1. Presentation: RMI Continued 2 Using The Registry & Callbacks

2. Goals of this lesson After this 1x35 lessons you will be Introduced to the RMI registry (rmiregistry) Introduced to RMI Callbacks Next time Java RMI over IIOP

3. Architecture ServerClient Stub Registry Interfaces Skeleton Activation Interfaces RMI Runtime (rmid,rmiregistry) coded manually rmic generated bind lookup RMI registry is light-weight naming service Independent proces with RMI interface

4. Naming in RMI: The RMI Registry package java.rmi.registry; public interface Registry extends java.rmi.Remote { public static final int REGISTRY_PORT = 1099; public java.rmi.Remote lookup(String name) throws java.rmi.RemoteException, java.rmi.NotBoundException, java.rmi.AccessException; public void bind(String name, java.rmi.Remote obj) throws java.rmi.RemoteException, java.rmi.AlreadyBoundException, java.rmi.AccessException; public void rebind(String name, java.rmi.Remote obj) throws java.rmi.RemoteException, java.rmi.AccessException; public void unbind(String name) throws java.rmi.RemoteException, java.rmi.NotBoundException, java.rmi.AccessException; public String[] list() throws java.rmi.RemoteException, java.rmi.AccessException; }

5. package examples.hello; import java.rmi.Naming; import java.rmi.RemoteException; import java.rmi.RMISecurityManager; import java.rmi.server.UnicastRemoteObject; public class HelloImpl extends UnicastRemoteObject implements Hello { public HelloImpl() throws RemoteException { super(); } public String sayHello() { return "Hello World! ; } public static void main(String args[]) { // Create and install a security manager //if (System.getSecurityManager() == null) { // System.setSecurityManager(new RMISecurityManager()); //} try { HelloImpl obj = new HelloImpl(); // Bind this object instance to the name "HelloServer" Naming.rebind("rmi://192.168.1.101/HelloServer", obj); System.out.println("HelloServer bound in registry"); } catch (Exception e) { System.out.println("HelloImpl err: " + e.getMessage()); e.printStackTrace(); } Server object (HelloImpl.java) Instantiate a new object and register (bind it) in the ”rmiregistry” Following methods available: bind, rebind, unbind, lookup Instantiate a new object and register (bind it) in the ”rmiregistry” Following methods available: bind, rebind, unbind, lookup

6. package examples.hello; import java.rmi.Naming; import java.rmi.RemoteException; public class HelloClient { public static void main(String args[]) { try { obj = (Hello)Naming.lookup("rmi://192.168.1.101/HelloServer"); String message = obj.sayHello(); System.out.println(message); } catch (Exception e) { System.out.println("HelloApplet exception: " + e.getMessage()); e.printStackTrace(); } ”lookup” the HelloServer – and call Method sayHello() on Stub ”lookup” the HelloServer – and call Method sayHello() on Stub Client object (HelloClient.java)

7. Limitations of RMI Registry Client always has to identify the server by name. obj = (Hello)Naming.lookup("rmi://192.168.1.101/HelloServer"); Inappropriate if client just wants to use a service at a certain quality but does not know from who DNS usage + load balancer will partly solve this Security Restriction: Name bindings cannot be created from remote hosts One Registry pr Server There has to be a registry on each host No composite names Alternatives: make your own or JNDI

8. Alternative Registry Use JNDI: Java Naming and Directory Interface Standard API for accessing naming and directory services (like JDBC to databases) Standard in Java: LDAP, RMI Registry, CORBA Naming service http://java.sun.com/products/jndi/tutori al/getStarted/overview/index.html intro to JNDIhttp://java.sun.com/products/jndi/tutori al/getStarted/overview/index.html http://java.sun.com/products/jndi/tutori al/objects/storing/remote.html JNDI and RMIhttp://java.sun.com/products/jndi/tutori al/objects/storing/remote.html

9. Nice feature – bootstrapping the Registry As until now, you have been manually starting the RMI Registry, which is a constant source of errors and other inconveniences. May be solved more elegantly: LocateRegistry.createRegistry(PORT); And you are up and running, ready to bind remote objects Access via class Naming as usual

10. // HelloServer.java import javax.naming.Context; import javax.naming.InitialContext; public class HelloServer { // The main method is used to bootstrap the HelloImpl and register it // with the Naming Service public static void main(String[] args) { try { // Start with instantiating the Hello servant HelloImpl hello = new HelloImpl(); // Next we publish the reference in the Naming Service Context initialNamingContext = new InitialContext(); initialNamingContext.rebind("HelloAppService", hello); System.out.println("Hello Server is up and running..."); } catch (Exception e) { System.out.println("An exception occurred: " + e); } BootStrap Server (HelloServer.java) We now use InitialContext as the Naming service We now use InitialContext as the Naming service Starting the server program with info on the ORBD Starting the server program with info on the ORBD ORBD is the Java CORBA Naming Service ORBD is the Java CORBA Naming Service

11. Callbacks Sometimes Client/Server is not enough Publish/Subscribe pattern / Observer CORBA has support for this An ORB is always both client and server Java RMI does not have support for this -BUT: turn the client object into a remote object -Web services -No support.

12. Data Collection & Presentation Server TRS RMI/CORBA Object New Reading DB PSP View Readings Classic Client / Sever model is sufficient for Data Collection & Presentation

13. Alarm level surveillance Server TRS New Reading DB PSP View Readings Problem: The client / server pattern breaks down when we want to notify FROM the server to the client. Solution: Client polling OR peer-to-peer model -> e.g. using Callbacks Present Alarm RMI/CORBA Object

14. Issues Distributed Deadlock If client and server are single-threaded Client calls server, server calls client back instantly Deadlock – both are blocked, waiting for a response Solution: Do not make single-threaded applications Problem: not all OS’s support multithreading Inconvenience : multithreading introduces new complexities Callback Persistence & Keep-Alive What happens if server breaks down? Server should store registered callbacks on persistent storage in case of server failure Consider implementing “ heart-beat ” or “ keep-alive ” Callback Failure As callback objects are transient, server should employ a “timeout” strategy for callback communication Coupling Callback objects comes at a price of higher coupling