1. Distributed Objects and Remote Invocation: RMI and CORBA Most concepts are drawn from Chapter 17 © Pearson Education Rajkumar Buyya, Xingchen Chu, Rodrigo Calheiros Cloud Computing and Distributed Systems (CLOUDS) Laboratory Department of Computing and Information Systems The University of Melbourne, Australia http://www.cloudbus.org/652

2. 2 Outline Introduction CORBA Architecture and Components CORBA Programming Advanced Topics in CORBA Extending File Server Example using CORBA RMI and CORBA comparison Summary

3. 3 Introduction on CORBA Common Object Request Broker Architecture It’s a specification rather than an implementation Defines the protocols and interfaces History about CORBA 1989, OMG (Object Management Group) initiated Aims on using object oriented model to construct distributed applications Object Request Broker (ORB) 1991, CORBA 1.0 specification CORBA Object model, Interface Definition Language (IDL), and Dynamic Interface Invocation 1996, CORBA 2.0 specification General Inter-ORB protocol (GIOP) An Internet version for GIOP: Internet Inter-ORB protocol (IIOP) 2002, CORBA 3.0 specification Real-time CORBA Interoperability consideration (objects by value, asynchronous method invocation) Current specification is 3.1 (2008); 3.2 is under development

4. 4 CORBA RMI CORBA RMI is a remote method invocation that the client and server can be implemented using different languages Proxy is generated in the client language, while skeletons are generated in the server language CORBA object model CORBA object can be implemented in non-OO languages (without the concept of class) The concept “class” do not appear in CORBA Various types of data can be passed as arguments Client is not necessarily an object – client can be any program that is used to refer to remote objects

5. 5 CORBA architecture client server proxy or dynamic invocation implementation repository object adapter ORB skeleton or dynamic skeleton client program interface repository Request Reply core for A Servant A Quite similar to Java RMI architecture Three additional components Object adapter instead of Dispatcher Implementation repository Interface repository

6. 6 CORBA components (1) ORB core Similar to the communication module in Java RMI Is responsible for communication of requests Transfers request to object implementation Object adapter Provides an interface between the ORB and the object implementation and enables their communication Maintains a mapping of object references to their implementations Creates remote object references for CORBA objects Dispatches client requests to server objects Activates and deactivates objects

7. 7 CORBA components (2) Skeletons Generated from the IDL compiler, in the server language Unmarshals the arguments in the request messages and marshals exceptions and results in reply messages Client stubs/proxies Generated from IDL, in client language Marshal the arguments in invocation requests and unmarshal exceptions and result in replies

8. 8 CORBA components (3) Implementation repository Allows the ORB to locate and activate implementations of objects Other information (e.g. access control) can also be recorded in implementation repository Example Implementation policy entry: Interface repository Provides information about registered IDL interfaces It can provide Interface name and methods For each method, names and types of arguments and exceptions Adds a facility for reflection to CORBA Object adapter namePath of object implementation Hostname/port number of server

9. 9 CORBA IDL language Different from C++ in several additional commonly used keywords interface module any attribute in, out, inout readonly oneway raises exception context

10. 10 IDL structure Modules Similar to packages in Java Define the naming scope Interfaces Inheritance interface B: A{ }; Multiple inheritance allowed interface Z: B, C { }; Structs Typedefs

11. 11 CORBA IDL struct Rectangle{ 1 long width; long height; long x; long y; } ; struct GraphicalObject {2 string type; Rectangle enclosing; boolean isFilled; }; interface Shape {3 long getVersion() ; GraphicalObject getAllState() ; // returns state of the GraphicalObject }; typedef sequence All; 4 interface ShapeList {5 exception FullException{ }; 6 Shape newShape(in GraphicalObject g) raises (FullException);7 All allShapes();// returns sequence of remote object references8 long getVersion() ; };

12. 12 IDL module Whiteboard module Whiteboard { struct Rectangle{...} ; struct GraphicalObject {...}; interface Shape {...}; typedef sequence All; interface ShapeList {...}; };

