Client/server computing and Middleware
By CHIN SIK SEAN
The client/server model evolves from two-tier to n-tier architecture. With the n-tier architecture that most of the enterprises are using now, it can ease the job of maintenance and modification of huge applications.
In order to implement the n-tier architecture, there is a need to “glue” the client and server with a middleware.
2.2 Types of Middleware
There are several kinds of middleware that have been developed. These vary in terms of programming abstractions as well as the heterogeneity they provide across the network.
i. Remote Procedure Call (RPC) - This type of middleware extends the procedure call interface to invoke a procedure across a network. RPC systems are usually synchronous. Hence, no potential for parallelism without using multiple threads.
ii. Message Oriented Middleware (MOM) - MOM provides programme-to-programme data exchange, enabling the creation of distributed applications. It supports asynchronous calls between client-server and requires the recipients of messages to interpret their meaning and to take appropriate action.
iii. Distributed Object-Oriented Middleware - Distributed Object-Oriented Middleware allows the objects that comprise an application to be distributed and shared across heterogeneous network.
2.3 Object-Oriented Framework
Object-oriented technology shows a shift of paradigm in the way software is developed. Hence, there is a need to develop object-oriented framework to act as a template to implement a complete application. From Ho & Jezequel (1998), object-oriented framework can be divided into 3 categories:
i. System Infrastructure - Provide basic template to build a new system and is normally used for internal software development. Examples of software development using this infrastructure are operating system, user interface and so on.
ii. Middleware Integration - Framework used to integrate software in a distributed environment.
iii. Enterprise Application - Normally this framework is based on a specific domain such as accounting, banking, telecommunications and so on.
2.3.1 Advantages of Distributed Object-Oriented Middleware
Advantages of object-oriented framework according to Fayad & Schmidt (1997) are: modularity, reusability, extensibility and inversion of control. Besides that, object-oriented framework produces higher quality applications out of reusable objects as all codes have been tested. Moreover, since existing objects can be used in different programmes for different purposes, the development process is faster and thus costs are reduced.
Object-oriented technology is also essential in integrating legacy systems. Legacy systems can be encapsulated as a single object and provide interfaces for client-side invocation.
Distributed object-oriented middleware have all the advantages suggested above as they are developed using object-oriented technology. Thus, it is feasible and practical to utilise distributed object-oriented middleware as the “glue” between client and server for components integration across the heterogeneous network.
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