What occurs when you visit a website?

When you visit a website, a series of processes and interactions occur behind the scenes to deliver the webpage content to your browser. Here's a detailed sequence of events and the technologies involved from the moment you enter a URL in your browser until the site is fully loaded and interactive:

1. DNS Resolution:

   • You begin by entering a URL in your web browser, like www.example.com.
   • The browser first needs to convert the human-readable domain name into an IP address that computers use to identify each other on the network.
   • This conversion is done by a Domain Name System (DNS) lookup. If the DNS information is not cached locally or in your recursive DNS server, the DNS query is sent out to resolve the domain name into an IP address.

2. TCP/IP Connection:

   • Once the IP address is obtained, your browser establishes a TCP (Transmission Control Protocol) connection with the web server hosting the website, using the server's IP address and the default port number 80 for HTTP or port 443 for HTTPS.
   • The connection involves a TCP handshake process: SYN, SYN-ACK, ACK. This ensures a reliable connection between client and server.

3. HTTPS and TLS/SSL Handshake:

   • If the website uses HTTPS, a secure connection is established using SSL/TLS protocols.
   • This involves the client and server agreeing on cryptographic keys to encrypt the data transmitted over the connection, ensuring privacy and integrity.

4. HTTP Request:

   • The browser sends an HTTP GET request to the web server to retrieve the desired webpage. This request includes headers with information like the browser type, language preferences, and any cookies associated with the website.

5. Server Processing:

   • The web server processes the request. This may involve running server-side scripts, interacting with databases, and retrieving resources necessary to construct the webpage.
   • The server then sends back an HTTP response, which includes status codes (e.g., 200 OK), headers, and the content of the webpage, commonly in HTML format.

6. HTML Parsing and Rendering:

   • The browser receives the HTML content and begins parsing it.
   • During parsing, the browser encounters additional resources it needs, such as CSS stylesheets, JavaScript files, images, and fonts, typically linked via <link>, <script>, and <img> tags within the HTML document.

7. Resource Request and Fetch:

   • The browser sends further requests for these resources, fetching them either from the server or its cache if they have been previously stored.
   • For each JavaScript and CSS file, it’s required to be fetched and executed/processed in the correct order due to dependencies.

8. DOM and CSSOM Construction:

   • As the HTML is parsed, the browser builds a Document Object Model (DOM) which represents the structure and content of the webpage.
   • Simultaneously, CSS is processed to construct the CSS Object Model (CSSOM), which determines how elements should be styled.

9. Render Tree Construction and Layout:

   • The browser combines the DOM and CSSOM to construct a Render Tree, detailing the visual elements of the page.
   • A layout process calculates the exact position and size of each element on the page based on the render tree.

10. Painting and Compositing:

    • Finally, the browser paints the pixels to the screen based on the layout calculations.
    • This involves compositing layers if there are elements like CSS animations or complex stacking contexts.

11. JavaScript Execution:

    • JavaScript files linked in the HTML or dynamically added to the page are executed, which can modify the DOM, handle user interactions, and update the page content dynamically.

12. Making the Page Interactive:

    • Once JavaScript execution is complete and event handlers are attached, the webpage becomes interactive.
    • Users can now interact with page elements like forms and buttons, triggering JavaScript events and further client-server interactions as needed.

Throughout this process, various optimizations (such as caching strategies, pipelining, and lazy loading) and network protocols (HTTP/2, HTTP/3) may be employed to enhance performance and user experience. The end result is a fully loaded and interactive website presented to you, the user, in your web browser.