Client–server model

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The client–server model of computing is a distributed application structure that partitions tasks or workloads between the providers of a resource or service, called servers, and service requesters, called clients.^[1] Often clients and servers communicate over a computer network on separate hardware, but both client and server may reside in the same system. A server machine is a host that is running one or more server programs which share their resources with clients. A client does not share any of its resources, but requests a server's content or service function. Clients therefore initiate communication sessions with servers which await incoming requests.

Contents 1 Description 2 Comparison to peer-to-peer architecture 3 Advantages 4 Disadvantages 5 See also 6 References

[edit] Description

The client–server characteristic describes the relationship of cooperating programs in an application. The server component provides a function or service to one or many clients, which initiate requests for such services.

Functions such as email exchange, web access and database access, are built on the client–server model. Users accessing banking services from their computer use a web browser client to send a request to a web server at a bank. That program may in turn forward the request to its own database client program that sends a request to a database server at another bank computer to retrieve the account information. The balance is returned to the bank database client, which in turn serves it back to the web browser client displaying the results to the user. The client–server model has become one of the central ideas of network computing. Many business applications being written today use the client–server model. So do the Internet's main application protocols, such as HTTP, SMTP, Telnet, and DNS.

The interaction between client and server is often described using sequence diagrams. Sequence diagrams are standardized in the Unified Modeling Language.

Specific types of clients include web browsers, email clients, and online chat clients.

Specific types of servers include web servers, ftp servers, application servers, database servers, name servers, mail servers, file servers, print servers, and terminal servers. Most web services are also types of servers.

[edit] Comparison to peer-to-peer architecture

This section requires expansion.

In peer-to-peer architectures, each host or instance of the program can simultaneously act as both a client and a server, and each has equivalent responsibilities and status.

Both client–server and peer-to-peer architectures are in wide usage today. Details may be found in Comparison of Centralized (Client-Server) and Decentralized (Peer-to-Peer) Networking.

[edit] Advantages

• In most cases, a client–server architecture enables the roles and responsibilities of a computing system to be distributed among several independent computers that are known to each other only through a network. This creates an additional advantage to this architecture: greater ease of maintenance. For example, it is possible to replace, repair, upgrade, or even relocate a server while its clients remain both unaware and unaffected by that change.
• All data is stored on the servers, which generally have far greater security controls than most clients.^{[citation needed]} Servers can better control access and resources, to guarantee that only those clients with the appropriate permissions may access and change data.
• Since data storage is centralized, updates to that data are far easier to administer in comparison to a P2P paradigm. In the latter, data updates may need to be distributed and applied to each peer in the network, which is time-consuming as there can be thousands or even millions of peers.
• Many mature client–server technologies are already available which were designed to ensure security, friendliness of the user interface, and ease of use.^{[citation needed]}
• It functions with multiple different clients of different capabilities.

[edit] Disadvantages

• As the number of simultaneous client requests to a given server increases, the server can become overloaded.^{[citation needed]} Contrast that to a P2P network, where its aggregated bandwidth actually increases as nodes are added, since the P2P network's overall bandwidth can be roughly computed as the sum of the bandwidths of every node in that network.
• The client–server paradigm lacks the robustness of a good P2P network.^{[citation needed]} Under client–server, should a critical server fail, clients’ requests cannot be fulfilled. In P2P networks, resources are usually distributed among many nodes. Even if one or more nodes depart and abandon a downloading file, for example, the remaining nodes should still have the data needed to complete the download.

[edit] See also

• Hybrid client
• Inter-server
• Mainframe computer
• Mobile software
• Name server
• Observer pattern
• Push technology
• Pull technology
• Servent
• Standalone server
• Thin client
• Multitier architecture
• Client (computing)
• Server (computing)

[edit] References