chp 3 - architectures of c-s systems

7/28/2019 Chp 3 - Architectures of C-S Systems

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Architectures of C/S Systems

Dr. Mohammad Faidzul Nasrudin


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Components of C-S Architectures

C-S architecture is mainly based on three

components

Hardware (for client and server)

Software (which make hardware operational)

The client (front-end-application) is any computer

process that requests services from server

The server (back-end-application) is any computerprocess providing the services to the client and also

supports multiple and simultaneous clients requests


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Components of C-S

Architectures (2) Communication middleware (which use to link the

client and server)

communication middleware is any computer process

through which client and server communicate


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Middleware


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Middleware (2)

A collection of technologies that support

interaction between C and S

An Application Programming Interface (API) that is

called by C to get services from S

Middleware tasks start from API call by C to get S

services, request sending through the network

untill the response received by the C Identity of middleware becomes vague because

many middlewares have been inserted into OS


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Principles Behind C-S System

C-S architecture principles constitute the

foundation on which most current generation

C-S system are built

Hardware independence

C, S, and communication middleware, processes run on

multiple hardware platforms (IBM, DEC, Compaq,

Apple, and so on) without any functional differences


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Principles Behind C-S System (2)

Software independence

C, S, and communication middleware processes

support multiple OS (such as Windows 98, Windows NT,

Apple Mac system, OS/2, Linux, and Unix) and multiple

network protocols (such as IPX, and TCP/IP)

Open access to services

All client in the system must have open (unrestricted)

access to all the services provided within the network These services must not be dependent on the location

of the client or the server


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Process distribution

Application-processing load of C and S must conform to

the following rules

Client and server processes must be autonomous entities with

clearly defined boundaries and functions

Local utilization of resources such as processes and memory

(at both C and S) must be maximized

Scalability and flexibility requires that the C and S process be

easily upgradeable to run on more powerful hardware andsoftware platforms

Interoperability and integration requires that C and S

processes be seamlessly integrated to form a system.

Swapping a server process must be transparent to C process


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Standards

All the principles that are formulated must be based on

standards applied within the C-S architecture

E.g. standard govern the user interface, data access,network protocols, interprocess communications, etc

The point is to ensure that all components (server,

clients, and communication middleware) are able to

interact as long as they use the same standards

Applications use many standards and middleware in

one time


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C App for Book Shop

ODBC

TCP

IP

IEEE 802.3 (Ethernet)

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical


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Client Side

The desirable client software and hardware

feature are

Suitable (powerful enough) hardware

Client processes typically requires a lot of hardware

resources, they should be stationed on a computer with

sufficient computing power

E.g. to open files, videos, images, etc

Client processes may require large amount of hard disk

space and physical memory, the more such a resource

is available, the better


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Client Side (2)

An OS capable of multitasking

Client should have access to an OS with at least some

multitasking capabilities

MS WinXP and Win7 are currently the most commonclient platforms

Communication capabilities

To interact efficiently in a C-S environment, the client

computer must be able to connect and communicatewith the other machines in a network environment

Need a network adapter to connect to a network


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Client Side (3)

A graphical user interface (GUI)

Client need GUI libraries in order to generate proper

form/window/presentation (GUI objects) to end user

DirectX, OpenGL, JavaFX, Applet, Flash, Silverlight


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Server Side

The desirable server services and hardware

feature are

Powerful hardware

server process should be the more powerful computer

(fast CPU) than the average client computer because

the server process must be able to handle concurrent

requests from multiple clients

include DVD-RW, video card, backup tape, etc

Fault tolerant: Dual power supply, UPC, ECC, RAID.

Expandable CPU, multiple bus, multiple communication


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Server Side (2)

A Server OS

is especially developed to serve as a platform for

running multi-user computer programs, applications

that are networked and programs critical to business

computing

FreeBSD, Linux, Mac OS X, Solaris, MS Windows Server,

etc

Provide file services: a client connected to the network can store files on the

file server as if it were another local hard disk


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Server Side (3)

Print services

a client can access any one of the printers as if it were

directly attached to its own computer

Manage print queues Fax services

a client able to submit data to be faxed to a fax server

that equipped with a fax device

Able to schedule the fax, dial the fax number, andtransmit the fax


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Server Side (4)

Communication services

A client can access other host computers or services to

which the client is not directly connected

E.g. access to telephony software Database services

The server act as a database server

Transaction services

A client can execute database transaction code or

procedures that manipulate the data in database


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Server Side (5)

