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

2. Ingeniørhøjskolen i Århus Slide 2 af 17 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 Activation and RMI IIOP

3. Ingeniørhøjskolen i Århus Slide 3 af 17 Architecture ServerClient Stub Registry Interfaces Skeleton Activation Interfaces RMI Runtime (rmid,rmiregistry) coded manually rmic generated bind lookup RMI registry is light-weight version naming service

4. Ingeniørhøjskolen i Århus Slide 4 af 17 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. Ingeniørhøjskolen i Århus Slide 5 af 17 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. Ingeniørhøjskolen i Århus Slide 6 af 17 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. Ingeniørhøjskolen i Århus Slide 7 af 17 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 will partly solve this No composite names Security Restriction: Name bindings cannot be created from remote hosts There has to be a registry on each host

8. Ingeniørhøjskolen i Århus Slide 8 af 17 Alternative Registry Use JNDI: Java Naming and Directory Interface –A standard API for accessing naming and directory services (like JDBC to databases) –Standard i Java: LDAP, RMI Registry, CORBA Naming service http://java.sun.com/products/jndi /tutorial/getStarted/overview/ind ex.html intro to JNDIhttp://java.sun.com/products/jndi /tutorial/getStarted/overview/ind ex.html http://java.sun.com/products/jndi /tutorial/objects/storing/remote.h tml JNDI and RMIhttp://java.sun.com/products/jndi /tutorial/objects/storing/remote.h tml

9. Ingeniørhøjskolen i Århus Slide 9 af 17 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

10. Ingeniørhøjskolen i Århus Slide 10 af 17 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.

11. Ingeniørhøjskolen i Århus Slide 11 af 17 Data Collection & Presentation Server TRS RMI/CORBA Object New Reading DB PSP View Readings Classic Client / Sever model is sufficient for Data Collection & Presentation

12. Ingeniørhøjskolen i Århus Slide 12 af 17 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

13. Ingeniørhøjskolen i Århus Slide 13 af 17 Issues Distributed Deadlock –If client and server 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 –Server should store registered callbacks on persistent storage in case of server failure 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