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Semantic Web Services SS 2018

This document discusses the motivation behind web services, the technical solution, the SOAP protocol and related specifications, WSDL interface definition language, additional related specifications, and possible extensions. It also provides an illustration by a larger example and a summary.

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Semantic Web Services SS 2018

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  1. Semantic Web ServicesSS 2018 Web services Anna Fensel 09.04.2018

  2. Where are we?

  3. Outline • Motivation • Technical solution • SOAP protocol and related specifications • WSDL interface definition language • Additional related specifications • Illustration by a larger example • Possible Extensions • Summary • References

  4. MOTIVATION

  5. Motivation: “What is a Web Service?” https://www.youtube.com/watch?v=u80uPzhFYvc

  6. MotivationSOA • Service-Oriented Architecture • For large-scale distributed computing • For interoperability within and among enterprises • Business-oriented services • Coarse-grained service interfaces • Improves evolvability, manageability • Needs a suitable set of technologies

  7. MotivationSolutions needed to implement SOA lifecycle • SOA is a programming style • An SOA application is a composition of services • A ‘service’ is a building block/unit of an SOA • Service encapsulates a business process • A Service Provider registers services • Service consumption involves Find-Bind-Execute pattern • Questions • How to specify data involved in the process? • How to specify the way to access a service functionality? • How to communicate to services? • Can we use some existing solutions? Service Registry Find Register Service Consumer Service Provider Bind Execute SOA Elements

  8. MotivationWeb Services • Technology for SOA • Client-server messaging approach • With predefined Message exchange patterns • Independent of network protocols • Application-specific interfaces • Similar to RPC, CORBA • Most data in XML • Descriptions in XML Schema

  9. MotivationRPC vs. Web Services • Remote Procedure Calls • Aim to hide the network • Distributed system looks like a local system • Some calls happen to go over the network • But network is important • Latency, bandwidth • Reliability – connection, nodes • RPC is harmful

  10. MotivationWhy is RPC considered harmful? • Introduces semantics of a synchronous, blocking invocation • SOA needs a more flexible implementation technology • Usage of statically typed interfaces • Late binding is one of the premises of SOA • Restrictive, inflexible and does not provide the required level of abstraction • Implicit introduction of the behavior which characterizes unified memory space solutions • Hinders resolution of the interoperability issues • Harder to adapt • An appropriate SOA underlying technology needs to shift from the single-machine-centric vision towards the network-centric vision

  11. MotivationCORBA vs. Web Services • CORBA = Distributed Objects • Encourages fine-grained design • Much more advanced than RPC • High barrier to entry • Caused by binary protocols, high core complexity • Initial big interoperability problems • Due to the above, and incomplete specifications

  12. TECHNICAL SOLUTIONWEB SERVICE SPECIFICATION AND PROTOCOL STACK

  13. Position in the Communication Architecture:TCP/IP vs. OSI Model See: http://www.tcpipguide.com/free/t_TCPIPArchitectureandtheTCPIPModel-2.htm

  14. Web Service StackOverview Service-based Processes • Compositions of Web Services • WS-BPEL • Registry of Web Services • UDDI Service Discovery • Describes the public interface to a specific Web Service • WSDL Service Description Service Messaging • XML-based message encoding • XML-RPC, SOAP Service Transport • Message transportation • HTTP, SMTP, FTP, BEEP Web Service Specification and Protocol Stack

  15. SOAPOverview • née Simple Object Access Protocol • In fact, simple messaging protocol • Current version: 1.2 @ W3C Recommendation 2007 • Message structure • Processing model • Protocol bindings

  16. SOAPMessages • XML Envelope • Body • Application payload • Faults • Headers • Metadata, processing instructions • May be marked as mandatory • Possibly targeted at intermediaries

  17. SOAPMessage Example <soap:Envelope namespaces…> <soap:Header> <wsa:MessageID> unique ID </wsa:MessageID> <wsa:ReplyTo> endpoint </wsa:ReplyTo> </soap:Header> <soap:Body> <bank:BalanceRequest> data </bank:BalanceRequest> </soap:Body> </soap:Envelope>

  18. SOAPMessage Envelope Body Header

  19. SOAPProcessing Model • Processing a message • Selecting headers targeted at me • The current intermediary or ultimate receiver • Checking for understanding • Do I understand all that is targeted at me and marked as mandatory? • Processing everything in some order

  20. SOAPMandatory Headers • mustUnderstand="true" • The recipient must understand them • Implies agreement to act in accordance to the spec • Non-mandatory headers can be ignored • This mechanism enables gracious evolution • If a new feature can be ignored, its introduction won't harm older nodes • If a new feature must be understood, its introduction will be discovered early by older nodes, without unexpected behavior

  21. SOAPProtocol Bindings • Transporting the message over a network • Addressing • What an endpoint address looks like • Serialization • How to put the XML message in on-the-wire bits and bytes • Connection • How to send the bits to the endpoint

  22. SOAPHTTP Binding • Addressing: URIs • Serialization: HTTP message body • Media type application/soap+xml • Connection: TCP • Possibly Web-friendly • SOAP 1.1 only used HTTP POST

  23. SOAPMEPs • Message Exchange Patterns • Request-Response • Input message followed by output or fault • SOAP-Response • Request without SOAP (e.g. HTTP GET) • SOAP output or fault

