Distributed Environments CORBA, JavaRMI and DCOM
Introduction to CORBA Distributed objects A mechanism allowing programs to invoke methods on remote objects Common Object Request Broker middleware - works over sockets alternative to RPC called a software bus
Difference between CORBA and RPC Client Server Client Server Object Call foo Proc Foo Invoke Foo on Object foo RPC Mechanism Object Request Broker
Components of the ORB nterface epository Client Object on Server Implementation Repository Dynamic Invocation Client IDL Stubs Orb Interface Static Skeletons Dynamic Skeleton Invocation Object Adapter Object Request Broker Core (IIOP)
Components of the ORB The Client IDL Stubs provide static interfaces to object services local proxy for the remote object Dynamic Invocation Interface lets the client find the methods to be invoked at run time Interface Repository APIs allows the client to obtain the description of all registered component interfaces
The Components of the ORB The ORB Interface access to local services that may be of interest to an application The Server IDL Stubs (Skeleton) static interface to each service exported by the server Dynamic Skeleton Interface provides a run-time binding mechanism
Components of the ORB The Object Adapter accepts requests for service on behalf of the server s objects provides a run time environment for instantiating server objects and giving them object IDs
The components of the ORB The Implementation Repository Provides a run-time repository of information about classes the server supports, the objects and Ids of objects that are instantiated, also administrative data The General Inter-ORB Protocol (GIOP) defines a set of message formats and common data representation for communications between ORBs. Like XDR
The components of the ORB The Internet Inter-ORB Protocol (IIOP) transport protocol that allows ORBs to communicate with each other using GIOP maps onto TCP/IP with some CORBA-defined message exchanges that serves as a common backbone protocol
Key Concept - Object References Uniquely identifies an object within a distributed ORB system unique name or identifier CORBA 2.0 defines a new standard to allow objects to be passed between ORBs Interoperable Object References (IOR)
Making a CORBA program Create your object classes using the Interface Definition Language (IDL) Run the IDL file through a language compiler to give skeleton files Add implementation code to the skeletons create and implement the server classes, etc
Making a CORBA Program Compile the code which should generate import files - describes the object to interface repository client stubs - invoked by the client program server skeletons Bind the class definitions to the Interface Repository load the object interface into the Interface Repository so that programs can access it at run
Making a CORBA program Register the run-time objects with the Implementation Repository Object Adapter registers object on server Used to locate objects or to activate objects on a given server Start object on the server Object Adapter starts the server object
Language Bindings CORBA IDL is language-independent However to write real programs you must use a language, so the CORBA specification must be written in that language Possible languages C - implementations around but because C does not directly support objects. It is difficult
Language Bindings C++ fast because compiled. C++ ORBs common good for server implementations Java very popular. Features of Java and CORBA go nicely together. Speed is an issue as Java is interpreted. JavaORBs around. Use Applets for Web Support. Good for client side implementations Python -extremely flexible scripting language. Python ORBs around
CORBA Services A collection of system-level services packaged with IDL-specified interfaces Object Services complement the functionality of the ORB Provides support to build large complex applications
CORBA Services Life Cycle Service defines operations for creating, copying, moving and deleting components on the bus Persistent Service persistent storage of objects on files or databases Naming Service allows objects and services to be located use X.500, Novell s NDS, Sun s NIS+
The Event Service CORBA Services allows clients to dynamically register their interest in specific events The Concurrency Control Service provides a lock manager that can maintain locks on behalf of either transactions or threads The Transaction Service provides a two-phase commit coordination among recoverable components using nested transaction
CORBA Services The Relationship Service creates dynamic associations between objects traversing object spaces The Query Service provides query operations on objects superset of SQL Object Query Language (OQL)
CORBA Services The Licensing Service metering the use of components and ensure fair compensation for their use The Properties Service can associate named values (properties) with any component The Time Service provides interfaces for synchronizing time in a distributed environments. Support for timetriggered events
CORBA Services The Security Service a complete framework for distributed object security. Supports authentication, access control lists,etc The Trader Service provides a Yellow Pages service for objects. Allows objects to publicize their services and bid for jobs. Usually most useful
CORBA Services The Collection Service CORBA interfaces to create and manipulate collections of objects The Startup Services starts up services automatically when an ORB is invoked
OmniORB Developed by AT&T Cambridge Labs led by Sai-Lai Lo Public C++ ORB freely available High Performance ORB implementation Support for Trader Service also comes with it.
Java RMI - Introduction RMI - remote method invocation Allows Java objects to be accessed from another Java Virtual Machine (JVM), even across a network No need IDL: all provided within Java object model
Java RMI RMI Objects talk to other RMI objects Does not naturally interact with objects outside Java model Merge RMI so that it can run over IIOP use pure Java systems to do programming Caffeine - such a platform RMI for Java Objects CORBA for other objects
Other Competitors DCOM, Active X, etc From Microsoft - desperately wants to dominate distributed computing DCOM - first attempt Like CORBA - separates interface from implementation
DCOM All interfaces must be declared using IDL Microsoft s IDL is not CORBA compliant DCOM objects can have multiple interfaces Interface - collection of function calls does not have real OO properties like inheritance DCOM interfaces are a number of pointers to functions keep in a virtual table called a vtable
DCOM cont d All DCOM objects must implement an IUknown interface which is used to manage the object and find out about its interfaces DCOM has class id which is unique with a class having more than one interface Active object is an instantiation of a class Not like CORBA - each object created has a unique ID
DCOM In DCOM servers are really class factories that create objects of a given class and returns the pointer to the primary interface The client using the pointer can then find out about the interfaces and invoke the relevant functions in the interface DCOM uses of Java to build applications
Comparison of CORBA and DCOM Client Client Invoke Foo on Object Server Object foo Client Need class Got object handle Invoke Foo on Object Class ID Object Handle Server Class Libraries Object Factory DCOM Object Object Request Broker CORBA DCOM
Neither of them DCOM abandoned Who won Microsoft Foundational Classes CORBA Object oriented Interface around Windows functions Led to.net Framework Still used in industry to build custom applications Did not take over the world as some predicted
The Web won Who won cont d Created an environment for e-commerce HTTP, Java, Apache, MySql, PHP, Python Development of Web 2.0 (Interactive Web) LAMP Linux, Apache, MySql, PHP Becoming the dominant environment for e- Commerce Microsoft trying to fight back using.net Framework
Sources for this lecture RPC: Internetworking with TCP/IP Volume III by Douglas Comer and David Stevens CORBA: Instant CORBA by Robert Orfali, Dan Harkey and Jeri Edwards William Stallings has a section on CORBA CORBA/Java: Client/Server Programming with Java and CORBA: Robert Orfali and Dan Harkey