13. 13 CORBA IDL struct is used to represent complex data structures C compatible Can also be compiled to OO class No method defined in, out, inout in: it’s a input parameter transferred from client to server out: it’s an output parameter returned from server to client, the return value will be treated as an output parameter. Set to void if no output parameter inout: both, seldom used Interface is similar to Java interface Only a set of methods defined Can be compiled to Java interface as shown below public interface ShapeList extends org.omg.CORBA.Object { Shape newShape(GraphicalObject g) throws ShapeListPackage.FullException; Shape[] allShapes(); int getVersion(); }

14. 14 Data representation in IDL Primitives – 15 primitive types Short (16-bit), long (32-bit), unsigned short, unsigned long, float (32-bit), double (64-bit), char, boolean (TRUE/FALSE), octet (8-bit) and any (which can represent any primitive or constructed type) Complex data Array, sequence, string, record (struct), enumerated, union object – CORBA object reference Is the common supertype of all of IDL interface types such as Shape and ShapeList in previous example

15. 15 IDL constructed types – 1 TypeExamplesUse sequence typedef sequence All; typedef sequence All bounded and unbounded sequences of Shapes Defines a type for a variable-length sequence of elements of a specified IDL type. An upper bound on the length may be specified. string String name; typedef string SmallString; unboundedand bounded sequences of characters Defines a sequences of characters, terminated by the null character. An upper bound on the length may be specified. array typedef octet uniqueId[12]; typedef GraphicalObject GO[10][8] Defines a type for a multi-dimensional fixed-length sequence of elements of a specified IDL type. this figure continues on the next slide

16. 16 IDL constructed types – 2 TypeExamplesUse record struct GraphicalObject { string type; Rectangle enclosing; boolean isFilled; }; Defines a type for a record containing a group of related entities. Structs are passed by value in arguments and results. enumerated enum Rand (Exp, Number, Name); The enumerated type in IDL maps a type name onto a small set of integer values. union union Exp switch (Rand) { case Exp: string vote; case Number: long n; case Name: string s; The IDL discriminated union allows one of a given set of types to be passed as an argument. The header is parameterized by an enum, which specifies which member is in use. };

17. 17 IDL methods General format [oneway] ([parameter1, …, parameterL]) [raises (except1, …, exceptN)] [context (name1, …, nameM)] Tags Oneway: non-blocked In, out, inout Raise-exception: throws user defined exceptions Exception can be empty, or have variables exception FullException{ GraphicalObject g;} Context: supply properties mappings (from string names to string values)

18. 18 Parameter passing in CORBA Pass By Reference Any parameter whose type is specified by the IDL interface, is a reference to a CORBA object and the value of a remote object reference is passed Pass By Value Arguments of primitive and constructed types are copied and sent to the recipient On arrival, a new value is created in the recipient’s process (new memory allocation).

19. 19 Example CORBA Application: Hello World Implementations (Java IDL) Server program Write HelloWorld.idl Generate classes from HelloWorld.idl Implement the Servant class Implement a Server class Client program Write a simple Client with main to lookup HelloWorld Service and invoke the methods

20. 20 Write IDL definition HelloWorld.idl module cs652{ module corba{ module server { interface HelloWorldService{ string sayHello(in string who); };

21. 21 Generate Java classes Command Line tool idlj -fall HelloWorld.idl Use idlj on CORBA IDL interface and generates the following items The equivalent Java interface: HelloWorldService.java The Portable Object Adapter (POA) abstract class HelloWorldServicePOA.java (since J2SE 1.4) for Servant class to extend The proxy class for client stub, _HelloWorldServiceStub.java Classes called helpers and holders, one for each of the types defined in the IDL interface Helper contains the narrow method, which is used to cast down from a given object reference to the class to which it belongs Holder deals with out and inout arguments, which cannot be mapped directly in Java Java classes corresponding to each of the structs defined within the IDL interface (not available for HelloWorld example)

22. 22 Implement the Servant HelloWorldServiceImpl.java package cs652.corba.server; public class HelloWorldServiceImpl extends HelloWorldServicePOA { public HelloWorldServiceImpl() { super(); } public String sayHello(String who) { return "Hello "+who+" from your friend CORBA server :-)"; } }

