CORBA and EJB
CORBA was originally developed to make applications communicate with each
other (using interfaces), with the Interface Definition Language
(IDL) to support the development of interfaces, and with the ORB
providing the infrastructure to facilitate the network-level communication.
This is essentially an integration problem. Let us consider two different
applications, one written in C and deployed on a Unix machine, and the
other developed in COBOL running on a mainframe. CORBA allows interface
wrappers on top of these applications. The purpose is to make these
applications communicate with each other without any dependency on the
programming language or the operating platform of these applications.
You can also use CORBA to develop distributed component applications. In
this case, we are not concerned about making applications communicate. The
goal is instead to build applications as a set of distributed components.
Although reusability is cited as one of the reasons for such an approach,
the main advantage is the ability to develop loosely coupled applications.
Distributed components can be easily distributed to suit specific
deployment and processing needs, without ever affecting the client
applications. Secondly, since distributed components are interface-centric,
interfaces remain independent of implementations, and implementations can
be changed without affecting the client applications.
On the other hand, the primary goal of EJB is not application integration.
The basic EJB technology is not meant for building interoperable
interfaces for applications developed on disparate platforms using
disparate programming languages. Instead, the purpose is to provide a
distributed component framework for building applications as a set of
distributed components. So, although CORBA meets both these requirements,
EJB is concerned only with the latter requirement – that is to build
distributed applications. The following figure illustrates these
differences:
The blocks on the left show how applications can be wrapped using CORBA.
The rectangular blocks are the applications, and with the help of CORBA,
these applications can expose language/platform independent interfaces to
client applications/components. Here the purpose of the interface is to
expose certain services.
Traditionally CORBA interfaces are coarse-grained, as they evolved from
interfaces to applications, and applications typically provide a number of
(often complex) services.
A coarse-grained interface is one that abstracts coarse concepts.
For instance, a Person interface with name and address is fine-
grained whereas a Customer interface with not only name and address,
but also abstracting the customer relation, history, account status etc. is
a coarse-grained interface.
The blocks on the right show the EJB approach. With EJBs, you design and
specify a set of interfaces, and provide implementations. The rounded
rectangles are objects that implement the interfaces. While traditional
object modelling practices focus on abstraction and encapsulation of domain
data/logic, such abstractions tend to remain fine-grained. In this case,
domain concepts such as Order, Customer, Placing an Order etc. get
translated into objects encapsulating respective details, and the remote
interfaces (the circles in the above figure) merely expose access to these
objects.
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