Web Services

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Web Services

• Evren Sirin
• MINDSWAP Research Group
• University of Maryland, College Park

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Programs on the Web
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Simple Calculations
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Slightly More Complicated
From NASA AIRS GPS Retrievals Matchup Demo
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Other Web Service Examples

• B2B applications
• Complex business transactions
• Grid applications
• Scientific computing
• Pervasive Computing
• Interact with devices using WS interfaces

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Web Services

• Programs on the Web
• Published, located, invoked across the Web
• Self-contained and self-describing applications
• Machine-to-machine interaction
• Machine processable descriptions
• Platform and language independent
• Provide interoperability between diverse
  applications
• Loose coupling between components
• Dynamic and flexible applications
• Service Oriented Architecture (SOA)

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Web Service Basics

• Communication protocol
• SOAP Simple Object Access Protocol
• Based on HTTP and XML
• Description language
• WSDL Web Services Description Language
• Describe interface, message structures and
  bindings
• Some Web Service examples

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Web Service Problems

• Discovery
• Matching service capability with goal
  requirements
• How to describe capabilities and requirements?
• How to efficiently query against them?
• Composition
• Combining multiple services to achieve an
  objective
• How to specify goals and constraints for a
  composition?
• How to build and analyze composition?
• Execution and Monitoring
• Automated invocation and execution tracking
• How to provide transactional guarantees?
• How to recovering from failed transactions?
• And others
• Security, Policy, Negotiation, ...

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Web Services Protocol Stack
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SOAP - Communication Protocol

• XML-based format for the specification of
  structured and typed messages

Header
Body
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WSDL Describing Services

• Types defined using a schema
• Typically XML Schema
• Interfaces to define abstract operations
• Message Exchange Pattern (MEP)
• In-only, in-out, out-only,
• Types of messages sent/received
• Binding for concrete message format and
  transmission protocol

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WSDL - Web Service Descriptions
Abstract Description
Concrete Description
From S. Chandrasekarans Talk
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Message Exchange Patterns
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Additional Machinery

• Reliable messaging
• Message delivery in the presence of software
  component, system, or network failures
• Security
• Message integrity and confidentiality
• Trust
• Issuance and dissemination of credentials within
  different trust domains
• Policy
• Policy assertions to express the capabilities and
  constraints of a particular Web service

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WS-Policy

• To use a web service a client needs more
  information than is provided in WSDL file.
• Does service support WS-Security?
• What encryption algorithm does it expect (or
  prefer)?
• Must messages be signed?
• What character encoding is used?
• What version of SOAP is supported?
• A policy is a set of conditions expected by a
  service
• WS-Policy provides an XML based declarative
  mechanism for expressing a policy that can be
• integrated with WSDL or attached as a tModel in
  UDDI
• enforced by web services infrastructure

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WS-Policy Example

• ltwspPolicygt
• ltwspExactlyOnegt
• ltwspAllgt
• ltwsseSecurityTokengt
• ltwsseTokenTypegtwsseKerberosv5TGTlt/w
  sseTokenTypegt
• lt/wsseSecurityTokengt
• lt/wspAllgt
• ltwspAllgt
• ltwsseSecurityTokengt
• ltwsseTokenTypegtwsseX509v3lt/wsseToke
  nTypegt
• lt/wsseSecurityTokengt
• lt/wspAllgt
• lt/wspExactlyOnegt
• lt/wspPolicygt

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Transactions and Coordination

• Atomic transactions
• Short-lived distributed activities
• All-or-nothing property
• Similar to ACIDity of database transactions
• Coordination
• Long-running distributed activities
• A set of messages constitute an activity
• Participants need to reach consistent agreement
  on the outcome
• A coordinator and set of coordination protocols

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Service Discovery

• Service registries, e.g. UDDI
• Similar to a traditional phone book's yellow and
  white pages
• Providers publish service descriptions
• Customers finds offered
• Directory of web service interfaces
• Contains business name, text description, contact
  info and other related info
• Classification information about the business
  entity and types of the services the entity
  offers
• Links to WSDL descriptions

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Orchestrating Services

• Combining services to achieve complex tasks
• Static composition
• Central agent coordinating execution
• Procedural descriptions, e.g. BPEL4WS
• Similar like a programming language
• Web Service class instead of function calls
• WSDL-based Web Services as building blocks
• Build executable business processes
• Composed using a control flow (sequence,
  conditional, ...)

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Choreography of Services

• Establish coordination between multiple
  participants
• Global perspective
• No single coordinating agent
• Contract between two or more parties
• Outline the obligations of the participants
• Related tasks
• Verification Dead-locks, live-locks, etc.
• Creating a conformant orchestration

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Web Services and Semantic Web

• Web Services
• Sharing programs on the Web
• Semantic Web
• Sharing information on the Web
• Common key features
• Interoperability between diverse applications
• Dynamic and extensible infrastructure
• Loose coupling between components

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Semantic Web Services

• Use ontologies to describe Web Services
• Not limited to syntactic (structural)
  descriptions
• Extend WSDL descriptions
• WSDL-RDF mapping
• OWL-S WSDL components described with Web
  Ontology Language (OWL)
• Richer descriptions supporting automation
• Discovery, selection, negotiation, coordination,
  composition, validation, execution, simulation
• Description provides info about
• Input/output types and logical constraints
• Internal structure of the process

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• Questions?