23. 23 Implement CORBA Server package cs652.corba.server; import org.omg.CosNaming.*; import org.omg.CosNaming.NamingContextPackage.*; import org.omg.CORBA.*; import org.omg.PortableServer.*; public class HelloWorldServer { public static void main(String[] args) { try{ // create and initialize the ORB ORB orb = ORB.init(args, null); // get reference to rootpoa & activate the POAManager POA rootpoa = POAHelper.narrow(orb.resolve_initial_references("RootPOA")); rootpoa.the_POAManager().activate(); // create servant and get the CORBA reference of it HelloWorldServiceImpl helloWorldImpl = new HelloWorldServiceImpl(); org.omg.CORBA.Object ref = rootpoa.servant_to_reference(helloWorldImpl); HelloWorldService helloWorldService = HelloWorldServiceHelper.narrow(ref); // get the root naming context and narrow it to the NamingContextExt object org.omg.CORBA.Object objRef = orb.resolve_initial_references("NameService"); NamingContextExt ncRef = NamingContextExtHelper.narrow(objRef); // bind the Object Reference in Naming NameComponent path[] = ncRef.to_name("HelloWorldService"); ncRef.rebind(path, helloWorldService); // wait for invocations from clients orb.run(); } catch (Exception e) {} }

24. 24 Commands explanation activate: make the object enabled in CORBA servant_to_reference: get the object reference from the servant class resolve_initial_references: first lookup of POA narrow: cast CORBA object reference to the preferred class to_name: convert between string value and name component path rebind: bind and rebind the object reference to the naming service

25. 25 Example CORBA client program package cs652.corba.client; import org.omg.CosNaming.*; import org.omg.CosNaming.NamingContextPackage.*; import org.omg.CORBA.*; public class HelloWorldClient { public static void main(String[] args) { try{ // create and initialize the ORB ORB orb = ORB.init(args, null); // get the root naming context org.omg.CORBA.Object objRef = orb.resolve_initial_references("NameService"); // Use NamingContextExt instead of NamingContext, part of the Interoperable naming Service. NamingContextExt ncRef = NamingContextExtHelper.narrow(objRef); // resolve the Object Reference in Naming HelloWorldService helloWorld = HelloWorldServiceHelper.narrow(ncRef.resolve_str("HelloWorldService")); System.out.println(helloWorld.sayHello("Raj")); }catch(Exception e){} }

26. 26 Run it Run the Object Request Broker Daemon (usedby clients for look up and object invocation on servers) orbd -ORBInitialPort 10000 & Run the server java cs652.corba.server.HelloWorldServer -ORBInitialPort 10000 & Run the client java cs652.corba.client.HelloWorldClient -ORBInitialHost localhost -ORBInitialPort 10000

27. Advance Aspects of CORBA

28. 28 Advanced Issues (1) Dynamic invocation interface (DII) Allows dynamic creation and invocation of object requests Makes use of interface repository Why use it? Clients using stubs have limitations only particular object types can be accessed interfaces must be known at compile time

29. 29 Advanced Issues (2) Dynamic skeleton interface Provides a runtime binding mechanism for the CORBA components that do not have an IDL-based compiled skeleton, When receives an invocation, it looks at the parameters of the request to discover (from the interface repository) its target object, the method to be invoked and the arguments, then invoke it Legacy code Legacy code refers to existing code that was not designed with distributed objects in mind CORBA enables them by defining and IDL and following the CORBA development steps

30. 30 CORBA language mappings Map to Java: idlj Primitives to Java primitives Structs, enums and unions are mapped to Java classes Sequence and arrays are mapped to Java arrays Exceptions are mapped to Java exception classes Multiple outputs are mapped to a class called Holder, since Java supports only single output Map to other languages IDL compile tool for each language

31. 31 CORBA services CORBA includes specifications for services that may be required by distributed objects The service themselves are provided and accessed as CORBA remote objects An index to documentation on all of the services can be found at OMG’s web site at www.omg.orgwww.omg.org Examples Naming service Security service Event service and notification service Persistent object service (POS) Transaction service and concurrency control service Trading service

32. 32 Name Service Most common used service to bind and discover objects according to specified names Provides a hierarchical structure to construct sub name context under a root naming context Objects within NameContext are composed of NameComponent arrays

33. 33 Security Service Provides a high-level security framework Supports authentication of remote users and services, access control for key objects and services, auditing functions, ability to establish secure communications channels between clients and object services Encryption functions are not included in the framework

