Design a Web Crawler: FAANG Interview Question

Quick Overview

Designing a robust, scalable web crawler requires addressing several complex challenges, including managing massive data volumes (billions of pages), ensuring politeness to avoid overloading hosts, implementing intelligent prioritization, avoiding duplicate content via URL and content seen checks, handling HTML parsing, and distributing the workload geographically for performance.

Key Points: The web crawler must handle massive scale, aiming for 400 pages per second, which translates to crawling one billion pages every month. Politeness is maintained by using a host-based queue system (e.g., B1, B2, Bn) managed by a Queue Router and Selector, ensuring requests to the same host are delayed by at least 2 seconds (1 req/2 sec). Prioritization is achieved by introducing a Prioritizer that routes URLs into different priority queues (f1, f2, fn) based on factors like page popularity, update frequency, and the number of incoming links. Duplicate avoidance uses two primary checks: a 'URL Seen' system (likely using a Bloom filter or hash set) and a 'Content Seen' system that hashes content to prevent crawling mirrored content. The core architecture flows from Seed URLs through a URL Frontier, HTML Downloader (which uses a DNS Resolver cache), Content Parser, and Link Extractor, before new links feed back into the URL Frontier. Scaling to billions necessitates distributing crawlers across regions geographically close to target servers, using local storage/queues, and implementing consistent checkpointing to allow recovery after crashes.

Context: This video provides a system design walkthrough for building a large-scale web crawler capable of indexing billions of web pages, a task undertaken by major search engines like Google, OpenAI, and Anthropic. The design focuses on handling high throughput (400 pages/sec), maintaining politeness towards web servers, intelligent prioritization of important pages, and robust mechanisms for deduplication and fault tolerance.

Detailed Analysis