1. EEC-681/781 Distributed Computing Systems Lecture 5 Wenbing Zhao Department of Electrical and Computer Engineering Cleveland State University wenbing@ieee.org

2. 2 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Outline Case study: Java RMI –Material taken from http://java.sun.com/developer/onlineTraining/rmi/RMI.html

3. 3 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Java RMI Architecture Design goal: to create a Java distributed object model that integrates naturally into the Java programming language and the local object model The definition of a remote service is coded using a Java interface The implementation of the remote service is coded in a class Interfaces define behavior and classes define implementation

4. 4 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Java RMI Architecture Layers Stub and Skeleton layer: intercepts method calls made by the client to the interface and redirects these calls to a remote RMI service Remote reference layer: to interpret and manage references made from clients to the remote service objects Transport layer: provides basic connectivity, as well as some firewall penetration strategies Transport Layer Remote Reference Layer Stubs & Skeletons Client ProgramServer Program RMI System

5. 5 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Stub and Skeleton Layer Java RMI follows the Proxy pattern –The stub class plays the role of the proxy –The remote service implementation class plays the role of the RealSubject > Subject Request() RealSubject Request() Proxy Request()

6. 6 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Remote Reference Layer Defines and supports the invocation semantics of the RMI connection –This layer provides a RemoteRef object that represents the link to the remote service implementation object –The stub objects use the invoke() method in RemoteRef to forward the method call –The RemoteRef object understands the invocation semantics for remote services

7. 7 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Transport Layer The Transport Layer makes network connection between JVMs Java Remote Method Protocol (JRMP): Java RMI’s wire protocol (proprietary) on top of TCP/IP

8. 8 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao RMI Registry RMI Registry ( rmiregistry ): a simple naming service that comes with Java RMI –It runs on each machine that hosts remote service objects, by default on port 1099 –It accepts registration request only from the local RMI servers –It accepts queries for services anywhere

9. 9 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao RMI Registry To access the remote server from client side: –Query a registry by invoking the lookup() method on the static Naming class –The method lookup() accepts a URL that specifies the server host name and the name of the desired service rmi:// [: ]/ –The method returns a remote reference to the service object

10. 10 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Steps to Create a Remote Service First create a local object that implements that service Next, export that object to Java RMI. When the object is exported, Java RMI creates a listening service that waits for clients to connect and request the service After exporting, register the object in the Java RMI Registry under a public name

11. 11 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Using Java RMI A working Java RMI system is composed of several parts: –Interface definitions for the remote services –Implementations of the remote services –Stub and Skeleton files –A server to host the remote services –An RMI Naming service that allows clients to find the remote services –A class file provider (an HTTP or FTP server) –A client program that needs the remote services

12. 12 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Steps to Build a System Design your system Write and compile Java code for interfaces Write and compile Java code for implementation classes Generate Stub and Skeleton class files from the implementation classes Write Java code for a remote service host program Develop Java code for Java RMI client program Install and run Java RMI system

13. 13 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Service Interface public interface Calculator extends java.rmi.Remote { public long add(long a, long b) throws java.rmi.RemoteException; public long sub(long a, long b) throws java.rmi.RemoteException; public long mul(long a, long b) throws java.rmi.RemoteException; public long div(long a, long b) throws java.rmi.RemoteException; } To compile it: > javac Calculator.java

14. 14 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Implementation of Remote Service public class CalculatorImpl extends java.rmi.server.UnicastRemoteObject implements Calculator { // Implementations must have an explicit constructor // in order to declare the RemoteException exception public CalculatorImpl() throws java.rmi.RemoteException { super(); } public long add(long a, long b) throws java.rmi.RemoteException { return a + b; } …

15. 15 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Stubs and Skeletons To generate the stub and skeleton files, invoke the RMI compiler, rmic The compiler runs on the remote service implementation class file > rmic CalculatorImpl After you run rmic you should find the file CalculatorImpl_Stub.class

16. 16 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Host Server Remote RMI services must be hosted in a server process import java.rmi.Naming; public class CalculatorServer { public CalculatorServer() { try { Calculator c = new CalculatorImpl(); Naming.rebind(" rmi://localhost:1099/CalculatorService ", c); } catch (Exception e) { System.out.println("Trouble: " + e); } } public static void main(String args[]) { new CalculatorServer(); } }

17. 17 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Client import java.rmi.Naming; import java.rmi.RemoteException; import java.net.MalformedURLException; import java.rmi.NotBoundException; public class CalculatorClient { public static void main(String[] args) { try { Calculator c = (Calculator) Naming.lookup( "rmi://localhost/CalculatorService"); System.out.println( c.sub(4, 3) ); System.out.println( c.add(4, 5) ); System.out.println( c.mul(3, 6) ); System.out.println( c.div(9, 3) ); } …

18. 18 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Running the RMI System Make sure you change to the directory that contains the classes you have written Launch a terminal, start Java RMI registry > rmiregistry Launch another terminal, start the server > java CalculatorServer Launch the 3 rd terminal, start the client > java CalculatorClient

19. 19 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Parameter Passing in Java RMI When Java RMI calls involve passing parameters or accepting a return value –How does RMI transfer these between JVMs? –What semantics are used? –Does RMI support pass-by-value or pass-by- reference?

20. 20 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Parameter Passing in Java RMI Primitive parameters: pass by value –For both input parameter and return type, Java RMI makes a copy of a primitive data type and send it to the destination Object parameters: pass by value –The object to be passed, together with all the objects it references, are serialized and copied over

21. 21 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Parameter Passing in Java RMI Remote object parameters: pass by reference –A client can obtain a reference to a remote object through the Java RMI Registry program –A client can obtain a remote reference as a result of making a remote method call

22. 22 Fall Semester 2008EEC-681: Distributed Computing SystemsWenbing Zhao Distributed Garbage Collection in Java RMI Java takes care of memory management Distributed garbage collection is needed in Java RMI –Many challenges to do so. Most prominent problem is that a client might quit without notice –Solution: lease based. The resource is granted to a client for certain period of time. It is the client’s responsibility to renew the lease. The resource is reclaimed if lease is not renewed