complete unit 1[1]

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Unit-1

What is client/server?

Clients and servers are separate logical things that work together over a network

To accomplish a task.

Example:

The "server" is the computer that "serves" data or services.

The "client" is the computer that asks for the service or data from the server.

Interestingly, the World Wide Web is probably one of the best examples of a

client/server architecture. When accessing the World Wide Web, you use client

software (usually Internet Explorer or Mozilla Firefox), which connects to the web

server, which holds the content. The web browser asks for the content, and the web

server serves it back to the browser.

What is meant by Middleware?

Middleware is distributed software needed to support interaction between clients and servers. In

short, it is the software that is in the middle of the Client/Server systems and it acts as a bridgebetween.

Client/server characteristics:

Service (request & response)

Shared resources (file sharing, data sharing)

Asymmetrical protocols (There is a many-to-one relationship between clients and server.

When, for every request there is a response that protocol is symmetrical protocol and when this

is not true, that protocol would be Asymmetrical protocol.)

Transparency of location: Transparency really means hiding the network and

its servers from the users and even the application programmers.

Mix and match :The ideal Client/Server software is independent of hardware and

Operating System software platform. It should be able to mix-and-match client and

server platforms.

Message based exchanges: we can exchange the information in terms of

messages over a distributed computer network.

Encapsulation of services: different types of services are encapsulated (bind)

into a
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Scalability : Horizontal scaling means adding or removing client workstations with

only a slight performance impact. Vertical scaling means migrating to a larger and

faster server machine or multiservers.

Integrity: client and server both will be integrated in computer

Types of client/server architectures:

File servers : In computing, a file server is a computer attached to a

network that has the primary purpose of providing a location for shared disk

access, i.e. shared storage of computer files (such as documents, sound files,

photographs, movies, images, databases, etc.) that can be accessed by the

workstations that are attached to the computer network

Database servers: A database server is a computer program that

provides database services to other computer programs or computers, as

defined by the clientservermodel

Transaction servers: A transaction server is a software component that

is used in implementing transactions.

Groupware servers: A groupware server is software designed to enable

users to collaborate, regardless of location, via the Internet or a corporate

intranet and to work together in a virtual atmosphere.

Object application servers:The Application Object Server (AOS) is used for

sharing application objects and information as an effective tool to increase

application performance. This section describes the AOS, how to develop

applications for the AOS, best practices, and security.

The following illustration shows the three-tier architecture.
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Web application servers:

This new model of Client/Server consists of thin, protable, "universal" clients that

talk to superfat servers. In the simplet form, a web server returns documents when

clients ask for them by

Monolithic computing:


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Early computing was performed on a single processor Makes use of a single

cpu to execute one or more programs for each application

Ex: A uni-processor

Distributed system:

A distributed system is a collection of independent computers

,interconnected via a n/w.

Ex: WWW,email,ftp

Historical trends:

Mainframe of 60s: The 3rd Generation computers

Multiprocessing and operating systems make the scene

1960's Burroughs B5000 mainframe introduced. The system can be considered the first of the"third generation" of computer systems. The most remarked-upon aspects are its use of a

hardware-managed stack for calculation, and the extensive use of descriptors for data access. It

included virtual memory -- perhaps the first commercial computer to do so -- as well as supportfor multiprogramming and multiprocessing. 1964 CDC (Computer Data Corp.) 6600 shipped;

100 nsec cycle time. First GE Time-sharing operation at Dartmouth College of the DTSS

Dartmouth time-sharing system on a GE-265 (GE-225 + Datanet-30) IBM announces the 360

family of computer systems. The Burroughs B5500, appeared. It also had multiprogrammingand virtual memory capabilities, but was three times faster than the B5000 1965 IBM ships the

midrange 360 model 40 computer which had COBOL and FORTRAN programming languages

available as well as the stock Basic Assembly Language (BAL) assembler. Introduction ofGECOS-II, a multi-programming operating system for the GE-600 1966 The Burroughs B6500,

which was actually an improved version of the B5500. 1967 First IBM 360/Model 91 shipped to

NASA GSFC. 1969 First shipment of the CDC 7600 computer system.

