Web Services: Building Blocks and Prospective

Yves Lafon, W3C <ylafon@w3.org>
Web Services Activity Lead

About W3C

International consortium, around 430 Members, started by Tim Berners-Lee, inventor of the Web, in 1994
Standardization of core Web technologies, publication of guidelines, etc.
Web Services work started in 2000
Semantic Web Activity

History

HTML scraping
mostly used for RPC operations
emergence of XML-based protocols
March 2000: XML Protocol Matrix
September 2000: XML Protocol WOrking Group
February 2002: Web Services Activity

Web Services

Web Services Challenges

Interoperability
Integration
Extensibility
Scalability
...

… translating to an architecture

… justifying some choices:

XML-based messages: for platform-independent machine processing
Framework focused on extensibility: facilitating interoperability
Machine-processable descriptions: facilitating integration
Loose coupling: maximizing scalability

The Big Picture ?

Web Services Work Items

Web Services Protocol
Core Protocol
- Messaging Framework: SOAP 1.2
- Web Services Addressing 1.0
Protocol extension
- Resource Representation SOAP Header Block
- Message Transmission Optimization Mechanism: XOP/MTOM
Web Services Description
- Web Services Description: WSDL 2.0
- Web Services Policy: WS-Policy 1.5
- Web Services Choreography Description: WS-CDL 1.0
- XML Schema Patterns for Databinding
- Semantic Annotations for WSDL

Web Services Protocol

Messaging Framework

SOAP 1.2:

basic XML Framework for exchanging structured and typed information in machine-to-machine interaction
necessitate the use of a transport protocol, such as HTTP/1.1
improved interoperability, processing model, and protocol independence since SOAP 1.1
already deployed in all major toolkits and several Web services extensions

W3C Recommendation since June 2003. Slow but steady transition.

SOAP Version 1.2

Core of the messaging framework
- XML-based
- Extensible
Defines:
- An envelope
- A processing model
- A binding framework

SOAP message

XML document
Represented abstractly as an XML Infoset

SOAP message path

One-way message
Going from a sender to a receiver, possible via intermediaries
SOAP body is for the ultimate recipient
SOAP header blocks may be targeted at anybody along the message path

SOAP message path

Example of a SOAP message

<?xml version='1.0' ?>
<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope"> 
 <env:Header>
  <m:reservation xmlns:m="http://travelcompany.example.org/reservation" 
          env:role="http://www.w3.org/2003/05/soap-envelope/role/next"
          env:mustUnderstand="true">
   <m:reference>uuid:093a2da1-q345-739r-ba5d-pqff98fe8j7d</m:reference>
   <m:dateAndTime>2001-11-29T13:20:00.000-05:00</m:dateAndTime>
  </m:reservation>
 </env:Header>
 <env:Body>
  <p:itinerary
    xmlns:p="http://travelcompany.example.org/reservation/travel">
   …
  </p:itinerary>
 </env:Body>
</env:Envelope>

Message exchange patterns

One-way exchanges can be combined
Template that establishes a pattern for the exchange of messages between SOAP nodes
Examples:
- Request response
- Response: SOAP message following a non-SOAP request

Underlying protocols binding

Need to carry the XML document that a SOAP message is
Duty of a binding to an underlying protocol:
- Serialization of the Infoset
- Transmission
- Reconstruction of the Infoset
Can implements features:
- Message exchange pattern (MEP)
- Optimization
- …

Underlying protocols bindings

SOAP 1.2 HTTP Binding

Carries SOAP messages in an HTTP message
Implements 2 MEPs
- SOAP Request-Response: HTTP POST
- SOAP Response: HTTP GET
Implements the SOAP Web Method feature:
- POST
- GET
Allows for REST-ful interactions

Addressing information: Web Services Addressing 1.0

SOAP is very generic: no addressing information provided in its core capabilities
Typical interaction: HTTP request-response
Other scenarios:
- protocol without appropriate addressing capabilities
- protocol which doesn't provide correlation facilities
- send the response to an alternate location

Addressing from and to endpoints

Messages travel between endpoints
Concept of an endpoint reference (EPR):
- destination address
- reference parameters: bag of information related to an interaction
- metadata about the endpoint

Addressing information: Web Services Addressing 1.0

SOAP is very generic: no addressing information provided in its core capabilities
Typical interaction: HTTP request-response
Other scenarios:
- protocol without appropriate addressing capabilities
- protocol which doesn't provide correlation facilities
- send the response to an alternate location

