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{{Short description|Distributed application structure in computing}}{{More citations needed|article (some sections)|date=March 2024}}[[File:Client-server-model.svg|thumb|250px|A computer network diagram of clients communicating with a server via the Internet]]
The '''client–server model''' is a [[distributed application]] structure that partitions tasks or workloads between the providers of a resource or service, called [[Server (computing)|servers]], and service requesters, called [[client (computing)|client]]s.<ref>{{cite web|url=http://java.sun.com/developer/Books/jdbc/ch07.pdf|publisher=Sun Microsystem|title=Distributed Application Architecture|archive-url=https://web.archive.org/web/20110406121920/http://java.sun.com/developer/Books/jdbc/ch07.pdf|access-date=2009-06-16|archive-date=6 April 2011}}</ref> Often clients and servers communicate over a [[Computer networking device|computer network]] on separate hardware, but both client and server may
Examples of computer applications that use the client–server model are [[email]], network printing, and the [[World Wide Web]].
==Client and server role==
Servers are classified by the services they provide. For example, a [[web server]] serves [[web page]]s and a [[file server]] serves [[computer file]]s. A [[shared resource]] may be any of the server computer's software and electronic components, from [[Computer program|programs]] and [[Data (computing)|data]] to [[Microprocessor|processors]] and [[Data storage device|storage devices]]. The sharing of resources of a server constitutes a ''service''.
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==Example==
When a [[bank]] customer accesses [[online banking]] services with a [[web browser]] (the client), the client initiates a request to the bank's web server. The customer's [[login]] [[credential]]s
In each step of this sequence of client–server message exchanges, a computer processes a request and returns data. This is the request-response messaging pattern. When all the requests are met, the sequence is complete
This example illustrates a [[design pattern]] applicable to the client–server model: [[separation of concerns]].
==Server-side==
{{See also|Backend (computing)|Server-side scripting|Server Side Includes|label 3=Server Side Includes (SSI)}}
{{More citations needed|section|small=y|date=December 2016}}
Server-side refers to programs and operations that run on the [[server (computing)|server]]. This is in contrast to client-side programs and operations which run on the [[client (computing)|client]].
=== General concepts ===
"Server-side software" refers to a [[computer application]], such as a [[web server]], that runs on remote [[server (computing)#Hardware|server hardware]], reachable from a [[user (computing)|user]]'s local [[computer]], [[smartphone]], or other device.<ref>{{cite web |url=https://www.cloudflare.com/en-gb/learning/serverless/glossary/client-side-vs-server-side/ |title=What do client side and server side mean? Client side vs. server side |publisher=Cloudflare |access-date=17 April 2025 }}</ref> Operations may be performed server-side because they require access to information or functionality that is not available on the [[client (computing)|client]], or because performing such operations on the [[client-side|client side]] would be slow, unreliable, or [[computer security|insecure]].
Client and server programs may be commonly available ones such as free or commercial [[web server]]s and [[web browser]]s, communicating with each other using standardized [[protocol (computing)|protocols]]. Or, [[programmer]]s may write their own server, client, and [[communications protocol]] which can only be used with one another.