First shipment of IBM 360 Model 85. The 360 family was intended to have 3 operating systems:

DOS/360 operating system for the small machines. It could run two "real-time" sessions

and one batch session.

OS/360 operating system for the midrange and high end.

TSS/360 operating system for Time-sharing Multi-user system

Introduction of Honeywell model 115 in the H-200 product line. The line was renamedH-2000 after models 115/2, 1015 and 2015 introduced in January 1971, and model 2020

and 2030 in December 1972 after the GE merger. The line was eventually merged into

Series6 0 NPL through a H-200 mode (emulator) on level 64.

Minicomputer:

Workstation:


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Pc

Community culture

web

Distributed computer advantages:

The affordability(low cost) of computers and availability of network

access

Resource sharing

Scalability: measurable

Fault tolerance:

The ability of a system to respond gracefully to an unexpected hardware orsoftwarefailure.

There are many levels of fault tolerance, the lowest being the ability to continue operation in the

event of a power failure. Many fault-tolerant computer systemsmirrorall operations -- that is,every operation is performed on two or more duplicate systems, so if one fails the other can take

over. )

Or

Fault-tolerant describes a computer system or component designed so that, in the event that acomponent fails, a backup component or procedure can immediately take its place with no loss

of service. Fault tolerance can be provided with software, or embedded in hardware, or provided

by some combination.

Distributed computer disadvantages:

Multiple prints of failure:

More Software Components

The more software components that comprise a system the greater chance of errors

occuring.

SecurityProviding easy distributed access increases the risk of a security breach occuring.

Networking

The underlying network can saturate or cause other problems .

Specific types of servers:
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App-servers: An application server is a software framework dedicatedto the efficient execution of procedures (programs, routines, scripts) forsupporting the construction of applications. The term was originally usedwhen discussing early clientserver systems to differentiate servers that runSQL services[1] and middleware servers from file servers.

Advantages of application servers

Data and code integrityBy centralizing business logic on an individual server or on a small number of server

machines, updates and upgrades to the application for all users can be guaranteed. Thereis no risk of old versions of the application accessing or manipulating data in an older,

incompatible manner.

Centralized configurationChanges to the application configuration, such as a move of database server, or system

settings, can take place centrally.

SecurityA central point through which service-providers can manage access to data and portions

of the application itself counts as a security benefit, devolving responsibility for

authentication away from the potentially insecure client layer without exposing thedatabase layer.

PerformanceBy limiting the network traffic toperformance-tiertraffic the clientserver modelimproves the performance of large applications in heavy usage environments.[citation needed]

Total Cost of Ownership (TCO)In combination, the benefits above may result in cost savings to an organizationdeveloping enterprise applications. In practice, however, the technical challenges of

writing software that conforms to that paradigm, combined with the need for software

distribution to distribute client code, somewhat negate these benefits.[citation needed]

Transaction SupportA transactionrepresents a unit of activity in which many updates to resources (on the

same or distributed data sources) can be made atomic (as an indivisible unit of work).End-users can benefit from a system-wide standard behaviour, from reduced time to

develop, and from reduced costs. As the server does a lot of the tedious code-generation,

developers can focus on business logic.

File servers: File servers are useful for sharing files across a network.With a file server, the client passes requests for file records over nerwork tofile server.

Mail servers: A mail server (also known as a mail transfer agentor MTA, a mail transport agent, a

mail routeror anInternet mailer) is an application that receives incoming e-mail fromlocal users (people within the same domain) and remote senders and forwards outgoing e-
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mail for delivery. A computer dedicated to running such applications is also called a mail

server. Microsoft Exchange, qmail, Eximand sendmail are among the more common

mail server programs. The mail server works in conjunction with other programs to make up what is sometimes

referred to as a messaging system. A messaging system includes all the applications

necessary to keep e-mail moving as it should. When you send an e-mail message, your e-mail program, such as Outlook or Eudora, forwards the message to your mail server,

which in turn forwards it either to another mail server or to a holding area on the same

server called a message store to be forwarded later. As a rule, the system uses SMTP(Simple Mail Transfer Protocol) orESMTP (extended SMTP) for sending e-mail, and

eitherPOP3(Post Office Protocol 3) orIMAP(Internet Message Access Protocol) for

receiving e-mail.

