Presentation: RMI Continued 2 Using The Registry & Callbacks

Presentation:RMI Continued 2Using The Registry & Callbacks

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

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

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; }

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

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(); } } } Client object (HelloClient.java) ”lookup” the HelloServer – and call Method sayHello() on Stub

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 • No composite names • Security Restriction: Name bindings cannot be created from remote hosts • There has to be a registry on each host • Alternatives: make your own or JNDI

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/index.html intro to JNDI • http://java.sun.com/products/jndi/tutorial/objects/storing/remote.htmlJNDI and RMI

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

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.

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

Alarm level surveillance TRS Server New Reading RMI/CORBA Object PSP View Readings Present Alarm DB 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

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

Presentation: RMI Continued 2 Using The Registry & Callbacks