1. 6/5/2016B.Ramamurthy1 Distributed Objects and Remote Invocation Chapter 5 B.Ramamurthy

2. 6/5/2016B.Ramamurthy2 Remote Method Invocation Remote Method Invocation (RMI) is Java’s implementation of object-to-object communication among Java objects to realize a distributed computing model. RMI allows us to distribute our objects on various machines, and invoke methods on the objects located on remote sites. Source code for the demo is a modified version of code in Chapter 20 of Deitel & Deitel’s text Java : How to Program.

3. The Object Model 1. Object references Objects are first-class values, meaning that they may be assigned to variables, passed as arguments and returned as results of methods. 2. Interfaces Interfaces provide signatures of the set of methods Signature is the type of arguments, return value and exceptions) 6/5/2016B.Ramamurthy3

4. The Object model (contd.) 3. Action: Action is initiated by an object invoking a method in another object. 4. Exceptions: These provide clean way to deal with error conditions without complicating the code. Each method heading explicitly lists the exceptions or error conditions that may occur allowing users of the method to deal with them A block of code may be defined to throw an exception when an error occurs. The control passes to another block of code that catches the exception 6/5/2016B.Ramamurthy4

5. The Object Model 5. Garbage Collection: it is necessary to provide means for freeing the space occupied by objects when they are no longer needed. 6/5/2016B.Ramamurthy5

6. The Distributed Object Model The invoker and the invoked are separated and remote to each other. Foundations of distributed systems Distributed objects may adopt client-server architecture. It may assume other architectural model – objects may be replicated for fault-tolerance and performance. Distributed object model allows for real encapsulation, heterogeneous systems, distribution. The concept of remote interfaces. 6/5/2016B.Ramamurthy6

7. 6/5/2016B.Ramamurthy7 RMI-based Distributed System 4. XYZ Implementation Client Host Server Host XYZ Client Stub XYZ interface uses implements 1. 2.3. 5.

8. 6/5/2016B.Ramamurthy8 Steps in RMI-based Application 1. Design the interface for the service. 2. Implement the methods specified in the interface. 3. Generate the stub, skeletons, and dispatchers (for enabling distributed access to the service). 4. Register the service by name and location. 5. Use the service in an application.

9. 6/5/2016B.Ramamurthy9 Compile and Register Commands Client Host Server Host XYZ Client Stub Skeleton XYZ interface uses implements 1. 2.3. 5. XYZ Implementation rmic rmiregistry Stores object by nameFinds object by name

10. 6/5/2016B.Ramamurthy10 More Details Once the object (or service) is registered, a client can look up that service. A client (application) receives a reference that allows the client to use the service (call the method). Syntax of calling is identical to a call to a method of another object in the same program.

11. 6/5/2016B.Ramamurthy11 Parameter Marshalling Transfer of parameters (or marshalling) is done by the RMI. Complex objects are streamed using Serialization. RMI model of networking for distributed system involves only Java. No need to learn IDL or any other language.

12. 6/5/2016B.Ramamurthy12 Case Study : Temperature Service Lets create a distributed system using RMI model for networking (remote access). Basically this program will download the weather (temperature) information from the site: http://iwin.nws.noaa.gov/iwin/us/traveler.html

13. 6/5/2016B.Ramamurthy13 Temperature Client/Server Distributed Application TempServerImpl Client Host Server Host TempClient TempImpl_Stub TempServer uses implements 1. 2.3. 5. TempImpl_Stub rmic 4 compiles generates uses

14. 6/5/2016B.Ramamurthy14 Defining Remote Interface import java.rmi.*; // the interface extends Remote interface // any class implementing Remote can be accessed remotely security permitting public interface TemperatureServer extends Remote { // specify methods that can be called remotely // each method “throws RemoteException” }

15. 6/5/2016B.Ramamurthy15 RemoteException Any time you depend on outside entities there is a potential for problems in communication, networking, server crash etc. Any exception due to these should be handled by the services. This feature imparts robustness to the application. Java mandates this feature for any RMI service.

16. 6/5/2016B.Ramamurthy16 Implementing the Remote Interface import java.rmi.*; import java.rmi.server.*; import java.net.*; // others as needed TemperatureServerImpl extends UnicastRemoteObject implements TemperatureServer {

17. 6/5/2016B.Ramamurthy17 TemperatureServerImpl This class’s constructor calls a private method which in turn: 1. Connects to the url specified 2. Streams into a buffer the page referenced. 3. Parses the buffer to get the required data. 4. Creates an array of weather information.

