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Design a Scalable and Polite Web Crawler

AmazonSoftware Engineer · Onsite
MediumSoftware EngineerOnsiteSystem Design

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This question tests the ability to design a distributed, fault-tolerant system that balances high-throughput data collection with ethical crawling constraints and resource efficiency.

Problem Statement

Design a distributed web crawler system capable of indexing a large subset of the internet while respecting domain-specific politeness policies. The system must operate at scale, handling billions of pages while maintaining freshness and avoiding redundant work.

Functional Requirements

  1. Scalable Crawling: Ability to scale horizontally to process millions of pages per hour.
  2. Politeness: Ensure no single domain is overwhelmed by requests; respect robots.txt directives.
  3. Deduplication: Avoid re-downloading identical content found at different URLs.
  4. Recrawl Strategy: Implement logic to prioritize high-change content over static pages.

Non-Functional Requirements

  1. Fault Tolerance: If a crawler node dies, the work currently in progress should be retrievable or re-assigned.
  2. High Availability: The frontier (the queue of URLs to visit) must be persistent.
  3. Backpressure: The system should handle network latency or slow responses from target servers without crashing downstream services.

Deliverables

  • Define the high-level architecture, including key components like the URL frontier, downloader pool, and content processor.
  • Explain how you would store and manage the URL frontier to ensure efficient scheduling and politeness.
  • Describe the strategy for identifying and preventing "crawler traps" (e.g., infinite URL cycles).
  • Discuss your approach to deduplication at scale.

Hints

Hint 1

Consider using a distributed priority queue (like Redis or Kafka) for the frontier and partitioning URLs by domain to enforce politeness.

Hint 2

Think about the difference between 'seen' URLs and 'seen' content; use Bloom filters for fast URL checking and content hashing (e.g., SimHash) for duplicate detection.

Hint 3

Use a lease-based system for workers to ensure that if a crawl task isn't completed within a set time, the URL is returned to the queue for another worker to pick up.

Follow-up questions

  • How would you modify your design to prioritize crawling reputable sites (e.g., news or government domains) over unknown or spammy sites?
  • What specific metrics would you monitor to detect if your crawler is being blocked by anti-bot systems?
  • How would you handle the storage and indexing of the collected content, and how does that impact the crawler's design?