The BFS structure came naturally, tracking visited URLs with a set, queuing up links as you fetch each page.
Start by clarifying requirements and constraints, then outline a BFS-based crawler with a queue and a visited set to ensure each URL is visited once and only within the seed domain. Discuss key components like URL normalization, politeness (rate limiting), and scalability considerations such as distributed crawling and storage.
Pro tip: Emphasize politeness and robustness: mention robots.txt, crawl-delay, and handling of dynamic content or infinite loops, as these show production-level awareness beyond basic BFS.
Ask about scale, depth limits, politeness policies, and whether dynamic pages or authentication are involved. Confirm that only same-domain URLs should be crawled and that BFS order is required.
Use a queue for BFS and a set (or Bloom filter for scale) to track visited URLs. Normalize URLs (e.g., remove fragments, resolve relative paths) before deduplication.
Dequeue a URL, fetch the page, parse links, filter to same domain, and enqueue unseen URLs. Include error handling, retries, and respect for robots.txt and crawl-delay.
Discuss distributed crawling with partitioned queues, rate limiting per host, and storage of visited URLs (e.g., Redis, Bloom filter). Mention handling of large-scale data and fault tolerance.
Compare BFS vs. DFS, single-machine vs. distributed, and exact vs. approximate deduplication. Suggest extensions like prioritizing important pages or handling JavaScript-rendered content.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
This is the part that actually matters and I wasn't fully ready for how deep they wanted to go.
Start by acknowledging the dual challenge: maintaining efficient crawling throughput while being a good API citizen. Then outline a multi-layered strategy covering rate limiting, concurrency control, backoff, and identification, and tie it back to system design principles like trade-offs between speed and politeness.
Pro tip: Mention that you would monitor for 429/503 responses and dynamically adjust concurrency, and that you'd respect robots.txt and API terms of service to avoid legal issues. This shows you think about both technical and non-technical aspects.
Determine the API's rate limits, terms of service, and the crawler's throughput needs. Understand the trade-off between speed and politeness.
Use a token bucket or leaky bucket algorithm to limit requests per second, and control the number of concurrent connections to avoid overwhelming the API.
On receiving 429 or 5xx errors, exponentially back off and retry with jitter. Dynamically reduce request rate if errors persist.
Set a custom User-Agent with contact info, and consider using an API key if provided. This helps the API owner distinguish you from malicious actors.
Track response times, error rates, and throughput. Use this data to tune rate limits and concurrency, and to detect if you're being throttled or blocked.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.