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Client/Server ModelsClient/Server Models

Client/Server Environment

LAN or WAN

Server DataBerson,Fig 1.4, p.8

clients

network

Client• Part of the local computing system • Functions• initiates contact with server (“speaks first”)• typically requests service from server, • for Web, client is implemented in browser; for e-

mail, in mail reader • Requesting the return of records

Client = “Origin”

Server• Functions• provides requested service to

client• e.g., Web server sends

requested Web page, mail server delivers e-mail

Server = “Target”

Client-Server Example• The ATM network:

– the clients are the ATM machines • user interfaces;

some simple application processing

– the server is at the bank • most application processing;

very large database of customer accounts

Client/Server Processing

• Application processing is usually distributed between a client and the server.

• Server controls services/data that the client accesses• Different client/server models are obtained by

locating different components and combinations of the application on the client and server(s).

• In general:– presentation logic stays on the client– DBMS and database move to the server– parts of the business and database logic that can be used by

several clients are placed on the server

Component Placement?• How much data is required by the local

application?• How many application users require the

same data?• How many interactions occur between

the application parts?• Technical issues

– platforms, networking

Database App.

Presentation Logic

Business Logic

Database Logic

DBMSDatabase

Stand-aloneApplication

The 2-tier Model• The database is on the server• Distributed database logic

– most of it is on the client• The client does the presentation and business logic.

Challenges• It is difficult to build heterogeneous database

environments.• Transaction processing is limited by the DBMS.• Asynchronous processing is difficult

– i.e. the client doesn’t wait for the server’s answer• Scalability

The 2-tier Model

Presentation Logic

Business Logic

Database LogicDBMS

Database

Database Logic

ClientServer

The 3-tier Model

ServerData

Application/Business logic Data Servers

ServerData

UNIX

Win/NTClients

The 3-tier Model

Server

Back-End ServerTier 1

Tier 2

Tier 3

Clients

Price/PerformanceFunctionalityLocal Autonomy

Greater IntegritySecurityCentral Control

3-Tier Architecture-Example

Database

XMLFile System(Web Site)

Or

Middle Tier“Business Logic”

dissociatesUser and Back End

Export/Import

Request

Information

ObjectRepository

3-tier over 2-tier• Benefits

– The application logic in the middle-tier is more independent of the client and the back-end server

– The application logic in the middle-tier can work more easily with data from multiple sources.

– Encourages multiple back-end servers.

Challenges• Much more complex:• network management, data integrity, maintenance,

development

• Still (partially) dependent on platformse.g. the client may still be restricted

to a certain backend application server, but not (maybe) to any data server

3 tier-ExampleATM machines are the clients (as before)

– the middle-tier servers provide certain processing• checking balances, money transfer requests• directing queries to the relevant back-end

server– back-end server(s)

• specialized by account type• very robust concurrency control, transaction

processing• Many Web applications are 3-tier:

– the Web browser is the client software– the embedded components in Web pages (e.g.

Java applets) come from the middle-tier– the back-end server contains the database

Distributing Business Logic

Three ways • Locate it entirely on the client

(‘fat’ client)• Locate it entirely on the server

(‘fat’ server)• Split it between the client and

server

Business Logic

Presentation Logic

Business LogicDBMS

Database

Database Logic

ClientServer

Business Logic

Fat Server Advantages• Easier to update the application logic since

clients not involved.

• Data is better hidden from clients.

• Easier to manage and debug since data and code is centrally located.

• Reduces bandwidth problems since data processing stays on the server.

• Better for mission-critical applications when fault-tolerance and stability are important.

Fat Client Advantages• The server is unaffected when

updates are done to the client’s application logic– the server will be more stable

• Easier to program– less networking– more direct access to client

platform features, such as GUI

Multi-tier Model

Middleware

Physical Network

Multi-tier Model• Asynchronous connectivity

• Data distribution using replication

• Name/directory services for resource location independence

• More complex data types• More complex analysis• Authentication services

– you must 'prove' who you are to the system

• Distributed file system(s)

• Time services

Mult-tier Web Applications

• The Web browser is the client software on the first tier.

• Web page components come from the second tier.

• The third tier is a database front-end for a series of fourth tier heterogeneous databases