CORBA - What is it all about?

CORBA, which stands for "Common Object Request Broker Architecture," is an open distributed object computing infrastructure that allows interoperation between distributed applications. It is a major initiative lead by the non-profit organization Object Management Group (OMG) that develops and maintains computer industry specifications for distributed enterprise applications. These specifications are free-of-charge and several IT companies have implemented them including IONA Technologies (Orbix ORB) and ObjecSpace (Voyager ORB). CORBA has been successfully used in various mission-critical applications such as healthcare and banking.

The main value-added by CORBA to distributed enterprise applications is the automation of network tasks. The development of such applications poses multiple challenges including how to locate appropriate services, how to guarantee a reliable delivery of information despite risks of network disconnections, and how to recover from disconnections with less impact on the business process under execution. These challenges result in an additional burden on application developers. To reduce the complexity of some of these technical challenges, CORBA as a middleware technology is deemed appropriate. Abstracting communication details, supporting communications in a transparent way, and recovering from potential crashes are some of the objectives of the CORBA technology. As a result, application developers can now focus on the underlying logic of the functionalities to offer to end-users, instead of low-level technical details.

The main elements in CORBA specification are:

• Object Request Broker (ORB): responsible for routing the invocation or request from the client (source) to the server (destination), and returning back the response.

• CORBAServices: a collection of packages that are used in the IDL interface to provide services to the CORBA system. These services are mainly used in ORB. Examples of these services are:
o Startup Service: a request could start automatically whenever it invokes the ORB.
o Life Cycle Services: an item on the bus can be identified with a process like create, copy, move, or delete.
o Naming Service: name can be given to an item on the bus for identification reason.
o Persistence Service: items can be saved in a storage area like Object Databases (ODBMS) through a single interface.
o Event Service: a registration to specific events can be given to any item on the bus.

• CORBAFacilities: a set of IDL-defined frameworks that are used as a services provider to the application objects. The two main facilities are horizontal and vertical facilities which specify regulations needed to manage business components.

As part of our senior capstone project, we designed, developed, and simulated the operation of a Distributed Online Library System, using both VisiBroker ORB (info.borland.com/techpubs/bes) for Java and Java as a middleware and implementation language respectively. The library object implementation, namely the server, offers various services to potential borrowers including search, borrow, and return resources to clients existing within the same Local Area Network. For data management purposes, the server was connected to a database through the Java Database Connectivity (JDBC). Moreover, a thread safe utility was used in order to isolate clients' concurrent requests and achieve data consistency and integrity. For communication to be established between a client and the server, CORBA middleware must run on both machines. The middleware hides the heterogeneity of the underlying platforms, programming languages, network, and hardware. It coordinates the connection and interoperation of separate applications across a network.

When a client submits a request to the server, the ORB is in charge of locating the object implementation, delivering the request to the object (and activates the object if needed), and returning the response to the client. The client is unaware of the location of the object whether it is on the same machine or across a network. The client can use a remote object by obtaining a reference to that object using binding. Known as Static Invocation Interface (SII), this type of binding is appropriate when the object type is declared during design-time. The client uses the object reference to invoke an operation on the object or to reference data contained by the object.

Despite some criticisms preventing the widespread adoption of CORBA by the IT community (e.g., complexity of use, learning curve, appropriate for large applications) and the availability of other competing technologies such Microsoft DCOM and Java RMI, CORBA has a larger scope (in terms of utilities, features, etc.), as it does not only use CORBA's distributed object abstraction, but also elements of the Object Management Architecture (OMA). CORBA is independent from any programming languages and bypasses the heterogeneity obstacle of programming languages and vendors' implementations of the CORBA specification.