Server-side operations include both those that are carried out in response to client requests, and non-client-oriented operations such as maintenance tasks.<ref>{{Cite web |date=2023-11-05 |title=Introduction to the server side - Learn web development {{!}} MDN |url=https://developer.mozilla.org/en-US/docs/Learn/Server-side/First_steps/Introduction |access-date=2023-11-13 |website=developer.mozilla.org |language=en-US}}</ref><ref>{{Cite web |date=2023-06-30 |title=Server-side website programming - Learn web development {{!}} MDN |url=https://developer.mozilla.org/en-US/docs/Learn/Server-side |access-date=2023-11-13 |website=developer.mozilla.org |language=en-US}}</ref>
=== Computer security ===
In a [[computer security]] context, server-side vulnerabilities or attacks refer to those that occur on a server computer system, rather than on the client side, or [[Man-in-the-middle attack|in between the two]]. For example, an attacker might exploit an [[SQL injection]] vulnerability in a [[web application]] in order to maliciously change or gain unauthorized access to data in the server's [[database]]. Alternatively, an attacker might break into a server system using vulnerabilities in the underlying [[operating system]] and then be able to access database and other files in the same manner as authorized administrators of the server.<ref name=oreilly>{{cite book |url=https://books.google.com/books?id=DyrLV0kZEd8C&q=client-side+OR+server-side&pg=PT17 |title=Computer Security Basics |edition=2nd |last1=Lehtinen |first1=Rick |last2=Russell |first2=Deborah |last3=Gangemi |first3=G. T. |publisher=[[O'Reilly Media]] |isbn=9780596006693 |date=2006 |access-date=2017-07-07}}</ref><ref name=n3tweb>{{cite web |url=https://n3tweb.wordpress.com/2015/10/15/week-4-is-there-a-difference-between-client-side-and-server-side/ |title=Week 4: Is There a Difference between Client Side and Server Side? |author=JS |website=n3tweb.wordpress.com |date=2015-10-15 |access-date=2017-07-07}}</ref><ref name=alpinesecurity>{{cite web |url=https://www.alpinesecurity.com/s/Alpine-Security-Decoding-the-Hack-Presentation-22-April-16.pdf |title=Decoding the Hack |last=Espinosa |first=Christian |website=alpinesecurity.com |date=2016-04-23 |access-date=2017-07-07 }}{{Dead link|date=March 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>
=== Examples ===
In the case of [[distributed computing]] projects such as [[SETI@home]] and the [[Great Internet Mersenne Prime Search]], while the bulk of the operations occur on the client side, the servers are responsible for coordinating the clients, sending them data to analyze, receiving and storing results, providing reporting functionality to project administrators, etc. In the case of an Internet-dependent user application like [[Google Earth]], while querying and display of map data takes place on the client side, the server is responsible for permanent storage of map data, resolving user queries into map data to be returned to the client, etc.
Web applications and [[web service|services]] can be implemented in almost any language, as long as they can return data to standards-based web browsers (possibly via intermediary programs) in formats which they can use.
==Client side==
{{refimprove|section|small=y|date=December 2016}}{{See also|Client-side prediction|Front-end (computing)|Pagination# In web browsers|label 3=Pagination § In web browsers}}
Client-side refers to operations that are performed by the [[Client (computing)|client]] in a [[computer network]].
=== General concepts ===
Typically, a client is a [[computer application]], such as a [[web browser]], that runs on a [[user (computing)|user]]'s local [[computer]], [[smartphone]], or other device, and connects to a [[server (computing)|server]] as necessary. Operations may be performed client-side because they require access to information or functionality that is available on the client but not on the server, because the user needs to observe the operations or provide input, or because the server lacks the processing power to perform the operations in a timely manner for all of the clients it serves. Additionally, if operations can be performed by the client, without sending data over the network, they may take less time, use less [[Bandwidth (computing)|bandwidth]], and incur a lesser [[Computer security|security]] risk.
When the server serves data in a commonly used manner, for example according to standard [[Protocol (computing)|protocols]] such as [[HyperText Transfer Protocol|HTTP]] or [[File Transfer Protocol|FTP]], users may have their choice of a number of client programs (e.g. most modern web browsers can request and receive data using both HTTP and FTP). In the case of more specialized applications, [[programmer]]s may write their own server, client, and [[communications protocol]] which can only be used with one another.
Programs that run on a user's local computer without ever sending or receiving data over a network are not considered clients, and so the operations of such programs would not be termed client-side operations.
=== Computer security ===
In a [[computer security]] context, client-side vulnerabilities or attacks refer to those that occur on the client / user's computer system, rather than on the [[server-side|server side]], or [[Man-in-the-middle attack|in between the two]]. As an example, if a server contained an [[encryption|encrypted]] file or message which could only be decrypted using a [[Key (cryptography)|key]] housed on the user's computer system, a client-side attack would normally be an attacker's only opportunity to gain access to the decrypted contents. For instance, the attacker might cause [[malware]] to be installed on the client system, allowing the attacker to view the user's screen, record the user's keystrokes, and steal copies of the user's encryption keys, etc. Alternatively, an attacker might employ [[cross-site scripting]] vulnerabilities to execute malicious code on the client's system without needing to install any permanently resident malware.<ref name=oreilly /><ref name=n3tweb /><ref name=alpinesecurity />
=== Examples ===
[[Distributed computing]] projects such as [[SETI@home]] and the Great Internet Mersenne Prime Search, as well as Internet-dependent applications like [[Google Earth]], rely primarily on client-side operations. They initiate a connection with the server (either in response to a user query, as with Google Earth, or in an automated fashion, as with SETI@home), and request some data. The server selects a data set (a [[server-side]] operation) and sends it back to the client. The client then analyzes the data (a client-side operation), and, when the analysis is complete, displays it to the user (as with Google Earth) and/or transmits the results of calculations back to the server (as with SETI@home).