Terminal servers:

Servers can be: Stateless servers: do not keep any information b/w requestsEx: http

Stateful servers : can remember informationEx: apache tomcat

Types of clients: Fat client: A fat/heavy client or thick client is a computer (client) in clientserver

architecture or networks which typically provide rich functionalityindependent of the central server. Originally known as just a 'client' or 'thick

client', the name is contrasted to thin client, which describes a computerheavily dependent on a server's applications.

Thin client:

Advantages of thick clients:

Fewer server requirements. A thick client server does not require as high a level of

performance as a thin client server (since the thick clients themselves do much of the

application processing). This results in drastically cheaper servers. Offline working. Thick clients have advantages in that a constant connection to the

central server is often not required. Better multimedia performance. Thick clients have advantages in multimedia-rich

applications that would be bandwidth intensive if fully served. For example, thick clientsare well suited forvideo gaming.

More flexibility. On some operating systems software products are designed for personal

computers that have their own local resources. Running this software in a thin clientenvironment can be difficult.
http://searchexchange.techtarget.com/sDefinition/0,,sid43_gci952894,00.htmlhttp://searchsoa.techtarget.com/sDefinition/0,,sid26_gci779358,00.htmlhttp://searchsoa.techtarget.com/sDefinition/0,,sid26_gci779358,00.htmlhttp://searchenterpriselinux.techtarget.com/sDefinition/0,,sid39_gci212961,00.htmlhttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci214219,00.htmlhttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci296370,00.htmlhttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci212805,00.htmlhttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci212805,00.htmlhttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci214022,00.htmlhttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci214022,00.htmlhttp://en.wikipedia.org/wiki/Client_(computing)http://en.wikipedia.org/wiki/Client%E2%80%93serverhttp://en.wikipedia.org/wiki/Server_(computing)http://www.techterms.com/definition/thickclienthttp://www.techterms.com/definition/thickclienthttp://en.wikipedia.org/wiki/Thin_clienthttp://en.wikipedia.org/wiki/Video_gaminghttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci952894,00.htmlhttp://searchsoa.techtarget.com/sDefinition/0,,sid26_gci779358,00.htmlhttp://searchenterpriselinux.techtarget.com/sDefinition/0,,sid39_gci212961,00.htmlhttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci214219,00.htmlhttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci296370,00.htmlhttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci212805,00.htmlhttp://searchexchange.techtarget.com/sDefinition/0,,sid43_gci214022,00.htmlhttp://en.wikipedia.org/wiki/Client_(computing)http://en.wikipedia.org/wiki/Client%E2%80%93serverhttp://en.wikipedia.org/wiki/Server_(computing)http://www.techterms.com/definition/thickclienthttp://www.techterms.com/definition/thickclienthttp://en.wikipedia.org/wiki/Thin_clienthttp://en.wikipedia.org/wiki/Video_gaming


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Using existing infrastructure. As many people now have very fast local PCs, they

already have the infrastructure to run thick clients at no extra cost. Higher server capacity. The more work that is carried out by the client, the less the

server needs to do, increasing the number of users each server can support.

Thin clients:

A thin client (sometimes also called a lean orslim client) is a computeror a computerprogram which depends heavily on some other computer (itsserver) to fulfill its

traditional computational roles. This stands in contrast to the traditional fat client

Eg: A thin client is computer which acts as a remote keyboard, video, andmouse terminal for another machine. The terminal server performs allcomputation and stores all data. Multiple thin clients can accesssimultaneously. Together, the clients and server make up the thin clientcomputing model. Key differences between this model and traditionalcomputing are that a thin client model can better utilize resources, improvecentralization and integrity of data, and reduce total hardware costs.

