Understanding the Web Stack: What Happens When You Type "https://www.google.com" and Press Enter

As a software engineer, a deep understanding of the web stack is crucial. One common interview question that tests this knowledge is: "What happens when you type https://www.google.com in your browser and press Enter?" This question helps interviewers gauge your understanding of various web technologies and protocols. In this post, we’ll break down the entire process step-by-step, from DNS requests to database interactions.

1. DNS Request

The first step in the process is the DNS (Domain Name System) request. When you type https://www.google.com into your browser, it needs to find the IP address of www.google.com. Here’s what happens:

- Browser Cache: The browser checks its cache to see if it already knows the IP address for www.google.com.

- Operating System Cache: If not found, the browser asks the operating system (OS) for the IP address, checking the OS cache.

- Router Cache: If the OS cache doesn’t have it, the query is sent to the local router, which may have its own cache.

- ISP DNS Server: The router forwards the query to your Internet Service Provider’s (ISP) DNS server.

- Recursive Query: If the ISP’s DNS server doesn’t have the IP address, it performs a recursive query, asking other DNS servers until it finds the authoritative DNS server for google.com, which returns the IP address.

2. TCP/IP Connection

Once the IP address is obtained, your browser initiates a TCP (Transmission Control Protocol) connection to the server:

- TCP Handshake: A three-step handshake process is used to establish a connection:

1. SYN: The browser sends a SYN (synchronize) packet to the server.

2. SYN-ACK: The server responds with a SYN-ACK (synchronize-acknowledge) packet.

3. ACK: The browser sends an ACK (acknowledge) packet back to the server.

- IP Packets: Data is sent over the internet as a series of IP packets, which are routed through multiple networks to reach the destination.

3. Firewall

During the TCP connection, data packets pass through firewalls that protect networks from unauthorized access:

- Inbound Firewall: The server’s firewall checks incoming packets to ensure they are from legitimate sources.

- Outbound Firewall: Similarly, your local firewall checks outgoing packets to prevent unauthorized data leaks.

4. HTTPS/SSL

With the TCP connection established, the browser initiates an HTTPS (Hypertext Transfer Protocol Secure) request:

- SSL/TLS Handshake: This involves a series of steps to establish a secure connection:

1. Client Hello: The browser sends a "Client Hello" message with its SSL/TLS version and supported cipher suites.

2. Server Hello: The server responds with a "Server Hello" message, selecting the SSL/TLS version and cipher suite.

3. Certificate Exchange: The server sends its SSL certificate, which includes its public key.

4. Key Exchange: The browser and server exchange keys to establish a secure session.

5. Session Established: Encrypted communication begins using the session key.

5. Load Balancer

After establishing a secure connection, the request reaches the load balancer:

- Traffic Distribution: The load balancer distributes incoming requests across multiple servers to ensure no single server is overwhelmed.

- Health Checks: It performs health checks on servers to ensure they are capable of handling requests.

6. Web Server

The load balancer forwards the request to one of the web servers:

- Request Handling: The web server (e.g., Apache, Nginx) handles the HTTP request, processing static content or forwarding dynamic content requests to the application server.

7. Application Server

If the request involves dynamic content (e.g., a search query), it’s passed to the application server:

- Business Logic: The application server (e.g., Node.js, Django) processes the request, executing the business logic.

- Database Queries: It may query the database for data required to generate the response.

8. Database

The application server interacts with the database to fetch or store data:

- SQL/NoSQL: Depending on the architecture, it could be a SQL (e.g., MySQL, PostgreSQL) or NoSQL (e.g., MongoDB) database.

- Data Retrieval: The database processes the query and returns the requested data to the application server.

Response Back to Client

Finally, the processed data travels back through the layers:

- Application Server: Sends the response back to the web server.

- Web Server: Sends the response back through the load balancer.

- Load Balancer: Forwards the response to your browser.

- Browser Rendering: The browser receives the data, processes HTML, CSS, and JavaScript, and renders the page.

This entire process happens in milliseconds, showcasing the complexity and efficiency of modern web technologies.

Conclusion

Understanding the intricate details of what happens when you type a URL and press Enter is crucial for any software engineer. It demonstrates a solid grasp of networking, security, and server infrastructure. Mastering this knowledge will not only prepare you for technical interviews but also enhance your problem-solving skills in real-world applications.

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