Editing Client–server model

{{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 be on the same device. A server [[Host (network)|host]]  runs one or more server programs, which share their resources with clients. A client usually does not share its computing resources, but it requests content or service from a server and may share its own content as part of the request. Clients, therefore, initiate communication sessions with servers, which await incoming requests.
Examples of computer applications that use the client–server model are [[email]], network printing, and the [[World Wide Web]].

==Client and server role==
The server component provides a function or service to one or many clients, which initiate requests for such services.
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''.

Whether a computer is a client, a server, or both, is determined by the nature of the application that requires the service functions. For example, a single computer can run a web server and file server software at the same time to serve different data to clients making different kinds of requests. The client software can also communicate with server software within the same computer.<ref>The [[X Window System]] is one example.</ref> Communication between servers, such as to synchronize data, is sometimes called ''[[inter-server]]'' or ''server-to-server'' communication.

==Client and server communication==
Generally, a service is an [[Abstraction (computer science)|abstraction]] of computer resources and a client does not have to be [[Concern (computer science)|concerned]] with how the server performs while fulfilling the request and delivering the response. The client only has to understand the response based on the relevant [[application protocol]], i.e. the content and the formatting of the data for the requested service.

Clients and servers exchange messages in a [[request–response]] [[messaging pattern]]. The client sends a request, and the server returns a response. This exchange of messages is an example of [[inter-process communication]]. To communicate, the computers must have a common language, and they must follow rules so that both the client and the server know what to expect. The language and rules of communication are defined in a [[communications protocol]]. All protocols operate in the [[application layer]]. The application layer protocol defines the basic patterns of the dialogue. To formalize the data exchange even further, the server may implement an [[application programming interface]] (API).<ref>{{Cite journal | last1 = Benatallah | first1 = B. | last2 = Casati | first2 = F. | last3 = Toumani | first3 = F. | title = Web service conversation modeling: A cornerstone for e-business automation | doi = 10.1109/MIC.2004.1260703 | journal = IEEE Internet Computing | volume = 8 | pages = 46–54 | year = 2004 | s2cid = 8121624 }}</ref> The API is an [[abstraction layer]] for accessing a service.  By restricting communication to a specific [[content format]], it facilitates [[parsing]]. By abstracting access, it facilitates cross-platform data exchange.<ref>{{Cite journal | last1 = Dustdar | first1 = S. | last2 = Schreiner | first2 = W. | doi = 10.1504/IJWGS.2005.007545 | title = A survey on web services composition | journal = International Journal of Web and Grid Services | volume = 1 | pages = 1 | year = 2005 | url = http://www.infosys.tuwien.ac.at/Staff/sd/papers/A%20survey%20on%20web%20services%20composition_Dustdar_Schreiner_inPress.pdf | citeseerx = 10.1.1.139.4827 }}</ref>

A server may receive requests from many distinct clients in a short period. A computer can only perform a limited number of [[Task (computing)|tasks]] at any moment, and relies on a [[Scheduling (computing)|scheduling]] system to prioritize incoming requests from clients to accommodate them. To prevent abuse and maximize [[uptime|availability]], the server software may limit the availability to clients. [[Denial of service attack]]s are designed to exploit a server's obligation to process requests by overloading it with excessive request rates.
Encryption should be applied if sensitive information is to be communicated between the client and the server.

==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 are compared against a [[database]], and the webserver accesses that [[database server]] as a client. An [[application server]] interprets the returned data by applying the bank's [[business logic]] and provides the [[Input/output|output]] to the webserver. Finally, the webserver returns the result to the client web browser for display.

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 |url-status=live}}</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==
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.

