1. WEB SERVICES 1 From Chapter 19 of Distributed Systems Concepts and Design,4 th Edition, By G. Coulouris, J. Dollimore and T. Kindberg Published by Addison Wesley/Pearson Education June 2005

2. WEB SERVICES 2 Topics Introduction Web Services SOAP (Simple Object Access Protocol) Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

3. WEB SERVICES 3 Introduction  A web service provides a service interface enabling clients to interact with servers in a more general way than web browsers do.  Clients access the operations in the interface of a web service by means of requests and replies formatted in XML and usually transmitted over HTTP. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

4. WEB SERVICES 4 Introduction  Users require a secure means for creating, storing and modifying documents and exchanging them over the Internet.  The secure channels of TLS do not provide all of the necessary requirements.  XML security is intended to breach this gap. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

5. WEB SERVICES 5 Introduction  Web services provide an infrastructure for maintaining a richer and more structured form of interoperability between clients and servers.  They provide a basis whereby a client program in one organization may interact with a server in another organization without human supervision. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

6. WEB SERVICES 6 Introduction  External data representation and marshalling of messages exchanged between clients and web services is done in XML. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

7. WEB SERVICES 7 Introduction  The SOAP (Simple Object Access Protocol) specifies the rules for using XML to package messages, for example to support a request-reply protocol.  Figure 1 summarizes the main points about the communication architecture in which web services operate. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

8. WEB SERVICES 8 Introduction Figure 1. Web services infrastructure and components Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

9. WEB SERVICES 9 Introduction  A web service is identified by a URL and can be accessed by clients using messages formatted in XML. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

10. WEB SERVICES 10 Introduction  SOAP is used to encapsulate these messages and transmit them over HTTP or another protocol, for example, TCP or SMTP.  A web service generally provides a service description, which includes an interface definition and other information, such as the server's URL. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

11. WEB SERVICES 11 Web Services  A web service interface generally consists of a collection of operations that can be used by a client over the Internet.  The operations in a web service may be provided by a variety of different resources, for example, programs, objects, databases.  A web service may either be managed by a web server along with web pages; or it may be a totally separate service. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

12. WEB SERVICES 12 Web Services  Many well-known commercial web services including Amazon, Yahoo, Google and eBay offer web service interfaces that allow client 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. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

13. WEB SERVICES 13 Web Services  The Amazon web services may be accessed either by SOAP or by REST (REpresentaional State Transfer).  The provision of a service interface allows its operations to be combined with those of other services to provide new functionality. Figure 2. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

14. WEB SERVICES 14 Web Services Figure 2. The ‘travel agent service’ combines other web services Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

15. WEB SERVICES 15 Web Services  A major task of many middleware platforms is to protect the programmer from the details of data representation and marshalling and sometimes with making remote invocations look like local ones.  These things are provided as a part of an infrastructure or middleware platform for web services. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

16. WEB SERVICES 16 Web Services  At the simplest level, clients and servers may read and write their messages directly in SOAP, using XML. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

17. WEB SERVICES 17 SOAP (Simple Object Access Protocol)  SOAP is defined to enable both client- server and asynchronous interaction over the Internet.  It defines a scheme for using XML to represent the contents of request and reply messages as well as a scheme for the communication of documents.  Originally, SOAP was based on HTTP, but the current version is designed to use a variety of transport protocols including SMTP, TCP or UDP. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

18. WEB SERVICES 18 SOAP (Simple Object Access Protocol)  The SOAP specification states:  How XML is to be used to represent the contents of individual messages.  How a pair of single messages can be combined to produce a request-reply pattern.  The rules as to how the recipients of messages should process the XML elements that they contain.  How HTTP and SMTP should be used to communicate SOAP messages. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

19. WEB SERVICES 19 SOAP (Simple Object Access Protocol)  A SOAP message is carried in an “envelope”.  Inside the envelope there is an optional header and a body as shown in Figure 3. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

20. WEB SERVICES 20 SOAP (Simple Object Access Protocol) Figure 3. SOAP message in an envelope Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

21. WEB SERVICES 21 SOAP (Simple Object Access Protocol)  Message headers can be used for establishing the necessary context for a service or for keeping a log or audit of operations.  The message body carries an XML document for a particular web service. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

22. WEB SERVICES 22 SOAP (Simple Object Access Protocol)  The XML elements envelope, header and body, together with other attributes and elements of SOAP messages are defined as a scheme in the SOAP XML namespace.  Figure 4 shows an example of a simple request message without a header. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

23. WEB SERVICES 23 SOAP (Simple Object Access Protocol) Figure 4. Example of a simple request without headers Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

24. WEB SERVICES 24 SOAP (Simple Object Access Protocol)  Figure 5 shows the corresponding successful reply messages, which contains the two output arguments. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