34. 34 Event and Notification Service Provides an asynchronous interaction between distributed objects Distinguishes an event as event consumers or event suppliers

35. 35 Persistent Object Service (POS) Provides ways for CORBA objects to interact with various underlying persistence engines Can be thought of as middleware between CORBA objects and database protocols

36. 36 Transaction and Concurrency Control Service Transaction Service defines interfaces that allow distributed objects to create and engage in transactional interactions Concurrency control service manages concurrent access to remote objects from multiple clients

37. 37 Trading Service Objects are described with additional attributes Clients send a query with desired requirements Trading service matches the client’s request and the objects attributes to find a proper one

38. 38 Sample Scenario: File Server (cont) Figure: File Server with Registry Additional Requirement File Server can be registered, unregistered and discovered via a Registry Server by Client

39. 39 Registry Server IDL module cs652{ module corba{ module server { typedef sequence StringArray; interface FileServerRegistry{ void registerFileServer(in string name,in string serverURI); void unregisterFileServer(in string name); string getFileServer(in string name); StringArray getAvailableFileServers(); }; Generate java files idlj -fall FileServerRegistry.idl

40. 40 Implement Servant FileServerRegistryImpl.java package cs652.corba.server; import java.util.Map; import java.util.HashMap; public class FileServerRegistryImpl extends FileServerRegistryPOA { private Map registry = new HashMap(); public void registerFileServer(String name, String serverURI) { registry.put(name,serverURI); } public String[] getAvailableFileServers() { String [] result = new String[registry.size()]; Object[] names = registry.keySet().toArray(); for(int i=0;i<names.length;i++){ result[i] = names[i].toString(); } return result; } public String getFileServer(String name) { return (String)registry.get(name); } public void unregisterFileServer(String name) { registry.remove(name); }

41. 41 Common Things The server and Client code are almost the same as the HelloWorld example. The only exception is the name to bind in the NameContext is FileServerRegistry rather than HelloWorldService. Run it Start CORBA services with default setting orbd -ORBInitialPort 10000 & Run the server java cs652.corba.server.FileServer -ORBInitialPort 10000 Run the Client java cs652.demo.FileBrowser -ORBInitialHost localhost -ORBInitialPort 10000

42. 42 Comparison : RMI and CORBA (I) Development of Application

43. 43 Comparison : RMI and CORBA (II) Similarities Both provide a framework for developing distributed applications Provides a lot of services to support and ease the development Compared with socket programming, developers concentrate more on business logic rather than low-level protocols Both provides tools to generate stubs and skeletons for application RMI use RMI compiler (rmic) and CORBA use various compilers (eg. idlj for java) Both provide name service to register and discover service by name Both support static and dynamic method invocation RMI use Java reflection and CORBA use DII and DSI via interface repository

44. 44 Comparison : RMI and CORBA (III) Differences Language and platform support RMI is designed only for Java and only works under JVM (exception: RMI-IIOP) CORBA is designed to work with multiple languages and platforms Communication Protocol RMI uses Java Remote Method Protocol (JRMP) which utilizes Java object Serialization CORBA uses language independent General Inter-ORB Protocol (GIOP) which defines common data representation (CDR) Internet Inter ORB Protocol (IIOP) Programming Model RMI is a pure object-oriented programming model CORBA supports both object-oriented programming and non object-oriented programing Object passing RMI sends object by value making use of the dynamic class load mechanism and also support automatic distributed garbage collection CORBA does not support distributed garbage collection Name Schema in Name Service RMI makes use of URL based name schema to look up object CORBA constructs a hierarchical structure of object’s name Security RMI utilizes the build-in Java security framework to grant various permissions CORBA has its own security service to handle security issue Simplicity RMI is much simpler to learn and use CORBA is a big specification and hard to learn Interoperability RMI supports if IIOP is used as transport CORBA supports interaction between implementations in various languages and platforms

45. 45 Summary CORBA Programming Define IDL Generate Stub and Skeleton Implement Servant and Server Implement Client CORBA and RMI Comparison RMI is much simpler CORBA is much more powerful Other Related Topics Service Oriented Architecture Jini http://www.jini.org/http://www.jini.org/ Web Services