EVOLUTION OF COMPUTING ARCHITECTURES

Aim: To discuss the evolution of client server architecture for various computing models.

There are three system architectures,

1. Centralized system

2. File Server Architecture

3. Client server system

CENTRALIZED SYSTEM ARCHITECTURE

Companies that needed real computing power turned to the mainframe computer, which is a centralized system architecture.

Salient feature: Only movement of marshalling are the keystrokes between the client and the host machine.

Marshalling is the process of packaging interface elements and sending them across process boundaries.

In Centralized System, keystrokes are marshalledfrom the terminal to the host.

Network Server

Terminal 1 Terminal 2 Terminal 3 Terminal 4

DATA

Merits Excellent security

Centralized administration as both application logic and data reside on the same machine

Demerits Expensive to buy, lease, maintain and use.

Both the application and the database are housed within the same machine process thereby offering no way to truly partition the application logic beyond the physical of the mainframe.

FILE SERVER ARCHITECTURE

• This brought a complete change in the implementation of the computer architecture

• In this system, the application logic was executed on the client workstation instead of the server.

• These servers provide access to computing resources like printers and large hard drives

File server architecture

Disk I/O Requests

Disk Blocks

DATA

Workstation 1 Workstation 4Workstation 3Workstation 2

ApplicationLogic 1

ApplicationLogic 4

ApplicationLogic 3

ApplicationLogic 2

Network Server

MERITS:

It has the low cost entry point with flexible arrangement

Computer resources can be added or reduced as and when necessary using this system.

DEMERITS:

All application logic is executed on the client machine. This results in the client machine’s need for a large amount of power to run the application.

CLIENT SERVER MODEL

This is a process involving a minimum of two independent entities, one is the client and the other is the server.

The client makes a request to the server and the server services the requests and the resulting data is sent to the client.

In this application, two separate applications operating independently could work together to complete a task.

SQL (Structured Query Language based Database Management System)is the implementation of this

concept.

Disk I/O

Requests

Disk

DATANetwork Server

Workstation 1 Workstation 4Workstation 3Workstation 2

ApplicationLogic

ApplicationLogic

ApplicationLogic

ApplicationLogic

Application logic

The most popular client server applications revolve round the use of DBMS such as Oracle

and Microsoft SQL server. These applications are referred to as back-ends and offer support for storage, manipulation and retrieval of business persistent data. They use SQL as a standard method for submitting client requests.

The significance of distributing the application logic between the client and server offers a faster and more cost-effective performance.

File Server v/s Client Server Database Deployment

With the advent and popularity of Microsoft Access using the Microsoft Database file(MDB) to store data, this is not a client server implementation but a file server implementation.

CASE STUDY

Assume that we have a set of data stored in an MDB file namely Student_Info.mdb. This file holds the details of the marks scored in different subjects by students. A client want to know as how many students scored 100% in more than two subjects.

A query is sent to obtain the results.

Query Sent to a File Server System

• In this case, the query is not sent to the server.

• The logic of the query is processed and evaluated at the client end itself.

• The query logic realizes that it needs to access a table namely Student_Info in the MDB in order to process the request.

• No logic is executed at the server end except the transferring of file disk blocks.

Query Sent to a Client Server System

• Here the actual SQL statement is sent across the network and processed by an application running locally on the server machine

• As the SQL statements is processed on the server, only the results need to be sent back to the client.

• Advantage: Reduced network traffic and an incredibly quick execution time.

To meet the challenges presented by a business environment, a new three-tier or n-tier client server approach has been introduced

Two-Tier Client Server Model

• In this mode, a desktop machine operates as a client and a network server functions as a back-end database engine.

• The logic in a two-tier model is split between the two physical locations namely the server and the client.

• In Visual Basic, using any of the data controls that provide a graphical link to the back-end data source creates a two-tier client server relationship.

Limitations of Two-Tier Client Server Model

• Not Scalable Inability of a two-tier approach to grow beyond the physical boundaries of a client and server machine

• Unmanageable Business rules cannot be encapsulated and deployed centrally because of which sharing common processes and re-using work becomes difficult

• Poor Performance The graphical interface binding to the data consumes major resources on the client machine which results in poor performance and dissatisfied clients.

Three-Tier Client Server Model

• This model is based on the ability to build partitioned applications.

• Partitioning an application breaks the code into logical components.

• Each tier is called a service. These services are shared between that client and the servers.

• The components can be logically grouped into three tiers: user services, business services and data services.

Service Location

User Services Client

Business logic Server

Data Services Server

The three-tier logic can be extended to any number of tiers/services known as Multi-Tier or n-tier architecture.

Values of the Three-Tier Client Server Development

• Reuse The time invested in designing and implementing components is not wasted as they can be shared among applications.

• Performance As components can be placed on machines other than the client workstation, load processing can be shifted from a client machine that may be underpowered to a server with extra horsepower. This offers the user best possible methods for each aspect of an application’s execution, resulting in better performance.

• Manageability Encapsulation of application’s services into components allows the user to break down large, complex application’s into more manageable parts.

• Maintenance Centralizing components for reuse has an added benefit. They become easier to redeploy after any amendments are made in the order to keep pace with business needs.