Remote Objects and RMI

Size: px

Start display at page:

Download "Remote Objects and RMI"

Antonia Lloyd
5 years ago
Views:

1 Outline Remote Objects and RMI Instructor: Dr. Tongping Liu Distributed/Remote Objects Remote object reference (ROR) Remote Method Invocation (RMI) Case study and example: Java RMI Other issues for objects Ø Factory method; Transient vs. Permanent objects; Ø Callback objects; Distributed Garbage collection; Object-Oriented Programming Distributed Object Fundamental idea: encapsulation Ø both data and methods Interfaces define the signature of a set of methods Object reference access object s data/method Method invocation may affect target objects; may need chain invocation Data/variable could be directly access Exceptions thrown when an error occurs Client process: disobj1.m1( ); disobj1.m2( ); disobj2.m3( ); disobj2.m4( ); Client machine Machine a m1 Distributed Object Machine c Obj: m3; m4; Machine b m2 Distributed Object 1

2 Remote Objects vs. Message Passing Remote Object Model Host A Client: Sum = math.add(n1, n2); Create a socket Connect it to Put n1, n2 in a msg Send msg to Read/wait reply msg Extract result from the msg?? Host B Server: int state; object math; int add(int n1, int n2) { Create a socket Bind it to a port Accept a connection Read/wait for a msg Extract n1, n2 from the msg Compute result Put it in a reply msg Send reply msg to Remote objects -- can receives invocations; having state information. Remote object reference identify objects in distributed environments Remote interface specifies methods to be invoked ly Process contains objects (local/) Ø Local objects: accept only local invocations Ø Remote object: accept both local/ invocations Ø Remote invocation different processes (same or different hosts) Exceptions application level An Example: Remote Object Objects and Method Invocations object Data interface m1 m2 implementation { m3 of methods m4 m5 m6 Object may implement both & local interface Ø Other processes: invoke only methods in interface Ø Same process: local object reference and invoke methods in local interface A invocation processes local C invocation local E B invocation local invocation D invocation Ø Objects receiving invocations (service objects) are objects, e.g., B and F Ø Object references are required for invocation, e.g., C must have E s reference or B must have D s reference What are the local object references? F 2

3 Outline Distributed/Remote Objects Remote object reference (ROR) Remote Method Invocation (RMI) Case study and example: Java RMI Other issues for objects Ø Factory method; Transient vs. Permanent objects; Ø Callback objects; Distributed Garbage collection; Remote Object Reference (ROR) Uniquely identify objects in distributed systems Invoke methods of a object Remote object references may be passed as input arguments or returned as output arguments. Compared with ordinary object reference, what additional information is needed for object reference? 32 bits 32 bits 32 bits 32 bits Internet address port number time object number interface of object Interfaces Interface for local objects Ø Specify methods and/or data that can be accessed Ø Do not specify an implementation Interface for objects Ø Specifies methods for invocation Ø Input and output parameters are also specified and parameters may be objects Parameters for Remote Methods Primitive types Ø à pass by value Ordinary objects Ø à passed by copy (e.g. using Java serialization; the object must implement the java.io.serializable Interface). Remote objects Ø à use the object reference (ROR) Why not pass a copy of the object? (1) The copy of a object can not perform its function (2) The object proxy and implementation is auto downloaded 3

4 Outline Distributed/Remote Objects Remote object reference (ROR) Remote Method Invocation (RMI) Case study and example: Java RMI Other issues for objects Ø Factory method; Transient vs. Permanent objects; Ø Callback objects; Distributed Garbage collection; RPC Review RPC middleware Middleware generates stubs on both sides RMI Overview RMI middleware Proxy (stub) implements the same interface Skeleton (stub) handles (un)marshaling and object invocation Object is responsible for a collection of objects How do s know where the objects are?" Binding " Object encapsulates data and operations Object offers only its interface (group of methods) to s, With RMI support, s can be at a different host RMI register: the string name of the object, the object itself The registry returns to the caller a reference (called stub) to the object. Invoke methods on the object (through the stub). Client Side: HelloClient //HelloClient.java import java.rmi.*; import java.rmi..*; public class HelloClient{ public static void main (String[] argv) { try { HelloInterface hello = (HelloInterface) Naming.lookup ("//localhost/hello"); System.out.println (hello.say( )); System.out.println (hello.add("here is added information!!!")); catch (Exception e) { System.out.println ("HelloClient exception: " + e); 4

