Client-server architecture

The client-server architecture consists of two components: a client and a server.

Client and server

Сlient is a computer on the user's side that sends a request to the server for information or to perform certain actions.

Server is a more powerful computer or equipment designed to solve certain tasks in executing program codes, performing service functions at the request of clients, providing users with access to particular resources, and storing information and databases.

Such a system is modeled on a client sending a request to the server, where it is processed and the finished result is sent to the client. The server can process several clients simultaneously. If more than one request arrives at the same time, they are queued and executed by the server in sequence. Sometimes requests can be prioritized. Requests with higher priorities should be executed earlier.

Functions implemented on the server:

• data storage, access, protection, and backup;
• processing of a client request;
• sending the result (response) to the client.

Functions that are implemented on the client side:

• providing a user interface;
• formulating a request to the server and sending it;
• receiving the results of the request and sending additional commands (requests to add, update, or delete data).

The client-server architecture outlines the principles of communication between computers, with the rules and interactions defined in a protocol.

Network protocol is a set of rules by which computers interact on a network.

Network protocols:

TCP/IP is a suite of communication protocols. TCP/IP stands for the entire network that operates on the basis of two protocols - TCP and IP.

TCP (Transfer Control Protocol) is a protocol that is used to establish a reliable connection between two devices, transfer information, and confirm its reception. 

IP (Internet Protocol) is an Internet protocol that is responsible for the correct message delivery to a specific address. In this case, data is divided into packets that can be transferred in different ways.

MAC (Media Access Control) is a protocol that helps identify network devices. All devices connected to the Internet have their own unique MAC address.

ICMP (Internet control message protocol) – a protocol which is responsible for the exchange of information, but is not used for data transmission.

UDP (User datagram protocol) is a protocol that controls the transfer of information, without the information being checked when it is received. This protocol is faster than TCP.

HTTP (Hypertext Transfer Protocol) – a hypertext transfer protocol used by all websites. It requests the necessary data from a remote system (web pages, files).

FTP (File Transfer Protocol) – a protocol for transferring files from a special file server to a user's computer..

SSH (Secure Shell) is a protocol used to provide remote control of the system through a secure channel.

POP3 (Post Office Protocol) is a standard mail connection protocol responsible for mail delivery.

SMTP (Simple Mail Transfer Protocol) is a protocol that defines the rules for mail transfer. It is responsible for returning, confirming delivery, or error notifications.

There are concepts for building a client-server system:

1. A weak client – a powerful server. In this model, all information processing is transferred to the server, and the access rights of the client are strictly limited. The server returns a response that does not require additional processing. The client interacts with the user: creates and sends a request, receives the result, and displays the information on the screen.
2. A strong client is a concept in which part of the information is processed by the client. In this case, the server acts as a data repository, and all the work of processing and presenting information goes to the client's computer.

A system (application) based on client-server interaction includes three main components: a presentation tier, an application tier and a data tier.

There are two-tier and three-tier client-server architectures.

The two-tier architecture consists of two components:

• A server that is responsible for receiving requests and sending responses to the client, using only its own resources;
• The client represents the user interface.

It works by having the server receive a request, process it, and respond directly, without using third-party resources.

The three-tier architecture consists of the following components:

• presentation tier – user interface;
• application tier – an application server;
• data tier – a database server that provides information.

It works on the basis of the principle that several servers process a client's request. The distribution of operations reduces the load on the server.

The three-tier architecture can be expanded to a multi-tier (N-tier) architecture by installing additional servers. 

The multi-tiered architecture allows for increasing the efficiency of the information system and optimizing the allocation of its software and hardware resources. 

Client-server interaction makes it possible to divide the functionality and computing load between client applications (service customers) and server applications (service providers). Knowledge of the application architecture allows the tester to more efficiently conduct functional, cross-browser, usability, and performance testing.