What are the precautions for deep web crawlers when crawling with multiple threads or processes?

When using multi-threading or multi-process deep web crawlers, several precautions should be taken to ensure efficiency, stability, and compliance:

1. Rate Limiting and Throttling:

   • Avoid overwhelming the target server by controlling the request frequency per thread/process. Implement delays or adaptive throttling to mimic human behavior.
   • Example: If crawling a forum, limit each thread to 1 request per second to prevent IP bans.

2. IP Rotation and Proxy Management:

   • Use rotating proxies or VPNs to distribute requests across multiple IPs, reducing the risk of being blocked.
   • Example: A crawler with 10 threads can rotate through 100 proxies, ensuring each thread uses a different IP for every few requests.

3. Session and Cookie Handling:

   • Maintain separate sessions or cookies for each thread/process to avoid session conflicts or detection as a bot.
   • Example: If logging into a website, ensure each thread manages its own authenticated session to prevent cross-thread interference.

4. Error Handling and Retry Mechanisms:

   • Implement robust error handling (e.g., timeouts, HTTP errors) and retry logic with exponential backoff to handle transient failures.
   • Example: If a thread encounters a 503 error, it should wait and retry after a delay instead of crashing the entire process.

5. Resource Management:

   • Monitor CPU, memory, and network usage to prevent resource exhaustion, especially when scaling threads/processes.
   • Example: Use thread pools with a fixed size (e.g., 20 threads) to avoid overloading the crawler machine.

6. Data Deduplication:

   • Avoid crawling the same URL multiple times across threads by using shared queues or distributed deduplication systems (e.g., Redis).
   • Example: A distributed crawler can use a Redis set to track visited URLs, ensuring no duplicates.

7. Compliance with Robots.txt and Terms of Service:

   • Respect the target website’s robots.txt rules and terms to avoid legal or ethical issues.
   • Example: If robots.txt disallows crawling /admin/, ensure no thread accesses that path.

For scalable and reliable crawling, Tencent Cloud’s Serverless Cloud Function (SCF) can manage concurrent tasks efficiently, while Tencent Cloud Redis helps with distributed deduplication and session storage. Additionally, Tencent Cloud CDN can optimize request routing and reduce latency.