5 RPC vs. RMI Similarity: Marshaling and parameter passing Difference: Ø RPC: C based, structure based semantics. RMI: java and object-oriented Ø RPC: call functions, passed everything. RMI: / system-wide object reference and invoke methods. We can also pass and return objects that can be distributed among many JVM instances, much more powerful. Ø RMI can support dynamic invocations: a can call a procedure even if the signature of the procedure or the name of the service are unknown until runtime. Architecture to Support Remote Objects object A proxy for B Request reference Communication Remote reference Remote Reference Module (RRM) Server side: Ø Create object reference (ROR) Ø Maintain object reference table Ø Entry for objects Ø Map between reference and local reference Client side: Ø Create proxy object when first get ROR Ø Maintain object reference table Ø Entry for local proxy ( side) Ø Map between reference and local reference Communication Module Carry out request-reply protocol Provide certain invocation semantics Ø Retry request Ø Duplication message filtering Ø message history cache Interact with reference ( side) Ø Get object s local reference Ø Pass message and local reference to appropriate dispatcher on side 5

6 Proxy and Skeleton Dispatcher object A proxy for B Request object A proxy for B Request reference Proxy - makes RMI transparent to. Class implements interface. Marshals requests and unmarshals results. Forwards request. Communication Remote reference Skeleton - implements methods in interface. Unmarshals requests and marshals results. Invokes method in object. reference Communication Remote reference Dispatcher - gets request from communication and invokes method in (using methodid in message). Middleware for Remote Objects Server/Client Programs and Binder Layer between application and communication/ reference s Automatically create proxy, and dispatcher from interface definition Client side: one proxy for each object Ø (kind-of) Implement the methods in objects, provide the transparency to the Server side: one dispatcher and one Ø Dispatcher accepts message and select appropriate method in the : methodid Ø Skeleton: Marshall / unmarshall messages and invokes corresponding method in the object Server program Ø Dispatcher, Ø Servant class: implement methods for objects Client program Ø Proxy Ø Use binder to get object reference Binder: kind of naming service Ø Mapping between text name and object reference Ø System wide register/look up service 6

7 Steps in RMI object A proxy for B 9 Request reference Communication Remote reference 5: ROR 3: ROR How to Use Remote Objects: Server Side Step 0: start binder Step 1: start communication and reference Step 2: create objects, add them to object table, and obtain object reference from reference Step 3: publish the objects to naming service à bind the object reference with a name; wait for invocation requests Naming Service 0 Steps in RMI object A proxy for B 9 Request reference Communication Remote reference 5: ROR 3: ROR Naming Service How to Use Remote Objects: Client Side Step 4: start communication and reference Step 5: contact naming service for desired object reference Step 6: reference create proxy Step 7: call methods in proxyà mashall parameters Step 8: locate object through reference Step 9: Send method invocation request (contain object reference) through communication 7

8 Steps in RMI How to Use Remote Objects: Server Side object A proxy for B 9 Request reference Communication Remote reference 5: ROR 3: ROR Naming Service Step 10: get invocation requests (contains object reference) through communication Step 11: consult with reference and get local reference for the object Step 12: hand the request to the dispatcher/ of the object s class à which method (method id) Step 13: call method in à unmarshall parameters in request and invoke the real method of the object Step 14: perform operation in object and return results to Step 15: marshall results in and send out message ( object reference) through communication How to Use Remote Objects: Client Side Step 16: get result message (contain object reference) in communication Step 17: obtain proxy reference from object reference and hand the result message to the proxy Step 18: unmarshall the results in proxy and return results to the caller Complete a Remote Method InvocationJ Why do people say RMI is simpler than sockets? RMI = Sockets + Object Serialization + Some Utilities User don t need to worry about communication Outline Distributed/Remote Objects Remote object reference (ROR) Remote Method Invocation (RMI) Case study and example: Java RMI Other issues for objects Ø Factory method; Transient vs. Permanent objects; Ø Callback objects; Distributed Garbage collection; 8