While formulating the client–server model in the 1960s and 1970s, [[computer scientist]]s building [[ARPANET]] (at the [[SRI International|Stanford Research Institute]]) used the terms ''server-host'' (or ''serving host'') and ''user-host'' (or ''using-host''), and these appear in the early documents RFC 5<ref name="rulifson">{{cite IETF |title=DEL |rfc=5 |last=Rulifson |first=Jeff |authorlink=Jeff Rulifson |date=June 1969 |publisher=[[Internet Engineering Task Force|IETF]] |accessdate=30 November 2013}}</ref> and RFC 4.<ref>{{cite IETF |title=Network Timetable |rfc=4 |last=Shapiro |first=Elmer B. |date=March 1969 |publisher=[[Internet Engineering Task Force|IETF]] |accessdate=30 November 2013}}</ref> This usage was continued at [[PARC (company)|Xerox PARC]] in the mid-1970s.

One context in which researchers used these terms was in the design of a [[computer network programming]] language called Decode-Encode Language (DEL).<ref name="rulifson"/> The purpose of this language was to accept commands from one computer (the user-host), which would return status reports to the user as it encoded the commands in network packets. Another DEL-capable computer, the server-host, received the packets, decoded them, and returned formatted data to the user-host. A DEL program on the user-host received the results to present to the user. This is a client–server transaction. Development of DEL was just beginning in 1969, the year that the [[United States Department of Defense]] established ARPANET (predecessor of [[Internet]]).

===Client-host and server-host===
''Client-host'' and ''server-host'' have subtly different meanings than ''client'' and ''server''. A host is any computer connected to a network. Whereas the words ''server'' and ''client'' may refer either to a computer or to a computer program, ''server-host'' and ''client-host'' always refer to computers. The host is a versatile, multifunction computer; ''clients'' and ''servers'' are just programs that run on a host. In the client–server model, a server is more likely to be devoted to the task of serving.

An early use of the word ''client'' occurs in "Separating Data from Function in a Distributed File System", a 1978 paper by Xerox PARC computer scientists Howard Sturgis, James Mitchell, and Jay Israel. The authors are careful to define the term for readers, and explain that they use it to distinguish between the user and the user's network node (the client).<ref>{{Cite web| title=Separating Data from Function in a Distributed File System |last1 = Sturgis |first1 = Howard E. |last2 = Mitchell |first2 = James George |last3 = Israel | first3 = Jay E. |publisher=[[PARC (company)|Xerox PARC]] |url=http://ip.com/IPCOM/000128883 |year = 1978}}</ref> By 1992, the word ''server'' had entered into general parlance.<ref>{{OEtymD|server|accessdate=30 November 2013}}</ref><ref>{{cite web |url=https://getinfo.de/app/Separating-data-from-function-in-a-distributed/id/TIBKAT%3A509976956 |title=Separating data from function in a distributed file system |author=<!--Staff writer(s); no by-line.--> |website=[[GetInfo]] |publisher=[[German National Library of Science and Technology]] |access-date=29 November 2013 |archive-url=https://web.archive.org/web/20131202233729/https://getinfo.de/app/Separating-data-from-function-in-a-distributed/id/TIBKAT%3A509976956 |archive-date=2 December 2013 |url-status=dead }}</ref>

==Centralized computing==
{{Further|History of personal computers|Decentralized computing|Computer cluster}}

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=[[SQL Server Pro]] |publisher=[[Penton Media]] |access-date=1 December 2013 |archive-url=https://web.archive.org/web/20131203011958/http://sqlmag.com/cloud/cloud-really-just-return-mainframe-computing |archive-date=3 December 2013 |url-status=dead }}</ref> During the 2000s, [[web application]]s matured enough to rival [[application software]] developed for a specific [[microarchitecture]]. This maturation, more affordable [[mass storage]], and the advent of [[service-oriented architecture]] were among the factors that gave rise to the [[cloud computing]] trend of the 2010s.<ref>{{Cite journal | last1 = Barros | first1 = A. P. | last2 = Dumas | first2 = M. | doi = 10.1109/MITP.2006.123 | title = The Rise of Web Service Ecosystems | journal = IT Professional | volume = 8 | issue = 5 | pages = 31 | year = 2006 | s2cid = 206469224 }}</ref>{{failed verification|date=March 2024}}

==Comparison with peer-to-peer architecture==
In addition to the client-server model, [[distributed computing]] applications often use the [[peer-to-peer]] (P2P) application architecture.