18. 6/5/2016B.Ramamurthy18 TemperatureServerImpl (contd.) It implements the service method getWeatherInfo which simply returns the weather data gathered. The main method instantiates an object for the service, and registers it with rmiregistry.

19. 6/5/2016B.Ramamurthy19 Streaming URLs Using the openStream of java.net.URL class you can stream in the file spefied by an universal resource locator(url). It can be streamed into a buffer where it can be analyzed for information. Any number of urls can be streamed in. Unicast Communication : When you are interested in a particular remote site you will direct your net connection to that particular site using unicast.

20. 6/5/2016B.Ramamurthy20 Server Object Name Syntax for the server object name is: //host:port/remoteObjectName Default port number for rmiregistry is 1099 For local host the object name: //localhost/TempServer For a remote host //127.0.0.1/TempServer

21. 6/5/2016B.Ramamurthy21 Name Binding rebind method binds a server’s object name to the object’s name as it is in the registry. Clients use the name in the registry. There is also a bind() method. But rebind is better since it binds the most recently registered object.

22. 6/5/2016B.Ramamurthy22 WeatherInfo class It is very traditional class for keeping the information about the temperature at a single location. It has data fields : cityName, temperature, and description and get methods for these. An array of objects of this class is used in the server implementation.

23. 6/5/2016B.Ramamurthy23 Temperature Client import java.rmi.*; // import other packages constructor calls a private method getRemoteTemp which takes care of lookup of remote object and access. In this application it also displays the information.

24. 6/5/2016B.Ramamurthy24 Temperature Client (contd.) The main method in this client can get the IP address of the remote host as a command line argument. Command line argument is an array of String of items in the command line after the name of the application.

25. 6/5/2016B.Ramamurthy25 Client Details The name of the server object along with the IP of the remote location is used in Naming class’s lookup method to get an object reference. This object reference is then used for remote method calls. Observe that there is no difference between the local and remote call. WeatherItem class used in the Graphical display of the weather information.

26. 6/5/2016B.Ramamurthy26 Preparing the Application 1. Compile all the class using javac. 2. Generate the stub and the skeleton: rmic -v1.2 TemperatureServerImpl 3. Then start the registry (this will be running as a daemon) rmiregistry &

27. 6/5/2016B.Ramamurthy27 Preparing the Application 4. Run the server which will register with the RMI registry. Java TemperatureServerImpl & 5. Run the client. Java TemperatureClient & or java TemperatureClient {IPAddress} java TemperatureClient 192.168.0.150

28. 6/5/2016B.Ramamurthy28 Inside RMI http://java.sun.com/j2se/1.5.0/docs/index.html Basic RMI classes: /usr/java1.1/src/java/rmi java.rmi.registry.* java.rmi.Naming class (static/class methods) java.rmi.Remote interface (marker interface) java.rmi.server.* Default RMI port 1099 Both lookup from local and remote are acceptable.

29. 6/5/2016B.Ramamurthy29 Implementation of RMI (5.2.5) AccessException.java RemoteException.java AlreadyBoundException.java ConnectException.java ServerException.java ConnectIOException.java ServerRuntimeException.java MarshalException.java StubNotFoundException.java UnexpectedException.jav ServerError.java UnknownHostException.java NoSuchObjectException.java UnmarshalException.java NotBoundException.java RMISecurityException.java RMISecurityManager.java Remote.java MarshalledObject.java Naming.java activation dgc Registry server

30. 6/5/2016B.Ramamurthy30 The role of proxy and skeleton in remote method invocation object A object B skeleton Request proxy for B Reply Communication Remote Remote referenceCommunication module reference module module for B’s class & dispatcher remote client server servant Object A invokes a remote object in Object B for which it holds a remote object reference. “System Model”

31. 6/5/2016B.Ramamurthy31 RMI Internals: Communication Module Carries out request-reply protocol; On the client side {message type, message id, remote reference to object} are gathered and sent out. At most once invocation semantics; On the server side, it gets local reference for remote reference from remote reference module, invokes a dispatcher with this reference. See UnicastRemote (implements UnicastRemote)

32. 6/5/2016B.Ramamurthy32 RMI Internals: Remote Reference module Responsible for translating between local and remote object references and for creating remote object references. A remote object table has a mapping between local and remote references. A table at server (entry for object ref for B) and a table at client (entry for object ref for proxy B).

