Architectures.pdf

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Distributed Computing

ARCHITECTURES

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Distributed system models



Architectural models:

– Placements of parts

– Relationship between parts



Examples

– Client /server

– Peer-to-peer model

– Proxy server



Architectural models (definition):

–

The way in which the components of systems interact with one

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Architectural models



Architecture:

– Structure of separately specified components



Over all goal:

– Structure should meet the present and future requirements on :



Reliability



Manageability



Adaptability

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The Client-Server Model



A server is a program that provide a service.

– It makes some resource available to other programs running somewhere on the network.

– The resource could be anything: a database, a file system, a printer, a modem, a screen, or a scanner.

– The server program runs on the machine which resource is attached to, and waits passively until the services are required.

– Servers are often started up at boot-time, via commands in a start-up script and Servers usually spent most of their time asleep, waiting for work.



A client is a program that uses a resource.

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Client-Server interaction (I)

• Synchronous remote procedure call

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Client-Server interaction (II)



An example of a server acting as a client (vertical distribution).

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Client-Server interaction (III)

•Asynchronous remote procedure call

(deferred synchronous RPC

)

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Client-Server interaction (IV)



An example of horizontal distribution .

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Client-Server Model



Server may in turn be client of other servers

– Web server may be client of local file server – Search engine is both a server and client

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A service provided by multiple servers

Server

Server Service

Client

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Example: Web proxy server

Client Proxy Web server Web server server Client



Cache: Store of reccently used data objects

– Cache

 collocated at each client (web browser)

 proxy server (shared cache)

– Web browser

 recently visited pages

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Peer-to-Peer Model

(not client-server model)

Coordination Application

code

Coordination Application

code Coordination

Application

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Peer-to-Peer Model



Peer processes: processes that play similar roles

– No absolute distinction between client/server

– May still assume client/server roles from time to time

– Peer processes maintain the synchronization & consistency of resources and actions



White board

– Viewing, editing a picture



Elimination of server processes reduces IPC on delay for

local object access



Increased fault-tolerance and scalability

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Variations on the Client-Server Model



Variations inside the client-server model from

following factors:

– the use of mobile code and mobile agents

– lightweight clients, based on users’ need for low-cost

computers and easy management

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

Mobile Code Example: Applet

a) client request results in the downloading of applet code

Web server

Client Applet _serverWeb Applet code

Client

b) client interacts with the applet

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

Example of a mobile agent

– a) The code and data are moved to the server

Web

server

Code + data

Users

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Mobile agents

–

A running program that travels between computers in a network



Carries out tasks on someone’s behalf

•

E.g., collect information, install programs



Has internal knowledge, beliefs and goals

–

Advantage: local access everywhere



Reduction in communication costs



Disconnected operation

–

Potential security threat

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

Network computer

– All files related to application is stored and managed remotely

– Application is present & run locally

– If allowed on local disk & space is available

 used as cache

 Minimum of local software is downloaded



Thin client

– Does not even run its own applications

– Programs are run by a powerful computer server

– Advantage

 Low cost

– Disadvantage

 Not good Highly interactive application like CAD

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• Thin Client / Compute Server:

Thin

Client

Application

Process

Network computer or PC

Compute server

network

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Mobile Devices and Spontaneous networking



Key features

– Easy connection to local network

 Transparently reconfigured devices

– Easy integration with local services

 Autonomous service discoveries



Issues

– Internet addressing, and routing assumption

 computers are at fixed locations

– Limited connectivity – Security and privacy



Discovery services

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The Client-Server Model: Application Layering



Many client-server applications are targeted toward

supporting user access to database, a distinction can be

drawn between the following three levels:



user-interface level

:

contains all that is necessary to directly

interface with the user, such as display management;



process level

:

typically contains the applications;

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Application Layering (search engine in Internet)

The general organization of an Internet search engine into three different layers

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Client-Server Addressing

C

1

S

2

C

3

S

4

1

2

S

3

C

1

NS

1. Machine.process / Machine.local-id addressing 2. Process addressing with broadcasting