Client–server model

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Client-side
)
A computer network diagram of clients communicating with a server via the Internet

The client–server model is a

computer network on separate hardware, but both client and server may reside in the same system. A server host
runs one or more server programs, which share their resources with clients. A client usually does not share any of its resources, but it requests content or service from a server. 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 "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. Servers are classified by the services they provide. For example, a

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.[2] 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

application protocol
, i.e. the content and the formatting of the data for the requested service.

Clients and servers exchange messages in a

application programming interface (API).[3] 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.[4]

A server may receive requests from many distinct clients in a short period. A computer can only perform a limited number of

Denial of service attacks
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 credentials may be stored in a database, and the webserver accesses the database server as a client. An application server interprets the returned data by applying the bank's business logic and provides the 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 and the web browser presents the data to the customer.

This example illustrates a design pattern applicable to the client–server model: separation of concerns.

Server-side

Server-side refers to programs and operations that run on the server. This is in contrast to client-side programs and operations which run on the client.[5] (See below)

General concepts

"Server-side software" refers to a

client side would be slow, unreliable, or insecure
.

Client and server programs may be commonly available ones such as free or commercial

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.[6][7]

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 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.[8][9][10]

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.

In the context of the World Wide Web, commonly encountered server-side computer languages include:[5]

However, web applications and 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