Addressing from and to endpoints

Messages travel between endpoints
Concept of an endpoint reference (EPR):
- destination address
- reference parameters: bag of information related to an interaction
- metadata about the endpoint

Addressing

Web Services Addressing 1.0:

defines service endpoint references
directs messages to the service endpoints, including replies or faults.
relates messages using URIs

W3C Recommendation (Core and SOAP Binding)

WSDL Binding interoperability ongoing

Handling non-XML data

SOAP messages are XML documents
What to do with legacy binary data?
- Bitmap images (e.g. PNG)
- Other document binary formats (e.g. PDF)
Need to be part of the SOAP envelope to benefit from the SOAP processing model
Solution: include Base64-encoded data of type xs:base64Binary in the SOAP envelope

XOP & MTOM

XML-binary Optimized Packaging
- An alternate serialization of the Infoset
- Optimizes serialization of binary content
SOAP Message Transmission Optimization Mechanism
- How to use XOP in SOAP

W3C Recommendation since January 2005.

Resource Representation SOAP Header Block

Associates a URI with some Base64 encoded data
Allow to carry a representation of a Web resource
Internet media type can be expressed: Describing Media Content of Binary Data in XML
Useful for receivers that have limited connectivity

RRSHB example

<soap:Envelope
    xmlns:soap='http://www.w3.org/2003/05/soap-envelope'
    xmlns:rep='http://www.w3.org/2004/08/representation'
    xmlns:xmlmime='http://www.w3.org/2005/05/xmlmime'>
 <soap:Header>
  <rep:Representation resource='http://example.org/me.jpg'>
   <rep:Data
   xmlmime:contentType='image/jpeg'>/aWKKapGGyQ=</rep:Data>
  </rep:Representation>
 </soap:Header>
 <soap:Body>
  <x:MyData xmlns:x='http://example.org/mystuff'>
   <x:name>John Q. Public</x:name>
   <x:img src='http://example.org/me.jpg'/>
  </x:MyData>
 </soap:Body>
</soap:Envelope>

Doing something with this messaging framework

SOAP+WS-Addressing+MTOM allows complex exchange of rich information
Message exchange can be enhanced with added processing

But:

How do you know what messages are accepted by an endpoint?
How to do things with Web services?

Describing Web services

Simple tasks are exposed as stand-alone services
In order to use those services and do useful things:
- Need to describe how to interact with a service: WSDL 2.0
- Need to describe interactions between a set of services: WS-CDL 1.0
- Need to define advanced databinding to XML schemas
- What's next?

Web Services Description

WSDL 2.0 Structure

Description document:

<description targetNamespace="…">

Message formats (schema types):

  <types> … </types>

Abstract interface of the service:

  <interface>
    <operation> … </operation>*
  </interface>

Binding of these messages to a protocol:

  <binding> … </binding>

Location of the service:

  <service>
    <endpoint> … </endpoint>*
  </service>

</description>

New Features in WSDL 2.0

Reusability: inheritance
Extensibility: MEPs, F&P
Web-friendly: safe operations, complete HTTP/1.1 support
SOAP 1.2 binding

Message Exchange Patterns (MEPs)

<operation> specifies logical message exchange
MEP defines a pattern of interaction between agents
- sequence
- cardinality
- direction
Base set of MEPs:
- In-Out, In-Only, Out-In, Out-Only
- Robust-In-Only, Robust-Out-Only (permit optional fault)
- Out-Optional-In, In-Optional-Out (reply is optional)
Extensible: other MEPs can also be used (identified by URI)

WSDL 2.0

Reusable definitions: import, include, inheritance, bindings
Advanced support for SOAP 1.2 and HTTP 1.1

Candidate Recommendation

Interoperability events

At risk: feature/properties, 6 MEPs

WS-Policy 1.5

Framework for policy assertions: All, ExactlyOne
Attachment mechanisms for policy: UDDI 2.0/3.0, WSDL 1.1/2.0
Not defining policy assertions
Not defining policy negotiation
Currently Working Draft (Nov 2)
W3C Recommendation by end of 2007

WS-Policy 1.5

<Policy>
  <mtom:OptimizedMimeSerialization wsp:Optional="true"/>
  <wsap:UsingAddressing />
  <ExactlyOne>
    <sp:TransportBinding>
      <Policy>
        <sp:TransportToken>
          <Policy>
            <sp:HttpsToken RequireClientCertificate="false" />
          </Policy>
        </sp:TransportToken>
        <sp:AlgorithmSuite>
          <Policy>
            <sp:Basic256Rsa15/>
          </Policy>
        </sp:AlgorithmSuite>
      </Policy>
    </sp:TransportBinding>
    <sp:AsymmetricBinding>…</sp:AsymmetricBinding>
  </ExactlyOne>
</Policy>

Choreography

WS-CDL 1.0: Describe interaction among atomic Web Services

Complex messages exchanges
Composition of Web Services: linking business via Web Services
Based on Pi Calculus
Choreography vs Orchestration

Candidate Recommendation phase

XML Schema Patterns for Databinding

Provides a set of simple XML Schema 1.0 patterns
Describe XML representations of commonly used data structures: Enumeration, Collection, Vector, etc.