  24. SOAPSummary • Still pretty simple • Extremely extensible • HTTP binding for easy communication • The value of SOAP: XML and Processing Model

  25. SOAPAttachments • XML is nice, but… not binary • XOP: XML-binary Optimized Packaging • Binary data in XML logically in the tree • On serialization, it is outside, raw, efficient • MIME multipart message • MTOM: XOP for SOAP • Message Transfer Optimization Mechanism • Extends SOAP HTTP binding

  26. WS-Addressing • Simple routing protocol • Endpoint References for addressing • With parameters or metadata in addition to the service address • Message headers for routing & correlation • Many headers use endpoint references

  27. WSA Msg Info Headers • To – let the middleware deliver the message • From, ReplyTo, FaultTo – channels back • MessageID, RelatesTo – simple correlation • Action – semantics implied by the message <S:Header> <wsa:MessageID> unique ID </wsa:MessageID> <wsa:ReplyTo> endpoint </wsa:ReplyTo> <wsa:To> address </wsa:To> <wsa:Action> URI </wsa:Action> </S:Header>

  28. WSDLOverview • Web Service Description Language • Interface Definition Language (IDL) for Web Services • Current version: 2.0 @ W3C Recommendation 2007 • Version 1.1 still in widespread use • Interface – reusable, abstract • Operations with MEPs • Binding – reusable, concrete • Service implements an interface • Endpoints use bindings

  29. WSDLComponent Structure service service service

  30. WSDLComponent Structure service service service endpoint endpoint

  31. WSDLComponent Structure interface interface interface service service service endpoint endpoint

  32. WSDLComponent Structure interface interface interface operation operation operation fault fault fault service service service endpoint endpoint

  33. WSDLComponent Structure interface interface interface operation operation operation fault fault fault binding interface interface service service service endpoint endpoint

  34. WSDLComponent Structure interface interface interface operation operation operation fault fault fault binding interface interface operation operation operation service service service fault fault fault endpoint endpoint

  35. WSDLComponent Structure interface operation operation operation fault fault fault

  36. WSDLComponent Structure interface operation operation operation message ref msg ref msg ref fault ref msg ref msg ref fault fault fault

  37. WSDLComponent Structure interface operation operation operation types message ref msg ref msg ref fault ref msg ref msg ref fault fault fault

  38. types element declaration operation operation type definition fault fault WSDLComponent Structure interface operation operation operation message ref msg ref msg ref fault ref msg ref msg ref fault fault fault

  39. types element declaration operation operation type definition fault fault WSDLComponent Structure interface operation operation operation message ref msg ref msg ref fault ref msg ref msg ref fault fault fault

  40. WSDLInterface • Design of application interface • Possibly extending other interfaces • Operations • Message exchange pattern (MEP) • Input/output messages, faults • Referencing XML elements defined in types • Faults • Used and reused by operations

  41. WSDLInterface Example <interface name=“Banking”> <operation name=“transfer” pattern=“http://www.w3.org/2004/08/wsdl/in-out”> <input element=“Transfer”/> <output element=“Balance”/> <outfault ref=“InvalidBankAccount”/> <outfault ref=“InsufficientFunds”/> </operation> <operation name=“balance” safe=“true” pattern=“http://www.w3.org/2004/08/wsdl/in-out”> <input element=“BalanceRequest”/> <output element=“Balance”/> <outfault ref=“InvalidBankAccount”/> </operation> <fault name=“InvalidBankAccount” element=“InvalidAccountInfo” /> <fault name=“InsufficientFunds” element=“InsufficentFundsInfo” /> </interface>

  42. WSDLPredefined MEPs Invocation: In-only In-out Out-only Out-in Messaging: Robust in-only In-optional-out Robust out-only Out-optional-in core additional, not formal standard

  43. WSDLInvocation MEPs • In-only: a single input message • No faults • In-out: a single input message, a single output message • A fault may replace output message • Out-only and Out-in mirror images of the above

  44. WSDLMessaging MEPs • Robust in-only: a single input message • May trigger a fault • In-optional-out: a single input message, possibly an output message • Either message may trigger a fault • Robust out-only, Out-optional-in mirrored

  45. WSDLBindings • Networking details necessary for accessing the service • Copies interface structure • SOAP and HTTP bindings provided

  46. WSDLBindings • SOAP binding: • XML message in SOAP envelope • Transport using a SOAP protocol binding (HTTP) • HTTP binding: • Web-friendly • XML message in payload, or as parameters in the URI

  47. WSDLService • A logical node of the application • One interface • Multiple alternate endpoints • Endpoints may have different bindings • E.g. SOAP over HTTP for a public endpoint, and SOAP over JMS for the intranet

  48. WSDLService Example <service name=“HypoTirol” interface=“Banking”> <endpoint name=“visible” binding=“HTTP” address=“http://ht.at/bankingSvc” /> <endpoint name=“tls” binding=“SecureHTTP” address=“https://ht.at/bankingSvc” /> </service>

  49. WSDLSummary • Concrete service • Abstract and reusable Interface • Network binding • WSDL does not imply implementation • CORBA IDL requires objects • Exchange of XML business documents • Extensible in many ways

  50. UDDI • Universal Description, Discovery and Integration • A Web service registry API specification • Business-oriented service publication, discovery (initially limited search capabilities) • Itself has Web service interface • Useful for intranet registries • A failed public service

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