Groupware services

Clients able to collaborate with each others using semi-

structured information like Text, image, mail, bulletin

boards, flow of work

Object application services

A client can access distributed objects in the server

Web application services

A client able to access shared documents across

intranets, or across the Internet via the HTTP protocol


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Complexity of Server

The server

processes one request at a time (sequential)

a big role in maintaining the request queue that

arrives for a server

usually much more difficult to build than clients

because they need to accommodate multiple

concurrent requests


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Complexity of Server (2)

Server tasks

1. Open port: The program opens a port at which

the client request reached

2. Wait for client: The program waits for a new

client to send a request

3. Choose port: If necessary, the program allocates

new local port for this request and informs theclient


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4. Start thread: The program starts an

independent, concurrent thread to handle this

request. Note that the thread handles one

request and terminated thread does not wait forrequests from other clients

5. Continue: The program returns to the wait step

and continues accepting new requests while the

newly created thread handles the previousrequest concurrently


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Besides the complexity of concurrent

requests, the server

have to protect the integrity of data/file. E.g. file

locking

must enforce authorization and protection rules

by reading system files, keeping logs, and secure

protected data access must protect themselves against malformed

request that will crash the server program


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Communication Middleware Side

Communication middleware software

provide the means through which clients and

servers communicate to perform specific actions

provides specialized services to the client process

that insulates/encapsulates/hides the

programmer from the internal working of the

server program and network protocols

In the past, programmers had to write code that would

directly interface with server program with specific

network protocol


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(2)

To accomplish its functions, the communica-

tion middleware software operates at 2 levels

The physical level

Deals with the communications between client and

server computers (computer to computer)

Deal with how the computers are physically linked

Include the network hardware and software (protocols)

E.g. TCP/IP, NetBIOS


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(3) The logical level

Deals with the communications about processes

between client and server

Deal with how the client and server processcommunicates (process to process)

Interprocess communications protocols that give the

signals meaning and purpose

level where the C/S conversation takes place E.g. ODBC


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Architecture for Business

Information System

Main question

How do I partition the business information

system into a number of client and server

components, so that my users functional andnon-functional requirements are met?

This is because

The business information system will be deployedacross a set of distributed processing units (e.g.,

machines in a network, processes on one

machine, threads within one process, etc)


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Information System (2) Suggestion

The information system

can be easily partitioned

using 3-tier model

Divide your system into

distributed views of

user interface, business

logic and data

management


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Information System (3)

You must consider

Business needs vs. construction complexity

allocating functionality and data to the places where it

is actually needed supports distributed businessprocesses but raises systems complexity

Reduce the complexity to a level where it can handled

properly

E.g. compute simple task locally (client side) instead ofremotely


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Information System (4) Processing style

Different processing styles require different distribution

decisions

Batch applications need processing power close to thedata

Interactive processing should be close to input/output

devices

Therefore, off-line and batch processing may conflictwith transaction and on-line processing


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Information System (5) Distribution vs. performance

We gain performance by distributed processing units

executing tasks in parallel, placing data close to

processing, and balancing workload between several

servers

But raising the level of distribution increases the

communication overhead, the danger of bottlenecks in

the communication network, and complicates

performance analysis and capacity planning


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Information System (6) Distribution vs. security

The requirement for secure communications and

transactions is essential to many business domains

In a distributed environment the number of possiblesecurity holes increases because of the greater number

of attack points (e.g. open port for communication)

Therefore, a distributed environment might require

new security architectures, policies and mechanisms


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Information System (7) Distribution vs. consistency

Abandoning a global state can introduce consistency

problems between states of distributed components

(e.g. database updating, mirroring, etc)

Relying on a single, centralized database system

reduces consistency problems, but inefficient


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Information System (8) Software distribution cost

The partitioning of system layers into client and server

processes enables distribution of the processes within

the network, but the more software we distribute the

higher the distribution, configuration management, and

installation cost

The lowest software distribution and installation cost

will occur in a centralized system

This is not happen when we use thin client such as web

browser that have well distributed in clients


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Information System (9) Reusability vs. performance vs. complexity:

Placing functionality on a server enforces code reuse

and reduces client code size

The server (where reuse code reside) must enable thehandling of requests by multiple clients (load balancing

issue)

The more you reuse, the more complex the system


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Before Ending

Please skip section 3.7.4 Distribution Pattern

You can read 3.8 Existing C/S Architecture

The main idea of this section had been discussed

in previous chapters


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The End

chp 3 - architectures of c-s systems

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