In the client-server model, the server is often designed to operate as a centralized system that serves many clients. The computing power, memory and storage requirements of a server must be scaled appropriately to the expected workload. [[Load balancing (computing)|Load-balancing]] and [[failover]] systems are often employed to scale the server beyond a single physical machine.<ref name="Cardellini Colajanni Yu 1999 pp. 28–39">{{cite journal | last1=Cardellini | first1=V. | last2=Colajanni | first2=M. | last3=Yu | first3=P.S. | title=Dynamic load balancing on Web-server systems | journal=IEEE Internet Computing | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=3 | issue=3 | year=1999 | issn=1089-7801 | doi=10.1109/4236.769420 | pages=28–39}}</ref><ref name="NGINX 2014">{{cite web | title=What Is Load Balancing? How Load Balancers Work | website=NGINX | date=June 1, 2014 | url=https://www.nginx.com/resources/glossary/load-balancing/ | access-date=January 21, 2020}}</ref>

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>{{Cite journal | last1 = Alharbi | first1 = A. | last2 = Aljaedi | first2 = A. | title = Peer-to-Peer Network Security Issues and Analysis: Review | journal = IJCSNS International Journal of Computer Science and Network Security | DOI = 10.1007/978-3-540-45172-3_6 | year = 2004 }}</ref> In peer-to-peer networking, an [[algorithm]] in the peer-to-peer communications protocol balances [[Load (computing)|load]], and even peers with modest resources can help to share the load.<ref>{{Cite journal | last1 = Rao | first1 = A. | last2 = Lakshminarayanan | first2 = K. | last3 = Surana | first3 = S.| last4 = Manning Karp | first4 = R. | title = Load Balancing in Structured P2P Systems | journal = IJCSNS International Journal of Computer Science and Network Security | volume = 20 | pages = 74–88 | year = 2020 }}</ref> If a node becomes unavailable, its shared resources remain available as long as other peers offer it. Ideally, a peer does not need to achieve [[high availability]] because other, [[Redundancy (engineering)|redundant]] peers make up for any resource [[downtime]]; as the availability and load capacity of peers change, the protocol reroutes requests.

Both client-server and [[Master/slave (technology)|master-slave]] are regarded as sub-categories of distributed peer-to-peer systems.<ref>
{{cite book
| last1 = Varma
| first1 = Vasudeva
| chapter = 1: Software Architecture Primer
| title = Software Architecture: A Case Based Approach
| chapter-url = https://books.google.com/books?id=jOMYtrJ6r_0C
| location = Delhi
| publisher = Pearson Education India
| date = 2009
| page = 29
| isbn = 9788131707494
| access-date = 2017-07-04
| quote = Distributed Peer-to-Peer Systems [...] This is a generic style of which popular styles are the client-server and master-slave styles.
}}
</ref>

==See also==
{{Columns-list|colwidth=30em|
* [[Endpoint security]]
* [[Front and back ends]]
* [[Modular programming]]
* [[Observer pattern]]
* [[Publish–subscribe pattern]]
* [[Pull technology]]
* [[Push technology]]
* [[Remote procedure call]]
* [[Server change number]]
* [[Systems Network Architecture]], a proprietary network architecture by [[IBM]]
* [[Thin client]]
* [[Configurable Network Computing]], a proprietary client-server architecture by [[JD Edwards]]
}}

==Notes==
{{Reflist}}

{{Inter-process communication}}
{{Authority control}}

{{DEFAULTSORT:Client-server model}}
[[Category:Servers (computing)]]
[[Category:Clients (computing)]]

[[Category:Application layer protocols|*]]
[[Category:Inter-process communication]]
[[Category:Network architecture]]