25. WEB SERVICES 25 SOAP (Simple Object Access Protocol) Figure 5. Example of a reply corresponding to the request in Figure 4 Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

26. WEB SERVICES 26 SOAP (Simple Object Access Protocol)  A transport protocol is required to send a SOAP message to its destination.  SOAP messages are independent of the type of transport used- their envelopes contain no reference to the destination address. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

27. WEB SERVICES 27 SOAP (Simple Object Access Protocol)  Figure 6 illustrates how the HTTP POST method is used to transmit a SOAP message. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

28. WEB SERVICES 28 SOAP (Simple Object Access Protocol) Figure 6. Use of HTTP POST Request in SOAP client-server communication Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

29. WEB SERVICES 29 SOAP (Simple Object Access Protocol)  The HTTP headers and body are used as follows:  The HTTP headers specify the endpoint address (the URI of the ultimate receiver) and the action to be carried out.  The HTTP body carries the SOAP message.  As HTTP is a synchronous protocol, it is used to return a reply containing the SOAP reply, as shown in Figure 5. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

30. WEB SERVICES 30 SOAP (Simple Object Access Protocol)  Figure 7 shows a web service interface. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

31. WEB SERVICES 31 SOAP (Simple Object Access Protocol) package ShapeListService; import java.rmi.*; public interface ShapeList extends Remote { int newShape(GraphicalObject g) throws RemoteException; int numberOfShapes()throws RemoteException; int getVersion() throws RemoteException; int getGOVersion(int i)throws RemoteException; GraphicalObject getAllState(int i) throws RemoteException; } Figure 7. Java web service interface ShapeList Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

32. WEB SERVICES 32 SOAP (Simple Object Access Protocol)  The Java interface of a web service must conform to the following rules, some of which are illustrated in Figure 7:  It must extend the Remote interface.  It must not have constant declarations, such as public final static.  The methods must throw the java.rmi.RemoteException or one of its subclasses.  Method parameters and return types must be permitted JAX-RPC types. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

33. WEB SERVICES 33 SOAP (Simple Object Access Protocol)  The class that implements the interface ShapeList is shown in Figure 8. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

34. WEB SERVICES 34 SOAP (Simple Object Access Protocol) package ShapeListService; import java.util.Vector; public class ShapeListImpl implements ShapeList{ private Vector theList = new Vector(); private int version = 0; private Vector theVersions = new Vector(); public ShapeListImpl(){ version = 0; theVersions = new Vector(); } public int newShape(GraphicalObject g) { version++; theList.addElement(g); theVersions.addElement(new Integer(version)); return theList.size(); } public int numberOfShapes(){ return theList.size(); } public int getVersion() { return version; } public int getGOVersion(int i){ return ((Integer) theVersions.elementAt(i)).intValue(); } public GraphicalObject getAllState(int i) { return (GraphicalObject)theList.elementAt(i); } } Figure 8. Java implementation of the ShapeListserver Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

35. WEB SERVICES 35 SOAP (Simple Object Access Protocol)  There is no main method, and the implementation of the ShapeList interface does not have a constructor.  In effect, a web service is a single object that offers a set of procedures. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

36. WEB SERVICES 36 SOAP (Simple Object Access Protocol)  Figure 9 shows the ShapeList client main a call through a proxy. Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

37. WEB SERVICES 37 SOAP (Simple Object Access Protocol) package staticstub; import javax.xml.rpc.Stub; public class ShapeListClient{ public static void main(String args[]){ try{ Stub stub = createProxy(); stub._setProperty (javax.xml.rpc.Stub.ENDPOINT_ADDRESS_PROPERTY, args[0]); aShapeList = (ShapeList) stub; System.out.println("Got stub"); int n = aShapeList.numberOfShapes(); System.out.println("Number of shapes=" + n); int v = aShapeList.getVersion(); System.out.println("Version=" + v); Vector sList = new Vector(n); for(int i=0; i<n; i++){ System.out.println("shape no =" + i); GraphicalObject g = aShapeList.getAllState(i); System.out.println("Got shape " + i); int vers = aShapeList.getGOVersion(i); sList.addElement(g); System.out.println("Version number " + vers); g.print(); } Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005

38. WEB SERVICES 38 SOAP (Simple Object Access Protocol) GraphicalObject g = new GraphicalObject(shapeType,30, 40, 3300, 5500, true); System.out.println("Created graphical object"); int listLength = aShapeList.newShape(g); System.out.println("Stored shape " + shapeType); }catch(Exception e) {System.out.println("allShapes: " + e.getMessage()); } private static Stub createProxy() { return (Stub) (new MyShapeListService_Impl().getShapeListPort()); } Figure 9. Java implementation of the ShapeListclient Couloris,Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4, Pearson Education 2005