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Design a distributed web crawler

AnthropicSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenSystem Design

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This question tests the ability to design a scalable, fault-tolerant distributed web crawler that handles URL management, distributed coordination, and domain politeness.

Problem Statement

Design a distributed web crawler system capable of fetching billions of webpages across the internet. The system must be scalable, handle dynamic content, and respect the politeness policies of target websites.

Functional Requirements

  1. URL Frontier: A mechanism to store and prioritize URLs to be crawled.
  2. Fetcher: A service that retrieves the raw HTML content from a given URL.
  3. Politeness: Ensure the system does not overwhelm any single domain with requests (e.g., respecting robots.txt and maintaining a delay between requests to the same host).
  4. Deduplication: Prevent crawling the same content or URL multiple times.

Non-Functional Requirements

  1. Scalability: Support scaling to thousands of concurrent worker nodes.
  2. Fault Tolerance: The system must not lose progress if a worker node crashes mid-fetch.
  3. Freshness: Highly ranked URLs should be refreshed more frequently than others.

Design Deliverables

  • Provide a high-level architectural diagram (in text) describing the components.
  • Explain how you would manage the URL frontier to ensure distributed worker nodes don't collide when fetching from the same domain.
  • Define the data model for storing URL states (e.g., pending, crawling, completed).
  • Describe the mechanism for handling distributed URL deduplication at scale.

Hints

Hint 1

Consider using a distributed message queue or a prioritized task system like Redis or Kafka to act as the buffer between the frontier and the fetchers.

Hint 2

To enforce politeness, think about consistent hashing or domain-based sharding to ensure all requests for a specific domain are routed to the same queue/worker.

Hint 3

For deduplication, a Bloom filter can provide a memory-efficient way to check if a URL has already been processed before checking a persistent database.

Follow-up questions

  • How would you handle scenarios where a website's `robots.txt` changes frequently or a server experiences a temporary outage?
  • If the crawler needs to index content from JavaScript-heavy pages, how would you modify the Fetcher service architecture?
  • How would you implement a mechanism to prioritize 'important' pages (like news or Wikipedia) over less relevant pages?