Introduction to CORBA Organizational Communications and Technologies Prithvi

Introduction to CORBA Organizational Communications and Technologies Prithvi N. Rao H. John Heinz III School of Public Policy and Management Carnegie Mellon University

Readings Posting on the Class Web Site

Objectives n Understand goals of distributed object technology n Examine evolution of Software Bus n Cite CORBA challenges n Review OO support in CORBA

Seven Layer Model Revisited OSI Model Revised Model Application Religion Presentation Politics Session Transport Finance Environment Network Stacks Data Link Interfaces Physical Substrates

Evolution of the Software Bus Remote Msgs Remote Procedure Distributed Objects DLL LPC DDE OLE 1. 0 DCE NCS RPC ONC+ UDP/TCP IPC OLE 2. 0 SHL DCOM COM/CORBA/IIOP RRBC JAVA/RMI

Terms Abbreviation Name LPC Local Procedure Call UDP User Datagram Protocol TCP Transmission Control Protocol IPC SHL RRBC DDE DLL OLE 1. 0 Purpose Parameterized code segments Intermachine messaging Reliable delivery of UDP messages Inter-process Communication Messaging between processes Shared Library/Object Dynamic Linking of shared code Release to release binary compat. Support for versioning of shared Libs. Dynamic data exchange u. Soft’s messaging between procs. Dynamic Link Library u. Soft’s shared library/object Object Linking and Embedding Extension of DDE for windows events

Terms Abbreviation Name COM Component Object Model DCOM Distributed COM RPC Remote Procedure Call Purpose Extension DLL/C++ supporting versioning Extension of OLE 2. 0 supporting IPC Proxies making RPC look like LPC NCS RPC Network Computing Service Apollo’s inter-machine marshalling ONC RPC Open Network Computing Common Object Request Broker Architecture CORBA IIOP RMI SNMP Sun’s inter-machine marshalling Extends NCS/DCE for dist. Object comm. Internet Inter-ORB Protocol CORBA 2. 0 standard marshalling protocol Remote Method Invocation Dist. Java-java ipc marshalling protocol Simple Network Management Protocol Remote attribute query mechanism

Software Interconnect Bus Application Objects Common Facilities Software Interconnect Bus Services

Distributed Objects CORBA brings together three technologies Objects + Remote Procedure Calls + Threaded Execution

Why do we care? n n Interested in heterogeneous distributed application development Advanced complexity of applications n n Security Transactions Naming Open standard development

Distributed Objects: Combining Three Technologies Objects Inheritance Encapsulation Polymorphism + Remote Procedure Calls Distribution Heterogeneity Procedural Model + Threaded Execution Concurrency Priorities Pre-emption

The Basic Idea n Take an Object n Make it distributable n Make it distributed

Terminology n Distributed Objects and components are dispersed across processes that may exist on the same machine or located across machines on a network. n Heterogeneous Application whose parts are located on machine that vary by hardware or by operating system or by programming language. n Object-Oriented Applications written using objects.

CORBA Challenges n Presents two challenges of developing distributed systems n n n Making distributed application development no more difficult than developing centralized programs Providing an infrastructure to integrate application components into a distributed application Challenges are easier said than done due to: n n Partial failures Impact of latency Load balancing Event ordering

General ORB Structure Op(args) Object Implementation CLIENT IDL Skeleton Dynamic II IDL Stubs ORB Interface Object Request Broker Object Adapter

CORBA Interface Definition Language (IDL) n OMG IDL is an object-oriented interface definition language n n n Used to specify interface containing methods and attributes OMG IDL support interface inheritance both single and multiple inheritance OMG IDL is designed to map onto multiple programming languages n C, C++, Smalltalk, COBOL, Modula 3, DCE

OMG IDL Compiler n OMG IDL compiler generates client stubs and server skeletons n Stubs and skeletons automate the following activities in conjunction with the ORB n n n Client proxy factories Parameter marshalling and unmarshalling Implementation class interface generation Object registration and activation Object location and binding

Requesting Service Using CORBA (and DCE) Application Code Client stub generated by IDL compiler Runtime System Client Server Code Server stub generated by IDL compiler Runtime System Server

CORBA Services (OMG specified) n Services essential for implementing objects n n Concurrency control service protects integrity of an object’s data when multiple requests to object are processed concurrently Event service that supports notification of interested parties when program-defined events occur Externalization service that supports conversion of object state to a form that can be transmitted between systems by means other than a request broker Life Cycle service that support creation, copying, moving and destruction of objects

CORBA Services (OMG specified) n n Naming service that permits object reference to be retrieved through associations between names and objects and for those associations to be created and destroyed Persistent Object service that supports persistence of an object’s state when the object is not active in memory and between application executions. Query service that supports operations on sets and collections of objects. Relationship services that provides for creating, deleting, navigating and managing relationships between objects n Containment relationship between a “folder” object and “document” objects contained in that folder.

CORBA Services (OMG Specified) n Transaction service provides support for ensuring that a computation consisting of one or more operations on one or more objects satisfies the requirements of: n n n Atomicity in case of failure Isolation – transactions run concurrently but results are the same as if they ran serially Durability – result of successful transaction is never lost

CORBA Services (OMG specified) n n n Security service that supports authentication, authorization, integrity and privacy to degrees and using mechanisms yet to be determined. Time service that provides synchronized clocks to all objects regardless of location. Collection service that support creation and manipulation of collections of objects. Replication service that would provide for the explicit replication of objects in a distributed environment for the purpose of fault tolerance and for management of consistency of replicated copies. Trader service that provides matchmaking service between clients seeking services and objects offering services

CORBA and Object Oriented Support n Encapsulation Data and functions manipulating data are “encapsulated” in object. This enforces data hiding since the only way to access an object’s data is through the operations in the object’s public interface.

CORBA and Object Oriented Support n Abstraction of common features shared by objects in classes. A class definition describes the data associated with each instance of the class, defines the set of operations that can be invoked on an instance of the class and prescribes the functions that are executed in response to requests for those operations.

CORBA and Object Oriented Support n Inheritance of interfaces and implementations. This is the mechanism that supports the specialization or refinement of classes into subclasses. It is also one example of reuse inn object-oriented programming.

CORBA and Object Oriented Support n Polymorphism is the ability for a request to specific operation to be handled differently depending on the type of object on which it is invoked. Example: subclasses of a common superclass may override functions defined by the superclass to differentiate how instances of the subclasses and superclass behave.

CORBA and Object Oriented Support n n Creation of new objects and classes Late binding of operation invocations to function calls allow programs to be written without regard for types objects they will manipulate

Summary n Examined evolution of distributed object technology n Introduced CORBA services n Examined OO support in CORBA