Client Dispatcher Pattern 2e323s

1) Client Dispatcher Pattern (Communication) Intent: The Client-Dispatcher-Server design pattern introduces an intermediate layer between clients and servers, the dispatcher component. It provides location transparency by means of a name service, and hides the details of the establishment of the communication connection between clients and servers. Structure:

Collaboration:

Participants: Client: The task of a client is to perform domain-specific tasks. The client accesses operations offered by servers in order to carry out its processing tasks. Before sending a request to a server, the client asks the dispatcher for a communication channel. The client uses this channel to communicate with the server. Server: A server provides a set of operations to clients. It either s itself or is ed with the dispatcher by its name and address. A server component may be located on the same computer as a client, or may be reachable via a network. Dispatcher: The dispatcher offers functionality for establishing communication channels between clients and servers. To do this, it takes the name of a server component and maps this name to the physical location of the server component. The dispatcher establishes a communication link to the server using the available communication mechanism and returns a communication handle to the client. If the dispatcher cannot initiate a communication link with the requested server, it informs

the client about the error it encountered. To provide its name service, the dispatcher implements functions for ing and locating servers.

The Class diagram:

Java Code: //CDS.java public class CDS { public static Dispatcher disp = new Dispatcher(); public static void main(String[] args) { Service sl = new PrintService("printSvc1","srvl"); Service s2 = new PrintService("printSvc2","srv2"); Client client = new Client (); client.doTask (); } } //Dispatcher.java import java.util.*; import java.io.*; public class Dispatcher { private Client client; private PrintService printservice; Hashtable registry = new Hashtable(); Random rnd = new Random(123456); // for random access public void Service(String svc, Service obj) { Vector v = (Vector) registry. get (svc) ; if (v == null) { v = new Vector (); registry.put(svc, v) ; } v.addElement(obj); } public void unService() { } public Service locateServer(String svc)throws NotFound { Vector v = (Vector) registry .get (svc) ; if (v == null) throw new NotFound ( ) ; if (v.size() == 0) throw new NotFound(); int i = rnd.nextInt () % v.size() ; return (Service) v. elementAt (i) ; } public void establishChannel() { } public void getChannel() { } }

//Service.java (an abstract class to provide runService method and to all the Services to the dispatcher, it is the server participant) public abstract class Service { String nameofservice; // service name String nameofserver; // server name public Service(String svc, String srv) { nameofservice = svc; nameofserver = srv; CDS.disp.Service(nameofservice,this) ; } abstract public void runService() ; // service provided } //PrintService.java (A real service class extending the abstract class Service) public class PrintService extends Service{ private Dispatcher dispatcher; private Client client; public PrintService(String svc, String srv) { super(svc, srv); } public void acceptConnection() { } public void runService() { //test output System.out.println("Service " + nameofservice + " by " + nameofserver); //here the service code would be implemented } public void receiveRequest() { } } //Client.java:( A client class with do Task operation) public class Client { private Dispatcher dispatcher; private PrintService printservice; public void doTask() { Service s; try { s = CDS .disp. locateServer ("printSvc1") ; s . runService ( ) ; }catch (NotFound n) { System.out .println ("Not available") ; } try { s = CDS .disp.locateServer("printSvc2"); s .runService ( ) ; }catch (NotFound n) { System.out.println("Not available"); } try { s = CDS .disp.locateServer("drawSvc"); s .runService () ; }catch (NotFound n) { System.out.println("Not available") ; } } public void sendRequest() { } } Output: Service printSvc1 by srvl Service printSvc2 by srv2

Not available