Introduction to CORBA

Objectives • CORBA technology introduction • CORBA terminology introduction • CORBA references introduction • CORBA IDL • CORBA Applications • CORBA Services • CORBA demonstration • Additional information ©2002-2003 G. Michael Youngblood

CORBA • Common Object Request Broker Architecture • Specification written and maintained by the Object Management Group (OMG) http://www.omg.org • First Specification: OMAObject Management Architecture ©2002-2003 G. Michael Youngblood

Object Management Architecture • Object: an encapsulated entity with an immutable distinct identity whose services are accessed only through well-defined interfaces. • Interface Categories: general groupings for object interfaces. Conceptually linked by an ORB. ©2002-2003 G. Michael Youngblood

OMA: Interface Categories • Object Services: domain-independent, or horizontally oriented, interfaces used by many distributed object applications.Ex: Naming Service, Trading Service • Domain Interfaces: domain-specific, or vertically oriented, interfaces usu. Industry specific.Ex: Person Identification Service • Application Interfaces: developed specifically for a given application.Ex: your own CORBA applications ©2002-2003 G. Michael Youngblood

OMA interface categories DomainInterfaces DomainInterfaces ApplicationInterfaces DomainInterfaces DomainInterfaces DomainInterfaces Object Request Broker ObjectServices ©2002-2003 G. Michael Youngblood

CORBA Terminology • CORBA Object: a “virtual” entity capable of being located by an ORB and having client requests invoked on it. • Target Object: within the context of a CORBA request invocation, it is the CORBA object that is the target of that request. • Client: an entity that invokes a request on a CORBA object, • Server: an application in which one or more CORBA objects exist. • Request: an operation invocation on a CORBA object by a client. • Object Reference: a handle used to identify, locate, and address a CORBA object. • Servant: a programming language entity that implements one or more CORBA objects. ©2002-2003 G. Michael Youngblood

CORBA major features • OMG Interface Definition Language • Language Mappings • Operation invocation and dispatch facilities (static and dynamic) • Object adapters • Inter-ORB Protocol ©2002-2003 G. Michael Youngblood

IDL • Interface Definition Language • Sole purpose is to allow object interfaces to be defined in a manner that is independent of any particular programming language. ©2002-2003 G. Michael Youngblood

Operation Invocation and Dispatch Facilities • Static invocation and dispatch • IDL translated into language-specific stubs and skeletons that are compiled into applications. • Dynamic invocation and dispatch • Construction and dispatch of CORBA requests at run time. • Interface and types determined by outside means (e.g., Interface Repository) ©2002-2003 G. Michael Youngblood

Object Adapters • Serve as glue between servants and the ORB • Fulfill 3 requirements • Create object references, which allow clients to address objects • Ensure each target object is incarnated by a servant • Takes requests dispatched by a server-side ORB and further directs them to servants incarnating each of the target objects • POA! ©2002-2003 G. Michael Youngblood

Inter-ORB Protocols • ORB-to-ORB communication (even between different implementations) • GIOP (“gee-op”): General Inter-ORB Protocol • An abstract protocol that specifies transfer syntax and a standard set of message formats to allow independently developed ORBs to communicate over any connection-oriented transport. • IIOP (“eye-op”): Internet Inter-ORB Protocol • GIOP over TCP/IP ©2002-2003 G. Michael Youngblood

Request Invocation • When a client invokes an operation via an object reference, the ORB does the following: • Locates the target object • Activates the server application if the server is not already running • Transmits any arguments for the call to the object • Activates a servant for the object if necessary • Waits for the request to complete • Returns any out and inout parameters and the return value to the client when the call completes successfully • Returns an exception (including any data contained in the exception) to the client when the call fails ©2002-2003 G. Michael Youngblood

Request Invocation Characteristics • The entire request invocation mechanism is completely transparent to the client, to whom it looks like an ordinary method invocation on a local object. • Request characteristics: • Location transparency • Server transparency • Language independence • Implementation independence • Architecture independence • OS independence • Protocol independence • Transport independence ©2002-2003 G. Michael Youngblood

Object Reference Semantics • Every object reference identifies exactly one object instance • Several different references can denote the same object • References can be nil • References can dangle • Reference are opaque (the client is not allowed to look at their contents) • References are strongly typed • References support late binding • References can be persistent • References can be interoperable ©2002-2003 G. Michael Youngblood

Reference Acquisition • Object references are the only way for a client to reach target objects. • How does a client get a server’s object reference? • Bootstrapping • Naming or Trading Service • Command-line or reading a file • Out-of-band method ©2002-2003 G. Michael Youngblood

References and Proxies • When a reference is received by a client, the client-side run time instantiates a proxy object in the client’s address space • This proxy is an instance that supplies an interface to the target object to the client. Client Server Proxy SkeletonInstance foo() foo() foo() ObjectReference foo() Servant ©2002-2003 G. Michael Youngblood

OMG’s Document Access Page • OMG Specificationshttp://www.omg.org/technology/documents/index.htm • Key Specifications • Architecture and Specification (CORBA/IIOP) • IDL • Language Mappings • Services ©2002-2003 G. Michael Youngblood

IDL Simple Example // description of the module AddressBook module AddressBook { interface Person { attribute string name; string sendEMail(in string fromWhom); }; } ; ©2002-2003 G. Michael Youngblood

General CORBA Application Development • Determine your application's objects and define their interfaces in IDL • Compile your IDL definitions into C++ stubs and skeletons • Declare and implement C++ servant classes that can incarnate your CORBA objects • Write a server main program • Compile and link your server implementation files with the generated stubs and skeletons to create your server executable • Write, compile, and link your code together with generated stubs ©2002-2003 G. Michael Youngblood

CORBA Services • OMG Naming Service • Basically, DNS for IORs • OMG Trading Service • Service discovery • OMG Event Service • Broadcast events • Interface Repository • Time Service • etc. ©2002-2003 G. Michael Youngblood

CORBA Examples • Simple Client-Server • Advanced Client-Server • Advanced Client-Server using NameService • Advanced Client-Server using NameService invoking a Callback ©2002-2003 G. Michael Youngblood

Introduction to CORBA