Hybrid clients:
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Clients are generally classified as either "fat clients", "thin clients", or "hybrid clients".

Local

storage

Local

processing

Fat Client Yes Yes

Hybrid

ClientNo Yes

Thin

ClientNo No

Fat

A fat-with low-fat client, also known as a rich-poor client orthick-thin clientthe personal

computers or laptops can operate independently.
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Programming languages and/or development tools for rich clients typically include Delphi, .NET

Framework,Java and Visual Studio.

Thin

A thin client is a minimal sort of client. Thin clients use the resources of the host computer. Athin client's job is generally just to graphically display pictures provided by anapplication server,

which performs the bulk of any required data processing. Programming environments for thinclients include JavaScript/AJAX (client side automation),ASP,JSP,Ruby on Rails, Python's

Django,PHP and other (depends on server-side backend and uses HTML pages or rich media

like Flash, Flex or Silverlight on client).

Hybrid

A hybrid client is a mixture of the above two client models. Similar to a fat client, it processes

locally, but relies on the server for storage data. This approach offers features from both the fat

client (multimedia support, high performance) and the thin client (high manageability,flexibility). The arrival of technologies such as Javaallows hybrid clients the high performance

required even for multimedia applications, with the data stored in theCloud. [2]

Skills required by client/server applicationsdevelopment

Transaction processing Database design Communications experience

Gui design Internet server Knowledge of distributed objects and component instructors

Comparison of 2-tier with 3-tier 2-tier

Security :lowPerformance :poorScale :poorApplication reuse :poor

Ease of development :highServer to server infrastructure :noLegacy application :noInternet support :poorSystem administrations :complex

3-tierSecurity :high
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Performance :highScale :goodApplication reuse :goodEase of development :lowServer to server infrastructure :yes

Legacy application :yesInternet support :goodSystem administrations :less complex

2 tier architecture is used to describe client-server system where the requests resources and theservers responds directly to the request using its own resources. 2 tier architecture is therefore a

client-server architecture where the server is versatile, as it is capable of directly responding to

all of the client server request

3 tier architecture, there is an intermediary level which means that it is splitted up between a

client i.e. the computer which request the resource equipped with a user interface for presentation

purpose. In 3 tier architecture however the server level applications are remote from one another.It provides a higher degree of flexibility, increased security and performance.

TIER-1

TIER-2


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Benefits of client/server programming You can develop big applications in small steps Applications can reuse components. Clients can access data and functions easily and safely Custom applications can incorporate off-the-shelf components Component environments dont get older they only get better Vendor independence as compared to the traditional mainframe

computing. This includes application development methodologies,programming paradigms, products and architectures

Organization have changed from steep hierarchies to flattenedhierarchies. Decisions making are carried out by many lower rankedmanagers across the organization rather than performed only by CEOsin the past.

Network management is replacing vertical management Faster response and flexibility to changing environment of business

world outside

TIER-1

TIER-2

TIER-3


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The customer has a single point of contact for all business with theorganization

The time required to complete the work will be minimized Better sharing of critical database resources and other application

software's among clients through the network.

Companies can cut costs on maintenance in employing Client- ServerComputing since servers are usually cheaper than mainframe (1/10 ofmainframe) and the performance of servers and mainframe are nearlyidentical.

Networked webs of small, powerful machines. If one machine goesdown, the organization can still function properly.

Systems grow easily. It is easy to updated and modernize system, bothhardware and software as the companies evolved and has newrequirements.

Individual client operating environments. You can mix and matchcomputer platforms to suit the needs of individual departments and

users.

Pitfalls of client/server computing Dependability - when the server goes down, operations cease Lack of mature tools - it is a relatively new technology and needed

tools are lacking e.g.. Automated client software distribution Lack of scalability - network operating systems (e.g.. Novell Netware,

Windows NT Server) are not very scalable. Higher than anticipated costs Can cause network congestion What some call advantages, others call disadvantages

Database server:

File server:


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