==Early history==
[[Category:Clients (computing)]]▼
An early form of client–server architecture is [[remote job entry]], dating at least to [[OS/360]] (announced 1964), where the request was to run a [[job (computing)|job]], and the response was the output.
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The client-server model does not dictate that server-hosts must have more resources than client-hosts. Rather, it enables any general-purpose computer to extend its capabilities by using the shared resources of other hosts. [[Centralized computing]], however, specifically allocates a large number of resources to a small number of computers. The more computation is offloaded from client-hosts to the central computers, the simpler the client-hosts can be.<ref name="Columbia">{{cite journal |last1=Nieh |first1=Jason |last2=Yang |first2=S. Jae |last3=Novik |first3=Naomi |title=A Comparison of Thin-Client Computing Architectures |journal=Academic Commons |date=2000 |doi=10.7916/D8Z329VF |url=https://academiccommons.columbia.edu/doi/10.7916/D8Z329VF |access-date=28 November 2018 |language=en}}</ref> It relies heavily on network resources (servers and infrastructure) for computation and storage. A [[diskless node]] loads even its [[operating system]] from the network, and a [[computer terminal]] has no operating system at all; it is only an input/output interface to the server. In contrast, a [[rich client]], such as a [[personal computer]], has many resources and does not rely on a server for essential functions.
As [[microcomputer]]s decreased in price and increased in power from the 1980s to the late 1990s, many organizations transitioned computation from centralized servers, such as [[Mainframe computer|mainframe]]s and [[minicomputer]]s, to rich clients.<ref>{{Cite book | last1 = d'Amore | first1 = M. J. | last2 = Oberst | first2 = D. J. | doi = 10.1145/800041.801417 | chapter = Microcomputers and mainframes | title = Proceedings of the 11th annual ACM SIGUCCS conference on User services - SIGUCCS '83 | pages = 7 | year = 1983 | isbn = 978-0897911160 | s2cid = 14248076 }}</ref> This afforded greater, more individualized dominion over computer resources, but complicated [[information technology management]].<ref name="Columbia"/><ref name="tolia">{{Cite journal |last1 = Tolia |first1 = Niraj |last2 = Andersen |first2 = David G. |last3 = Satyanarayanan |first3 = M. |title = Quantifying Interactive User Experience on Thin Clients |journal = [[Computer (magazine)|Computer]] |volume = 39 |pages = 46–52 |number = 3 |date = March 2006 |publisher = [[IEEE Computer Society]] |url = https://www.cs.cmu.edu/~dga/papers/tolia06-ieee.pdf |doi = 10.1109/mc.2006.101 |s2cid = 8399655 }}</ref><ref>{{cite web |url=http://sqlmag.com/cloud/cloud-really-just-return-mainframe-computing |title=Is the Cloud Really Just the Return of Mainframe Computing? |last=Otey |first=Michael |date=22 March 2011 |website=
==Comparison with peer-to-peer architecture==
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Load balancing is defined as the methodical and efficient distribution of network or application traffic across multiple servers in a server farm. Each load balancer sits between client devices and backend servers, receiving and then distributing incoming requests to any available server capable of fulfilling them.
In a [[peer-to-peer]] network, two or more computers (''peers'') pool their resources and communicate in a [[decentralized system]]. Peers are coequal, or equipotent [[Node (networking)|nodes]] in a non-hierarchical network. Unlike clients in a client-server or [[client-queue-client]] network, peers communicate with each other directly. <ref>{{
Both client-server and [[Master/slave (technology)|master-slave]] are regarded as sub-categories of distributed peer-to-peer systems.<ref>
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{{DEFAULTSORT:Client-server model}}
▲[[Category:Clients (computing)]]
[[Category:Application layer protocols|*]]
[[Category:Inter-process communication]]
▲[[Category:Servers (computing)|*]]
[[Category:Network architecture]]
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