Web Services - Concepts, Architecture and Applications Part 6: Service Description (WSDL) Gustavo Alonso and Cesare Pautasso Computer Science Department ETH Zürich [email protected]http://www.inf.ethz.ch/~alonso Some slides were modified or added by: Nilufer Onder Computer Science Department Michigan Technological University http://www.cs.mtu.edu/~nilufer
26
Embed
Web Services - Concepts, Architecture and Applications Part 6: Service Description (WSDL) Gustavo Alonso and Cesare Pautasso Computer Science Department.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Web Services - Concepts, Architecture and ApplicationsPart 6: Service Description (WSDL)Gustavo Alonso and Cesare PautassoComputer Science DepartmentETH Zü[email protected]://www.inf.ethz.ch/~alonso
Some slides were modified or added by: Nilufer OnderComputer Science DepartmentMichigan Technological Universityhttp://www.cs.mtu.edu/~nilufer
What is WSDL? Web Services Description Language (WSDL) is an XML format for describing
network services as a set of endpoints operating on messages containing either document-oriented or procedure-oriented communication.(www.w3.org/TR/wsdl)
The original document was prepared by Ariba, IBM, and Microsoft. The Web Services Description Language specification is in working draft 2.0
(November 2003, June 2007) WSDL discusses how to describe the different parts that comprise a Web
service. A WSDL document defines services as collections of network endpoints, or
ports. The general framework is to store WSDL descriptions where potential clients
WSDL definitions In WSDL, abstract definition of endpoints and messages is separated from their
concrete network deployment or data format bindings Abstract definition
Types: a container for data type definitions (e.g., using XSD) Message: a typed definition of the data being communicated Operation: description of an action supported by the service Port type: set of operations supported by one or more endpoints
Concrete description Binding: protocol and data format specification for a particular port type Port: a single endpoint defined as a combination of a binding and a network
address Service: a collection of related endpoints
The types in WSDL are used to specify the contents of the messages (normal messages and fault messages) that will be exchanged as part of the interactions with the Web service
The type system is typically based on XML Schema (structures and data types) - support is mandatory for all WSDL processors
An extensibility element can be used to define a schema definition language other than XML Schema
component”, it contains three elements: message reference: indicating
the message pattern used for this message
direction: whether it is an inbound or outbound message
message: the actual contents of the message expressed in terms of the types previously defined
Messages are divided into parts, each of them being a data structure represented in XML. Each part must have a type (basic or complex types, previously declared in the WSDL document).
If a SOAP binding is used, a WSDL message element is meant to match the contents of the body of a SOAP message. By looking at the types and looking at the message, it is possible to build a SOAP message that matches the WSDL description (and this can be done automatically since the description is XML based and the types also supported by SOAP)
Called the “fault reference component”, it contains: a name message reference: the message
to which the fault refers to direction: whether the fault is
and faults. The sequencing and number of messages in the operation is determined by the message exchange pattern (would add a line containing <fault message=… to the operation)
An operation has: name message exchange pattern message references: the
messages involved (input, output)
fault references: the faults involved (infault, outfault)
style (optional): RPC, set-attribute or get-attribute
features and properties
Style: RPC = implies interactions
mirroring the behavior of RPC set- and get- attribute = implies
interactions of the type commonly found in object oriented languages
Features and properties: used to specified characteristics of the message exchange implied by an operation. Examples include reliability, security, routing, etc
Bindings and ports A binding defines message formats
and protocol details for the operations and messages of a given Port Type
A binding corresponds to a single Port Type (obvious since it needs to refer to the operations and messages of the Port Type)
A Port Type can have several bindings (thereby providing several access channels to the same abstract service)
The binding is extensible with elements that allow to specify mappings of the messages and operations to any format or transport protocol. In this way WSDL is not protocol specific.
A port specifies the address of a binding, i.e., how to access the service using a particular protocol and format
Ports can only specify one address and they should not contain any binding information
The port is often specified as part of a service rather than on its own
Interfaces in WSDL 2.0 An interface defines the messages a
service sends or receives by grouping the messages into operations
An interface can extend the operations of other interfaces (inheritance)
An interface has: name extended interfaces: other
interfaces that this one extends style default: default style for
operations operations features and properties
An interface corresponds to the abstract description of the Web service, it does not contain any information about where the service resides or what protocols are used to invoke the Web service
Conversations WSDL is in its current version an extension of the IDL model to support
interaction through the Internet: XML as syntax and type system possibility of grouping operations into a service different options for accessing the service (addresses and protocols)
This is its great advantage … it is straightforward to adapt existing middleware platforms to use or
support WSDL automatic translation from existing IDLs to WSDL is trivial
… but also the disadvantage electronic commerce and B2B interactions are not single service calls WSDL does not reflect the structure of the procedures to follow to correctly
interact with a service (conversations)• business protocol = set of valid conversations
Without a business protocol, most of the development work is still manual
could be a possible evolution path for WSDL (or the type of technology that is being built on top of WSDL)
ebXML does not consider a client/ server model but an interaction between partners (peer-to-peer)
Consequently, the service description model for ebXML is the description of how two business processes interact with each other: partners publish their processes
(an external view over them) a collaboration agreement is
drawn based on those processes the collaboration agreement
describes the business protocol between those partners
Automatic development The ultimate goal of WSDL is to
provide support for automating as much as possible for the development process for Web services: given the WSDL description, a
WSDL compiler generates the stubs or skeletons necessary to develop clients that can interact with the service
for that purpose, WSDL must rely on a standard protocol so that generic stubs can be created, this is where SOAP comes into the picture
WSDL is meant as a bridge between internal services and external (Web) services
Similarly, the ultimate goal in ebXML is to automate the process of developing a collaboration agreement, deploying it and enforcing its rules: given a collaboration agreement
(possibly a standard one), one should be able to automatically generate a stub or skeleton for the individual business processes at the ends of the agreement
partners need only to extend the stub process with their own internal logic
this is why ebXML needs more than SOAP as the agreement is used to control and direct the flow of messages between partners at the platform level
WSDL Summary The goal of WSDL is to describe Web services, in particular describe service
interfaces. The WSDL data structure has interesting implications on how services are
described and how they interact Services can initiate operations like a client (blurred notion of client and
service provider) WSDL does not presume a particular form of communication (first defines
the abstract interface, then the location and protocol) Abstract interfaces are reusable: different services could combine different
interfaces using different bindings, and could make them available at different addresses
WSDL documents can import other WSDL documents The WSDL description is correlated with SOAP. If the messages defined in
WSDL are exchanged using SOAP, then the InterfaceBindings contain all the information needed to infer or automatically construct SOAP messages (interaction style, XML encoding, transport bindings)