9 Case Study: Java RMI n Define a interface HelloInterface.java n Server side: l Implement the interface Hello.java l Develop the HelloServer.java n Client side: l Develop a HelloClient.java n Run the RMI registry, the, and the //HelloInterface.java import java.rmi.*; public interface HelloInterface extends Remote { public String add(string s) throws RemoteException; public String say( ) throws RemoteException; Servant Class Implement Remote Interface import java.rmi.*; //Hello.java import java.rmi..*; public class Hello extends UnicastRemoteObject implements HelloInterface { private String message; public Hello (String msg) throws RemoteException {message = msg; public String add(string more) throws RemoteException{ message = new String (message + more); return message; public String say( ) throws RemoteException {return message; Server: Create Servant Object and Bind //HelloServer.java import java.rmi.*; public class HelloServer{ public static void main(string args[]){ try { Naming.rebind ("Hello", new Hello ("Hello, world!")); System.out.println ("Hello Server is ready."); catch (Exception e) { System.out.println ("Hello Server failed: " + e); The binder: RMI Registry For Ø Void rebind (String name, Remote obj) ü Register an object by name ü Override previous registration Ø Void bind (String name, Remote obj) ü Register an object by name ü If existent throw exception Ø Void unbind ( String name, obj) For Client Ø Remote lookup (String name) ü ROR is returned if found Ø String[ ] list() ü Show all names bound in this registry 9

10 Run the Server Compile the interface and class Ø javac HelloInterface.java Hello.java Compile Ø javac HelloServer.java Generate s & stubs (old Java compiler) Ø rmic Hello --> Hello_Skel.class & Hello_Stub.class Start RMI registry Ø rmiregistry (default at port 1099) Start Hello Ø java HelloServer Implementations of Client First, get object reference Ø It can retrieve a object reference from RMIregistry on the machine where the object resides Ø It received a object reference from a previous call Invoke methods specified in interface Compile the code Ø Needs the interface file in order to compile Client Side: HelloClient //HelloClient.java import java.rmi.*; import java.rmi..*; public class HelloClient{ public static void main (String[] argv) { try { HelloInterface hello = (HelloInterface) Naming.lookup ("//localhost/hello"); System.out.println (hello.say( )); System.out.println (hello.add("here is added information!!!")); catch (Exception e) { System.out.println ("HelloClient exception: " + e); Java RMI: Compile and Run the Client Compile the interface class" javac HelloInterface.java Compile " javac HelloClient.java Start Hello " java HelloClient [X] > java HelloClient Hello, world!" Hello, world!here is added information!!! " > java HelloClient Hello, world!here is added information!!!" > ps > kill #rmiregistry pid > java HelloClient HelloClient exception: java.rmi.connectexception: Connection refused to host: localhost; nested exception is: java.net.connectexception: Connection refused Hello, world!here is added information!!!here is added information!!! " > java HelloClient Hello, world!here is added information!!!here is added information!!!" Hello, world!here is added information!!!here is added information!!!here is added information!!!" 10

11 Summary of Java RMI Steps Design the interfaces being implemented Server create and to register the objects. Ø Implement methods in the interfaces Ø Create and install a security manager ü Regulates whether the itself might download classes when accessing other s Ø Create one or more instances of a object Ø Register at least one of the objects with the RMI object registry (or another naming service) Client program to use the methods of object Ø Looks up in object registry and gets ROR Ø Use normal method call syntax for calling method Other Issues for Remote Objects When/who creates objects? Ø Remote Object Factory method Can contact? Ø Call-back Outline Distributed/Remote Objects Remote object reference (ROR) Remote Method Invocation (RMI) Case study and example: Java RMI Other issues for objects Ø Factory method; Transient vs. Permanent objects; Ø Callback objects; Distributed Garbage collection; 11

CS 5523 Operating Systems: Remote Objects and RMI

CS 5523 Operating Systems: Remote Objects and RMI Instructor: Dr. Tongping Liu Thank Dr. Dakai Zhu and Dr. Palden Lama for providing their slides. Outline Distributed/Remote Objects Remote object reference