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Client Server Paradigm

Objectives: Taxonomy of distributed systems

service models point of view Client-server architecture

Thin versus thick clients Two and three tier architectures Tradeoffs between these different

architectures

Software Architectures for Clients and Servers

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Distributed Systems: Service-models Taxonomy

Centralized model Client-server model Cluster-based model Grid-based model Peer-to-peer model

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Distributed Systems: Service-models Taxonomy

Centralized model This the classic mainframe

time-sharing system The computer may contain

more than one CPU Terminals have serial

connections

Drawbacks Single-point of failure Not scalable Resource contention

Are there any advantages?

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Distributed Systems: Service-models Taxonomy

Client-server model A networked model

consisting of three components

• Server• Client• Service

The labels client and server are within the context of a particular service

Advantages? Disadvantages?

The labels client and server are within the context of a particular service

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Distributed Systems: Service-models Taxonomy

Cluster-based model Tightly-coupled Closely linked via LAN networking

Grid-based model Focuses on the ability to support computation across

administrative domains

Peer-to-peer model Example: Gnutella, Kazaa Peers are intermittently connected and change IP

addresses

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Distributed Systems: comparisonparadigm node

ownershipnode management

controlling policies

discovery mechanisms

peer-to-peer computing

local local none centralized or distributed

Cluster computing

global (single ownership)

global global job scheduling

Grid computing

local global management under local policies

single controlling policy

centralized or distributed

Where does a client server computing fit?

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Client Server Architecture It is way of designing an application in which clients

contact well-known servers to access resources What fraction of the task do clients process before

giving the work to the server? Thin clients

• Clients are information appliances • Servers are resource-rich• Must have a resource-rich connectivity

Thick clients• Clients perform the bulk of data processing operations• Servers perform rudimentary tasks

Trade-offs between thin and thick clients?

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Two-tier Client Server Architecture Traditional client-

server architecture A good solution for

small-scale group sizes This architecture has

limitations • Does not scale

• Restricts flexibility – Moving or

repartitioning program functionalities

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Multi-tier Client Server Architectures Example: three-tier

architecture The three-tier design

has many advantages over traditional two-tier

• Added modularity

• Function isolation • Scalability

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Client-server system architecture vs.

Client-server distributed computing

In the client-server system architecture, the terms clients and servers refer to computers

In the client-server distributed computing paradigm, the terms refer to processes

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Client-server, an overloaded term

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Client-server system The interprocess communications and event

synchronization Typically, the interaction of the client and

server processes follows a request-response pattern. clie n t s e rv e r

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Software Architectures for Clients and

Servers The Software architecture of client-server

application consists of Presentation layer Application layer Service layer

Any client-server software must have the three-layer functionalities

Is this a good approach? Why?

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Software Architectures for Clients and Servers: Example

We will look at a Daytime client-server software

Daytime service [RFC867]:Client: Hello, <client address> here. May I have a timestamp please.Server: Here it is: (time stamp follows)

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Software Architectures for Clients and Servers: Separating the layers

Allows each module to be developed by appropriate people People have different skills

Allows modifications to be made in isolation

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Client-Server Paradigm Issues

A service session Same service might be requested by

multiple clients Client sessions need to be kept separated

and isolated The service protocol

How the service is to be located The sequence of the IPC Data syntax and semantics

IPC and event synchronization

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Testing a Network Service

Since network software is notoriously difficult to test Use the three-layered software architecture and

modularize each layer on both the client and the server

Use an incremental or stepwise approach in developing each module

Develop the client first Test the client independent of the server Test the client-server suite on one machine before

running the programs on separate machine

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Client-Server Paradigm: Server Types

Connection-oriented server Connectionless-oriented servers Iterative servers Concurrent servers Stateful servers Stateless serves

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Client-Server Paradigm: Server Types

Connection-oriented server Connectionless-oriented servers Iterative servers Concurrent servers Stateful servers Stateless serves

We will look at the tradeoffs of these different server types

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Connection-oriented communication

A separate connection is maintained for each session

Once the connection is established, data can be sent until the session is over

The connection needs to be explicitly torn down

Imagine that we have n processes What happens if a connection is established between

a sender and every other process? What happens if all the n processes is a sender?

Connection-oriented servers rely on connection-oriented communication

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Connection-oriented: Daytime Server Example

… theServer = new ServerSocket(thePort);

p = new PrintWriter(System.out); try { p.println("Echo Server now in business on port " + thePort ); p.flush(); theConnection = theServer.accept(); // read a line from the client theInputStream = new BufferedReader (new InputStreamReader (theConnection.getInputStream())); p = new PrintWriter(theConnection.getOutputStream()); while (!done){ theLine = theInputStream.readLine(); if (theLine == null) done = true; else{ p.println(theLine); p.flush(); } } theConnection.close();

Connection acceptance

Protocol processing

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Connectionless-oriented communications Involves no connection Packets are explicitly addressed by the

sender The connection needs to be explicitly torn

down Connectionless communications are

simpler to provide Packets can be delivered out of order

Connectionless-oriented servers rely on connectionless-oriented communication

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Iterative Servers

An iterative server in unable to overlap client sessions

Is a connection-oriented server an iterative server? If yes, why?

Suppose that n clients have requested connection at a given time, and each session is expected to last t time units. What will happen to request n+1?

What is the solution???

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Concurrent Servers

A concurrent server in capable of conducting multiple client sessions at once

A concurrent server can be provided by using Threads or Asynchronous IPC operations

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Stateful servers

A stateful server maintain stateful information on each active client

Stateful information can reduce the data exchanged, and thereby the response time

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Stateful vs. Stateless server Stateless server is straightforward to code, but the

state information maintained by the server can reduce the data exchanged

Are there any problems with stateful servers?

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Stateful vs. stateless server

In actual implementation, a server may be Stateless Stateful A hybrid, wherein the state data is distributed on

both the server-side and the client-side

Which type of server is chosen is a design issue

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Global state information

Information maintained by a server for all the clients throughout the lifetime of a service

The global state information needs to be synchronized for mutual exclusion

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Session state information

Information maintained specific to a client session

Two schemes to maintain session information Session information maintained by the client

• The server processes each request in the same manner• The complexity of the server’s application logic is reduced• Such a server is called stateless

Session information maintained by the server• Server keeps track of the session progress of the client• Server is more complex to design and implement• Failure provisions

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Summary You have been introduced to the client-server

paradigm in distributed computing. Topics covered include The difference between the client-server system

architecture and the client-server distributed computing paradigm

Definition of the paradigm and why it is widely adopted in network services and network applications

The issues of service sessions, protocols, service location, interprocess communications, data representation, and event synchronization in the context of the client-server paradigm

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Summary: Con.

The three-tier software architecture of network applications: Presentation logic, application logic, and service logic

Connectionless server versus connection-oriented server

Iterative server versus concurrent server and the effect on a client session

Stateful server versus stateless server In the case of a stateful server: global state

information versus session state information

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Summary: Con.

You are required to read the following: The paper entitled “A taxonomy of distributed

computing” Chapter 5 of the Distributed Computing book Chapter 2 of the Distributed Computing book