Client Server computing is rapidly changing as three main forces attempt
to set a standard. One camp is Microsoft with its Windows operating system and DCOM.
Another is the Object Management Group (OMG) and its CORBA-based distributed-
objects model. The third group is SUN and the Enterprise JavaBeans (EJB) environment based
on the Java programming language.
The material presented in this node is derived primarily from an
article on the CORBA-based model published in ACM Computing Surveys
(Lewandowski 1998). Mr. Scott Lewandowski presents a technologically superior model
for Client/Server computing, but goes on to imply that technological superiority does not
guarantee dominance. 1 The EJB environment is described separately in
I. Definition of Client/Server
Client/Server is an interrelated, but independent, set of software and hardware
components that dynamically open and close transparent relationships, based on
the request/response paradigm, that are mutually beneficial to the participating
An interrelated set of software and hardware components means there are at least 3-tiers
of interacting entities: a client, a server, and a data repository. A client is a net consumer
requesting services from a server, which is a net provider. The server may in turn obtain
informational resources from a data repository in order to meet the client request. The
entities are independent and self-contained and may interact with any number of other
Client/Server entities. The relationships are dynamic in that they begin and end based on
the request and response of the entities. When the request has been fulfilled the
relationship ends. The relationships are transparent in that each entity is self-contained.
The method by which each entity carries out its part in the relationship is not visible to
any other entity.
Lewandowski presents this vision of Client/Server based on Distributed Objects
interconnected via the Common Object Request Broker Architecture (CORBA). Objects
encapsulating data and functionality would be created new or be "wrapped around"
legacy informational systems. Under a distributed environment these diverse objects
would take responsibility for managing their own resources. Distributed objects could
then form relationships ad hoc in diverse configurations creating applications to fill
specific requests. CORBA provides the language- and system implementation-neutral
communications mechanism to allow such relationships.
Compare the CORBA vision to the Microsoft ideal. The latter wish to maintain
control of their installed base of Windows operating systems. Distributed Component
Object Model (DCOM) is actually a set of services or interfaces and does not follow the
traditional, independent-object model. This version of Client/Server computing tries to
incorporate too much functionality in a central location - the operating system.
The technologically superior version is the CORBA-based distributed object
model. It will allow the type and number of interactions between objects to grow
exponentially developing new relationships and ideas that will benefit commerce. The
DCOM model is too centralized and will in the long run constrain the relationships that
can enhance commerce.
II. Glossary of Client/Server Terms
- 1. Client
- A net consumer of services. It is presentation software that
makes a request for information and then displays it in a user-desired
manner. Most often via a graphical user interface (GUI).
- 2. Server
- A net provider of services. It generally is software running on a
separate machine from the client. It is encapsulated so that its service-
providing mechanisms are hidden from clients. A server may assist
- 3. Middleware
- A framework of software that facilitates the communication
between the parts of Client/Server systems. Examples include
CORBA, DCOM, ODBC, SQL and EJB.
- 4. Objects
- A software abstraction that couples data and the operations to
process that data together into a self-contained entity. "Objects provide
a clean way separating related data and functionality from other parts of
a system in order to ease system construction and maintenance"3
- 5. Distributed Objects
- An extension to the Object paradigm. The objects may reside
anywhere on a network. They will retain their existence independent of
any program, application, or other objects. They may be accessed by
other objects or programs via standard message protocols contained in
- 6. Business Objects
- Self managing components representing key objects or
processes of a real-life system. They are composed of three
cooperating components: Presentation Objects to represent the Business
Object to a user; Business Logic Object to embody functionality and
data of the Business Object; and Business Process Object to interact
with the system as a whole and maintain long-lived processes.
- 7. CORBA
- Common Object Request Broker Architecture. It is middleware
software that provides structure for the client, server, data repositories,
and other distributed objects to interact seamlessly. CORBA supports
the traditional notion of encapsulated, self-operating objects.
It is an open-standard supported by many competing companies
in the industry, but not by Microsoft.
- 8. IDL
- Interface Definition Language. It is a language-neutral
mechanism to define the services an object provides to a CORBA based
Client/Server system. IDL is part of the CORBA specification for the
seamless interaction between objects written in different computer
languages or structured in diverse ways: from current object formats
back to mainframe data structures on legacy business systems.
- 9. COM/DCOM
- Component Object Model and Distributed Component Object
Model. This is the Middleware standard supported by Microsoft and
competes directly against CORBA. DCOM does not support the
traditional notion of objects. Instead it is a collection of interfaces to
services. DCOM objects do not provide "state": the results of a
request/response interaction are not available to other clients.
- 10. Tiers
- Tiers refer to the number of interacting components in a
2-Tier refers to a two-component architecture predominant in
the early 1990s. The two components were just the client and the
server. A data repository was an integrated part of the server, if
present. The business logic portion of the interaction was placed in
either the client, the server, or both. This led to undesirable higher
coupling between the client and server.
3-Tier refers to the current predominant model involving three
interacting components: client, server, and data repository. This is a
better model because the three components are encapsulated and hide
the mechanisms by which each operates.
A variation on 3-Tier is N-Tier. N-Tier indicates that a higher
number of interacting components may cooperate to fulfill a request.
N-Tier describes the multi-object relationships that exist under a
CORBA-based distributed component architecture.
1. Page 27, Lewandowski, 1998. As with VHS and BETA video technologies in the 1970-80s, dominance
may result from the superior marketing efforts, in the present case, of Microsoft (DCOM) rather than
technological superiority of OMG's CORBA/JAVA/Distributed Objects model.
2. Page 8, Lewandowski 1998.
3. Page 20, Lewandowski 1998.
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ACM Computing Surveys, Vol. 30, No. 1, March, pages 3 - 27.
Linthicum, D., 1997, David Linthicum's Guide to Client/Server and Intranet
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