Working Draft of XML Schema Patterns for Databinding

XML Schema Patterns example

<xs:complexType name="Collection">
  <xs:sequence>
    <xs:element name="name" type="xs:string"/>
    <xs:element name="shade" type="xs:string"/>
    <xs:element name="length" type="xs:int"/>
  </xs:sequence>
  <xs:attribute name="id" type="xs:string"/>
  <xs:attribute name="inStock" type="xs:int"/>
</xs:complexType>

Describing Web services

Need to describe how to interact with a service: WSDL 2.0
Need to describe interactions between a set of services: WS-CDL 1.0
Need to define advanced databinding to XML schemas
What's next? Semantics!

Semantics: setting expectations

Will make people's life easier
Examples of use:
- Connect two services, ensuring that they speak the same language with same meaning
- Merge data from different sources (e.g. title & título)
- Out of a list of 50 existing services, get only the 2 candidates that actually do what the user wants
- Automatic recovery from failure, etc.

Semantic Web Services

Generic Semantic Annotation mechanism for WSDL

Working Group started in April

Mechanism to enable annotation of Web services descriptions: operations, inputs, outputs, types

<interface name="CheckItemAvailability">
        sawsdl:modelReference="PurchaseOrderServices#ItemAvailability"
    <operation name="InStock"
       pattern="http://www.w3.org/2006/01/wsdl/in-out">
        <input messageLabel="In"
            element="tns:ItemDescription"/> 
        <output messageLabel="Out"
           element="tnsItemDescriptionAvaibility"/>
    </operation>

</interface>

Semantic Annotations Usage

Registries, Categories
- Defining Annotations to publish a Web Service, e.g. to a UDDI registry
- Adding Categorization information, defining a taxonomy if it does not exist
Matchmaking and Composing Web services
- Matching Web Service Interfaces using a Shared Ontology
- Matching Web Service Interfaces with Ontology Mediation
- Composing Web Services with/without Ontology Reasoning
- Matching Using Multiple Annotations
Defining Schema Mappings to enable Web service Invocation

Semantic Annotations example

<xs:element name="CheckInventoryServiceRequest">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="UPC" type="xsd:string"
        sawsdl:modelReference="SampleRetailOntology#UPCCode"/>
      <wsdl:element name="DeliveryDate" type="xs:string"
        sawsdl:modelReference="SampleRetailOntology#DeliveryDate"/>
      <wsdl:element name="numBundles" type="xs:float"
        sawsdl:modelReference="SampleRetailOntology#NumBundles/>
    </xs:sequence>
  </xs:complexType>
</xs:element>

<SampleRetailOntology.owl#> rdf:type owl:Ontology  .
  :PartNumber rdf:type owl:Class;
     rdfs:subClassOf owl:Thing;
     owl:equivalentClass :UPC .
  :UPC rdf:type owl:Class;
     owl:equivalentClass :PartNumber .
  :NumBundles rdf:type owl:Class;
     owl:equivalentClass :Quantity .
  :Quantity rdf:type owl:Class;
     rdfs:subClassOf owl:Thing;
     owl:equivalentClass :NumBundles; .
  :DeliveryDate rdf:type owl:Class;
     rdfs:subClassOf owl:Thing;
     owl:equivalentClass owl:DueDate .
  :DueDate rdf:type owl:Class;
     owl:equivalentClass :DeliveryDate .

Internationalization

Web Services Internationalization: WS-I18N
Preferences and configuration information to provide culturally sensitive or multi-lingual operation
SOAP and WSDL extensions

<env:Envelope>
 <env:Header>
  <i18n:international>
   <i18n:locale>en-US</i18n:locale>
   <i18n:preferences>
    <ldml:collation>
     <ldml:alias source="de_DE" type="phonebook"/>
    </ldml:collation>
   </i18n:preferences>
  </i18n:international>
  …
 </env:Header>
 <env:Body>
  …
 </env:Body>
</env:Envelope>

Thank you

More information at:

http://www.w3.org/2002/ws/