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

SeniorPrefer not to say

Jun 2026

Summary

Anthropic system design round, one big question about building a multithreaded web crawler from scratch. Went deep on concurrency and I felt okay about the implementation side but the discussion about when to use threads vs async vs processes is where I started fumbling a bit.

Questions Asked (1)

Design a multithreaded web crawler that starts from a seed URL, crawls all reachable pages under the same hostname without revisiting any URL, and uses a worker pool pulling from a shared queue. Cover thread-safe deduplication, queue design, graceful shutdown, rate limiting, and when you'd choose threads vs async vs processes.

System DesignTechnical Trade-offsAlgorithms & Data Structures

Author's notes

This is a meaty one and it looks straightforward until you actually have to talk through all the moving parts at once.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then sketch a high-level architecture with a shared URL frontier, a thread-safe visited set, and a worker pool. Walk through each component (deduplication, queue, shutdown, rate limiting) and discuss trade-offs, ending with a comparison of threads vs async vs processes and when to choose each.

Pro tip: Emphasize that the visited set must be checked and updated atomically to avoid race conditions, and that graceful shutdown requires a way to signal workers and drain the queue without losing in-flight work. Also, mention that rate limiting should be per-host to avoid overwhelming servers.

1. Clarify Requirements and Constraints

Ask about scale (pages, hosts), politeness (rate limits, robots.txt), and whether the crawler should be distributed. Confirm that only same-host pages are crawled and no URL is revisited.

2. Design Core Components

Outline the shared queue (thread-safe, blocking), visited set (thread-safe, e.g., concurrent hash set or lock-based), and worker pool. Explain how workers fetch URLs, download pages, parse links, and enqueue new URLs after deduplication.

3. Address Thread-Safety and Deduplication

Detail how to avoid race conditions: use atomic check-and-set for visited URLs, and ensure the queue is thread-safe. Discuss memory considerations for the visited set and potential use of bloom filters for large-scale crawling.

4. Implement Graceful Shutdown and Rate Limiting

Describe a shutdown mechanism: a sentinel value or a flag with condition variable to wake workers, and a way to wait for all workers to finish. For rate limiting, use a per-host token bucket or delay, and consider robots.txt.

5. Compare Concurrency Models and Trade-offs

Discuss threads vs async vs processes: threads for I/O-bound with shared memory, async for high concurrency with less overhead, processes for CPU-bound or isolation. Explain when to choose each based on workload and constraints.

Key Points to Mention

Thread-safe deduplication using a concurrent set or lock, with atomic check-and-set to prevent duplicate crawling.
Queue design: blocking queue with backpressure, possibly per-host queues for politeness, and handling of empty queue conditions.
Graceful shutdown: sentinel values or a shutdown flag, joining workers, and ensuring no URLs are lost.
Rate limiting: per-host token bucket or delay, respecting robots.txt, and avoiding overwhelming servers.
Concurrency models: threads for I/O-bound tasks with shared state, async for scalability, processes for CPU-bound or isolation.
Trade-offs: memory usage of visited set, complexity of distributed crawling, and handling of dynamic content or JavaScript.

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