Client-Server

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Client–server model — Wikipedia

Client-Server model is a type of architecture, typically used in distributed computing, where there exist two entity, namely client and server. A client can request a service or resource, and the server will be the one that response to the request, such as processing or computing the request, or retrieving the requested data.

Client requests a webpage
Source: https://www.scaler.com/topics/nodejs/client-server-architecture/

One of the biggest application that uses client-server model is the World Wide Web (WWW or the web). For example, when we open a website from our browser, we are essentially sending a request to a server. The browser act as the client that requests to that server. In the case of opening a website, we are requesting webpage resources (HTML, CSS, JavaScript code), which the browser will use to show us the website. The server is run by the organization or individual responsible for hosting the website or providing the service.

Communication

The interaction between client and server is done in a specific protocol. They are specific rules and format that controls how the two should exchange data. Typically, we are only concerned with the application-level protocol.

The examples are:

HTTP/HTTPS: HTTP is a protocol used for communication between web browsers (clients) and web servers. HTTPS is the secure version of HTTP that adds encryption and authentication using SSL/TLS protocols.
FTP: FTP is a protocol used for transferring files between a client and a server. The protocol will specify commands, which are used for file operations such as uploading, downloading, renaming, and deleting files on a remote server.
SMTP: SMTP is a protocol for sending email messages between clients and servers. It provides a set of commands and responses for the transfer of email across networks.

For example, the HTTP protocol specify that the client (web browser) must send a request consisting:

Request Line: Specify the HTTP method, which specifies our desired action of requesting, target URL or resource, and HTTP version.
Headers: Provide additional information about the request, such as content type, authentication credentials, or cookies.
Body (optional): If we are sending data to the server (e.g., submitting a form), they are sent along with the request.

The server that responses to the request must also follow the format rules:

Status Line: Includes the HTTP version, a three-digit status code indicating the result of the request (e.g., 200 for OK, 404 for Not Found), and a brief status message.
Headers: Additional information about the response, such as content type (specify what type of data is returned), caching directives, or authentication challenges.
Body (optional): Contains the requested resource or additional data sent as part of the response.

Here are a simple HTTP request and its response:

GET /example-page HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.1234.56 Safari/537.36
Accept: text/html,application/xhtml+xml

This request specifies that we are requesting the www.example.com webpage. The User-Agent is a header that provides information about the client application making the request (in this case, a web browser).

HTTP/1.1 200 OK
Date: Fri, 19 Feb 2024 12:00:00 GMT
Server: Apache/2.4.29 (Ubuntu)
Content-Type: text/html; charset=utf-8
Content-Length: 1234

<!DOCTYPE html>
<html>
<head>
   <title>Example Page</title>
</head>
<body>
   <h1>Welcome to the Example Page</h1>
   <p>This is a sample page for demonstration purposes.</p>
</body>
</html>

The response is an HTML page, which the browser will render and show it to us.

Communication​

Communication