33. 6/5/2016B.Ramamurthy33 RMI Internals: Remote References Action of remote reference module: See RemoteRef.java interface When a remote object is to be passed as argument or result for the first time, the remote ref is asked to create a remote ref object which is added to the table. When a remote object reference arrives in a request or reply, the remote ref module is asked for corresponding local object ref, which may either a proxy or remote object. If it is not in the table RMI runtime creates it and asks remote ref module to add it to the table.

34. 6/5/2016B.Ramamurthy34 RMI Internals: RMI software Layer of software between application level objects and communication and remote reference modules: “Middleware” Proxy: provides remote access transparency. One proxy for every remote object in the client. Dispatcher: A server has one dispatcher and skeleton for each class representing a remote object. It receives request message from comm. Module It used MessageId to select appropriate method in skeleton. Proxy and dispatcher use same MessageId. Skeleton: A class of remote object has a skeleton that implements of the remote interface. All the access dependencies are hidden in this class. A remote object has a servant that directly implements the methods. Java 5 creates this dynamically. Proxies, dispatcher and skeleton are automatically generated by interface compiler. Binder: binds textual names to remote object references. RMIRegistry is a binder; Naming class; see fig.5.13 Server Threads: one thread per invocation Distributed garbage collection: See Andrew Birell’s paper [1995].

35. 6/5/2016B.Ramamurthy35 RMI Internals: Distributed Garbage Collection Based on reference counts. Local garbage collectors and a distributed support. Each server holds the list of processes that hold remote object references: for example, B.Holders When a client C first receives a remote reference to a particular remote object, say B, it makes a addRef(B) invocation to server of that remote object and then creates proxy; server adds C to B.Holders. When client C’s garbage collector finds that proxy is no longer reachable (ref count), it makes a removeRef(B) invocation to server and then deletes proxy; the server removes C from B.Holders. When B.Holders is empty, server’s local garbage collector will reclaim the space occupied B unless there are any local holders. These extra calls for updates occur during proxy creation and deletion and do not affect normal opertion. Tolerates communication failures: addRef() and removeRef() are idempotent: effects of N > 0 identical requests is the same as for a single request. If addRef() fails with an exception, proxy is not created, removeRef() is transmitted; removeRef() failures are dealt with by “leases” (Jini kind).

36. 6/5/2016B.Ramamurthy36 RMI Internals: Use of Reflection What is reflection? See Reflection package Reflection enables Java code to discover information about the fields, methods and constructors of loaded classes, and To use reflected fields, methods, and constructors to operate on their underlying counterparts on objects, within security restrictions. http://java.sun.com/docs/books/tutorial/reflect/class/index.html Reflection feature allowed for dynamic creation of skeleton and proxy in Java 2 version onwards. Read more about reflection model of computing.

37. 6/5/2016B.Ramamurthy37 A Little bit of Reflection Method class, invoke method Invoke method requires two parameters: first the object to receive invocation, second an array of Object parameters. Invoke executes the method on the object and returns result as Object. Method m; Object result = m.invoke(String ObjRef, Args);

38. Reflection Java Reflection is a mechanism for inspecting the variables and methods of an unknown class at runtime. Reflection allows for –finding methods, variables, constructors –analyzing their types, parameters, results, modifiers –changing variables –calling methods or constructors 6/5/2016B.Ramamurthy38

39. Reflection (contd.) Reflection uses methods of java.lang.Class(and java.lang.reflect.*) e.g. java.lang.ClassforName(StringclassName) java.lang.reflect.Contructor[] getConstructors() java.lang.reflect.Field[] getFields() java.lang.reflect.Method[] getMethods() 6/5/2016B.Ramamurthy39

40. Reflection (contd.) Java Reflection: Example... Class clazz = Class.forName("MyClass"); Object myObject= clazz.newInstance(); Method[] methods= clazz.getMethods(); Method firstMethod= methods[0]; Objectresult= null;// try to invoke a method on myObject if("testMethod".equals(firstMethod.getName())) result= firstMethod.invoke(myObject, null); //...obtain class of specified name; create a new instance; obtain all public methods; obtain a method's name; invoke the method with no parameters on the class instance J2EE implementations use immense amounts of reflection! 6/5/2016 B.Ramamurthy40

41. 6/5/2016B.Ramamurthy41 Using Reflection in RMI Proxy (on the client side) marshals info. about a method and its arguments into a request message. For a method it marshals an object of class Method into the request. It then adds an array of objects for the method’s arguments. The dispatcher unmarshals the Method object and its arguments from request message. The remote object reference is obtained from remote ref. table. The dispatcher then calls the “invoke” method on the object reference and array of arguments values. After the method execution the dispatcher marshals the result or any exceptions into the reply message.

42. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Figure 5.1 Middleware layers Applications Middleware layers Request reply protocol External data representation Operating System RMI, RPC and events

43. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Remote and local method invocations invocation remote invocation remote local invocation A B C D E F

44. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 A remote object and its remote interface interface remote m1 m2 m3 m4 m5 m6 Data implementation remoteobject { of methods

45. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Instantiation of remote objects

46. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Invocation semantics Fault tolerance measures Invocation semantics Retransmit request message Duplicate filtering Re-execute procedure or retransmit reply No Yes Not applicable No Yes Not applicable Re-execute procedure Retransmit replyAt-most-once At-least-once Maybe

47. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Role of client and server stub procedures in RPC in the context of a procedural language client Request Reply Communication module dispatcher service client stub server stub procedure client processserver process procedure program

48. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Dealing room system

49. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Architecture for distributed event notification subscriberobserverobject of interest Event service object of interest observer subscriber 3. 1. 2. notification

50. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Java Remote interfaces Shape and ShapeList import java.rmi.*; import java.util.Vector; public interface Shape extends Remote { int getVersion() throws RemoteException; GraphicalObject getAllState() throws RemoteException;1 } public interface ShapeList extends Remote { Shape newShape(GraphicalObject g) throws RemoteException;2 Vector allShapes() throws RemoteException; int getVersion() throws RemoteException; }

51. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 The Naming class of Java RMIregistry void rebind (String name, Remote obj) This method is used by a server to register the identifier of a remote object by name, as shown in Figure 15.13, line 3. void bind (String name, Remote obj) This method can alternatively be used by a server to register a remote object by name, but if the name is already bound to a remote object reference an exception is thrown. void unbind (String name, Remote obj) This method removes a binding. Remote lookup(String name) This method is used by clients to look up a remote object by name, as shown in Figure 15.15 line 1. A remote object reference is returned. String [] list() This method returns an array of Strings containing the names bound in the registry.

52. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Java class ShapeListServer with main method import java.rmi.*; public class ShapeListServer{ public static void main(String args[]){ System.setSecurityManager(new RMISecurityManager()); try{ ShapeList aShapeList = new ShapeListServant();1 Naming.rebind("Shape List", aShapeList );2 System.out.println("ShapeList server ready"); }catch(Exception e) { System.out.println("ShapeList server main " + e.getMessage());} }

53. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Java class ShapeListServant implements interface ShapeList import java.rmi.*; import java.rmi.server.UnicastRemoteObject; import java.util.Vector; public class ShapeListServant extends UnicastRemoteObject implements ShapeList { private Vector theList; // contains the list of Shapes1 private int version; public ShapeListServant()throws RemoteException{...} public Shape newShape(GraphicalObject g) throws RemoteException {2 version++; Shape s = new ShapeServant( g, version);3 theList.addElement(s); return s; } public Vector allShapes()throws RemoteException{...} public int getVersion() throws RemoteException {... } }

54. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Java client of ShapeList import java.rmi.*; import java.rmi.server.*; import java.util.Vector; public class ShapeListClient{ public static void main(String args[]){ System.setSecurityManager(new RMISecurityManager()); ShapeList aShapeList = null; try{ aShapeList = (ShapeList) Naming.lookup("//bruno.ShapeList");1 Vector sList = aShapeList.allShapes();2 } catch(RemoteException e) {System.out.println(e.getMessage()); }catch(Exception e) {System.out.println("Client: " + e.getMessage());} }

55. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005 Classes supporting Java RMI RemoteServer UnicastRemoteObject Activatable RemoteObject

56. 6/5/2016B.Ramamurthy56 Summary We discussed designing a distributed system using RMI We also looked at RMI internal We also learned about marker interface, distributed garbage collection, object marshalling, registry, server-port binding, Naming class of RMI, …