← mercor Interview Insights

mercor·Software Engineer·Onsite - System Design / Architecture·Senior

Senior
Apr 2026

Summary

Mercor infrastructure engineer interview, one big system design question that basically ate the whole session. They wanted the full browser-to-server-to-browser story and weren't satisfied with surface-level answers.

Questions Asked (1)

Q1

Walk through everything that happens, end to end, when you type amazon.com into a browser and hit Enter. Cover DNS resolution, TCP and TLS handshakes, HTTP, load balancers, app servers, caching, and how the browser actually renders the page.

System DesignTechnical Trade-offsAPI & Integrations
Author's notes

This question sounds broad until you're actually in it and realize they want every layer.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Structure your answer as a chronological narrative from the browser's perspective, covering each layer of the stack in order: DNS, TCP, TLS, HTTP, server-side processing, and rendering. Balance breadth and depth by briefly explaining each step and then diving deeper into areas where you have expertise, while connecting concepts to real-world systems like CDNs, load balancers, and caching.

Pro tip: Mention that the browser may use a cached DNS entry or an existing TCP/TLS connection (HTTP keep-alive) to skip steps, and that modern stacks often use QUIC/HTTP3 over UDP. This shows you understand real-world optimizations beyond the textbook sequence.

1. DNS Resolution

Explain how the browser checks its cache, then queries the OS resolver, which may hit a recursive DNS resolver that traverses root, TLD, and authoritative nameservers to get the IP address for amazon.com.

2. TCP and TLS Handshakes

Describe the three-way TCP handshake (SYN, SYN-ACK, ACK) to establish a connection to the server's IP on port 443, followed by the TLS handshake (ClientHello, ServerHello, certificate exchange, key derivation) to secure the connection.

3. HTTP Request and Server-Side Processing

Detail the HTTP GET request sent over the established connection, and how it reaches a load balancer (e.g., AWS ELB) that routes to an application server; mention caching layers (CDN, reverse proxy, application cache) and how the server generates a response.

4. HTTP Response and Browser Rendering

Explain how the browser receives the HTML response, parses it to build the DOM, fetches subresources (CSS, JS, images), constructs the CSSOM, and renders the page through layout, paint, and composite steps.

Key Points to Mention

  • DNS caching at multiple levels (browser, OS, resolver) and TTL
  • TCP three-way handshake and TLS 1.3 handshake (including session resumption)
  • HTTP/1.1 vs HTTP/2 vs HTTP/3 (QUIC) and their impact on performance
  • Load balancers (L4 vs L7) and their role in distributing traffic
  • Caching strategies: CDN edge caching, reverse proxy caching, and application-level caching (e.g., Redis)
  • Browser rendering pipeline: DOM, CSSOM, render tree, layout, paint, and composite

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.