1. From Coulouris, Dollimore, Kindberg and Blair Distributed Systems: Concepts and Design Edition 5, © Addison-Wesley 2012 Slides for Chapter 9 Web Services

2. Introduction web service provides a service interface enabling clients to interact with servers in a more general way than web browsers. Clients access the operations in the interface of a web service. External data representation and exchanging messages between clients and web services is done in XML Web services also provide the middleware for cloud computing. 2

3. Introduction Commercial web servers including Amazon, Yahoo, Google and eBay, offer web service interfaces that allow clients to manipulate their web resources. As an example, the web service offered by Amazon.com provides operations to allow clients to get information about products, to add an item to a shopping cart or to check the status of a transaction. 3

4. Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 9.1 Web services infrastructure and components Security Service descriptions (in WSDL) Applications Directory service Web Services XML Choreography SOAP URIs (URLs or URNs)HTTP, SMTP or other transport

5. Figure 9.1 Web services infrastructure and components Directory service: to allow clients to find out about services. Security: documents or parts of documents may be signed or encrypted. Choreography: provide access to resources for remote clients Service description: interface definition and other information, such as the server’s URL in Web Services Description Language (WSDL). The SOAP(Simple Object Access protocol) specifies the rules for using XML to package messages and transmit them over HTTP 5

6. hire car booking Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 9.2 The ‘travel agent service’ is an example of combining web services hotel booking a Travel Agent flight booking a hire car booking a Service Client flight booking b hotel booking b b

7. Figure 9.3 SOAP message is in an envelope To enable both client-server and asynchronous interaction over the Internet using SMTP, TCP or UDP protocols. envelope header body header element body element header element body element

8. Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 9.10 The main elements in a WSDL description The abstract part of the description includes a set of definitions of the types used by the service The Concrete part includes the binding (the choice of protocols) and the service (the choice of endpoint or server address) abstract concrete howwhere definitions types target namespace interfacebindings services message document stylerequest-reply style

9. A directory service for use with web services Universal Description, Discovery and Integration service (UDDI) provides both a name service and a directory service WSDL service descriptions may be looked up by name or by attribute They may also be accessed directly via their URLs, which is convenient for developers Clients may use the attribute approach to look up a particular category of service, such as travel agent or bookseller, or they may use the name approach to look up a service with reference to the organization that provides it. 9

10. XML security XML security is used for signing, key management and encryption. As an example of a context in which XML security would be useful, consider a document containing a patient’s medical records. It will be updated by doctors, nurses and consultants making notes on the patient’s condition and treatment It is essential for those parts to be secured and not viewed except for the specialized person. Securing parts of documents is available by XML security algorithms. 10

11. Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 9.18 Travel agent scenario 1. The client asks the travel agent service for information about a set of services; for example, flights, car hire and hotel bookings. 2. The travel agent service collects prices and availability information and sends it to the client, which chooses one of the following on behalf of the user: (a) refine the query, possibly involving more providers to get more information, then repeat step 2; (b) make reservations; (c) quit. 3. The client requests a reservation and the travel agent service checks availability. 4. Either all are available; or for services that are not available; either alternatives are offered to the client who goes back to step 3; or the client goes back to step 1. 5. Take deposit. 6. Give the client a reservation number as a confirmation. 7. During the period until the final payment, the client may modify or cancel reservations