Client — Server Flow in a Developer Perspective

When you connect to a web server for the first time and make subsequent calls, the request flow involves several steps. Here’s a breakdown of the process:

First-Time Connection to a Web Server

1. URL Entry: You enter a URL (e.g., www.example.com) in your web browser.
2. DNS Resolution: The browser checks its cache to see if it has the IP address for the domain. If not found, it sends a request to a DNS (Domain Name System) server to resolve the domain name into an IP address. The DNS server responds with the corresponding IP address.
3. Establishing a Connection: With the IP address, the browser establishes a connection to the web server using the Transmission Control Protocol (TCP). This usually involves a three-way handshake (SYN, SYN-ACK, ACK).
4. Sending the HTTP Request: The browser sends an HTTP request (e.g., GET request) to the web server to retrieve the desired resource (like a web page).
5. Server Response: The web server processes the request and sends back an HTTP response, which includes the requested resource and a status code (like 200 for success).
6. Rendering the Page: The browser receives the response and renders the web page for you to view.

Subsequent Calls to the Same Web Server

1. Cached DNS Entry: For subsequent requests, the browser may use the cached IP address from the first request, so it may not need to query the DNS server again.
2. Reusing the TCP Connection: If the server supports persistent connections (HTTP/1.1 default behavior), the browser may reuse the existing TCP connection for faster communication.
3. Sending Additional HTTP Requests: The browser may send additional HTTP requests for other resources (like images, stylesheets, scripts) as needed. These requests may still use the cached IP address.
4. Receiving Responses: The server responds to each request, and the browser continues to render the page or update content as necessary.

Does Every Request Pass Through DNS?

Not Every Request: No, not every request passes through DNS. Once the IP address is resolved for the first request, the browser caches that IP address for a certain period (defined by the DNS Time-to-Live, or TTL). Subsequent requests to the same domain can use the cached IP address without needing to query DNS again, unless the cache expires or is cleared.

• DNS TTL: Defined in the DNS zone file for each record, indicating how long clients can cache the record.
• TCP Connection: Lifetime varies based on factors like idle timeout, keep-alive settings, application layer protocols, and server configurations, without a fixed duration.