When You Open a Website What Exactly happens Internally

Understanding the Webpage Loading Process

Accessing a website, such as amazon.in, involves a complex interplay between your browser and the website's server. Let's break down this process step-by-step.

1. URL Input and Validation:

You enter "amazon.in" and press Enter. The browser first validates the URL, ensuring it has a correct protocol (http:// or https://). If missing, it defaults to https://.

2. DNS Resolution:

The browser needs the server's IP address. It performs a DNS lookup:

• Checks its cache for a recent resolution.
• If not found, checks the operating system's cache.
• Finally, it queries a DNS server (your ISP's or a public one like Google's 8.8.8.8) to get the IP address (e.g., 54.239.33.123).

3. TCP Connection:

A secure connection is established using TCP:

• Three-way handshake: SYN, SYN-ACK, and ACK packets are exchanged to ensure reliable communication.

4. TLS/SSL Handshake (for HTTPS):

For HTTPS (the default for most sites), encryption is set up:

• Encryption protocols are negotiated.
• The server's SSL/TLS certificate is verified to guarantee a secure connection.

5. HTTP Request:

The browser sends an HTTP(S) request:

• Method: Usually GET to retrieve the webpage.

• Headers: Metadata including browser type, language preferences, cookies, and cached data. A sample request:

  <code>GET / HTTP/1.1
  Host: amazon.in
  User-Agent: Mozilla/5.0</code>

6. Server Response:

The Amazon server processes the request and sends back:

• Status Code: Indicates success (200 OK), redirection (301), or error (404).
• Headers: Metadata like content type (text/html), caching instructions, and cookies.
• Body: The HTML, CSS, JavaScript, and other data needed to display the page.

7. Client-Side Rendering:

The browser renders the webpage:

1. HTML Parsing: The HTML is parsed into a DOM (Document Object Model) tree. External resources (CSS, JS, images) are identified.
2. CSS Parsing: CSS files are fetched and parsed, creating a CSSOM (CSS Object Model).
3. JavaScript Execution: JavaScript files are downloaded and executed (order matters unless async or defer is used). JavaScript can dynamically modify the DOM and CSSOM.
4. Render Tree: The DOM and CSSOM are combined to create the render tree, determining what's displayed.
5. Layout and Painting: Element positions and sizes are calculated, and the page is rendered on the screen.

8. Resource Loading:

Images, videos, fonts, etc., are downloaded concurrently. Some might be served from a CDN (Content Delivery Network).

9. Browser Caching:

The browser caches parts of the page (images, stylesheets, JavaScript) for faster loading on subsequent visits. Server headers control caching.

10. User Interaction:

The browser maintains a connection to handle user actions (clicks, form submissions), triggering new HTTP requests.

Simplified Diagram:

1. Browser: URL input → DNS lookup → TCP/TLS handshake → HTTP request
2. Server: Processes request → Sends response (HTML, CSS, JS)
3. Browser: Parses → Renders → Loads resources → Displays page

Website Optimizations:

Major websites like Amazon use numerous optimizations:

• CDNs: Distribute content geographically for faster delivery.
• Lazy Loading: Load resources only when needed.
• Minification: Reduce file sizes.
• Preconnect/Prefetch: Improve resource loading speed.
• Caching: Maximize resource reuse.

This detailed explanation covers the complete lifecycle of loading a webpage. Let me know if you need more information on any specific step!

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