

Semantic Web• Slow adoption• Why?

– concepts are quite heavy– RDF XML is ugly– requires taxonomies (ontologies) and

agreement between them– very different from folksonomies– But history doesn’t stop. More and more

people are becoming interested in it– HTML5+RDFa

Next…• Structured Data• Web Services technology deep dive