Chapter 1: Computing with Services

1

• Chapter 1Computing with Services

Service-Oriented Computing Semantics, Processes,
Agents Munindar P. Singh and Michael N. Huhns,
Wiley, 2005
2
Highlights of this Chapter

• Visions for the Web
• Open Environments
• Services Introduced
• The Evolving Web
• Standards Bodies

3
The Web As It Is

• Not easy to program
• Designed for people to get information
• Focuses on visual display (as in HTML)
• Lacks support for meaning
• Supports low-level interactions
• HTTP is stateless
• Processing is client-server
• Creates avoidable dependencies among what should
  be independent components

4
The Web As It Is Becoming

• Enable interactions autonomous, heterogeneous
  parties (information providers and users)
• Go beyond visual display to capture meaning ?
  Semantic Web
• Support standardized interfaces ? Web services
• Support complex activities ? processes
• Support rich interactions among autonomous
  parties ? agents

5
Historical View of Services over the Web
6
Viewpoints on Services

• Traditionally, a capability that is provided and
  exploited, often but not always remotely
• Networking bundle of bandwidth-type properties
• Telecom features (caller ID, forwarding)
• Systems operational functions (billing,
  storage) parceled up into operation-support
  systems
• Web or Grid Web pages or Grid resources
• Wireless Wireless access messaging
• By contrast, we treat services as resembling
  real-life services or business partners

7
What is a Web Service?

• A piece of business logic accessible via the
  Internet using open standards (Microsoft)
• Encapsulated, loosely coupled, contracted
  software functions, offered via standard
  protocols (DestiCorp)
• A set of interfaces providing a standard means of
  interoperating between different software
  applications, running on a variety of platforms
  and frameworks (W3C)
• Our working definition A service is
  functionality that can be engaged

8
Scope

• Includes wherever Internet and Web technologies
  are employed
• Internet
• Intranet network restricted within an enterprise
• Extranet private network restricted to selected
  enterprises
• Virtual Private Network (VPN) a way to realize
  an intranet or extranet over the Internet

9
Service Composition

• Vision
• Specify and provide services independently,
  hiding implementations
• Use services in combination in novel ways
• Going beyond the idea of a passive object
• Obviously desirable and challenging
• But is this what we want?
• Can or should implementations be hidden?
• What about organizational visibility?
• How to assess risk? How to handle exceptions?

10
Applications of Composable Services

• Portals
• Legacy system interoperation
• E-commerce
• Virtual enterprises
• Grid computing

11
Autonomy

• Independence of business partners (users and
  organizations)
• Political reasons
• Ownership of resources
• Control, especially of access privileges
• Payments
• Technical reasons
• Opacity of systems with respect to key features,
  e.g., precommit in distributed databases

12
Heterogeneity

• Independence of component designers and system
  architects
• Political reasons
• Ownership of resources
• Technical reasons
• Conceptual problems in integration
• Fragility of integration
• Difficult to guarantee behavior of integrated
  systems

13
Dynamism

• Independence of system administrators
• Needed because the parties change
• Architecture and implementation
• Behavior
• Interactions
• Make configurations dynamic to improve service
  quality and maintain flexibility

14
Locality How to Handle the Above

• Reduce sharing of data and metadata to reduce
  inconsistencies and anomalies
• Reduce hard-coding, which reflects out-of-band
  agreements among programmers
• Bind dynamically to components
• Use standardized formats to express data
• Express important knowledge as metadata
• Use standardized languages to express metadata
• Relax consistency constraints
• Obtain remote knowledge only when needed
• Correct rather than prevent violations of
  constraints often feasible

15
System Architectures Centralized
Terminal
Terminal
3270
Terminal
Terminal
Terminal
Mainframe
Terminal
Terminal
Terminal
Terminal
Terminal
Terminal
16
System Architectures Client-Server
Workstation Client
PC Client
PC Client
PC Client
E-Mail Server
Web Server
Database Server
Master-Slave
17
System Architectures Peer-to-Peer
Application
Application
Application
Application
E-Mail System
Web System
Database System
18
System Architectures Cooperative
Agent
Application
Application
Application
Agent
Agent
Agent
Application
Agent
Agent
E-Mail System
Agent
Agent
Database System
Web System
(Mediators, Proxies, Aides, Wrappers)
19
Chapter 1 Summary

• Evolving perspectives on the Web
• Evolutions in IT architectures
• Open environments challenge some fundamental
  assumptions of computer science
• Autonomy
• Heterogeneity
• Dynamism
• Services, if understood correctly, can support IT
  in open environments