January 2, 2022 7:00 PM PST
This document summarizes a mock system design interview focused on creating a distributed crawler. The interview evaluated the candidate's ability to design a scalable and efficient system for crawling a large number of URLs while addressing various technical challenges and requirements.
Interview Details
- Target Level: L5 (Senior)
- Duration: 1 hour
- Topic Covered: Crawler
- Drawing Tool Used: Whimsical
Requirements
Functional Requirements
- Design a distributed crawler.
Non-Functional Requirements
- Determine latency requirements.
- Define the number of websites to crawl and the frequency of crawling.
Key Considerations
- Latency is flexible and depends on content change frequency.
- The system should handle approximately 1 trillion URLs.
System Design
Components
- URL Retriever
- URL Downloader
- Content Parser
- Indexer
Improvement Suggestions
- Control crawling speed using Kafka to prevent data loss.
Metadata Handling
- Metadata indicates how often a client should access the website.
- To manage duplicate URLs, maintain a metadata database with URL and update time. Use hashing to check for content changes.
Database Schema
- ID
- Link
- Status
- Update Time (e.g., last crawl time)
Sharding Strategy
- Calculate the distribution of URLs:
- 1 trillion websites
- Crawl rate: 11k-12k URLs per second if crawling once a week.
- Shard URLs based on ID and location, distributing them across multiple machines.
Failure Handling
- Implement retries for when a website is down.
- Use a message queue to manage failures.
- Use a write-ahead log to maintain state.
Failure Detection
- Use heartbeat signals to monitor the status of the retriever and scheduler.
- Consider using a distributed coordination service for process management.
Avoiding Infinite Loops
- Implement BFS to track crawled URLs and avoid duplicates.
System Extension
- Save data in a distributed file system (DFS) for different content processing.
Audience Feedback
- The design was clear, and the candidate effectively explained each component.
- Demonstrated strong technical knowledge and scalability considerations.
- The distinction between the retriever and downloader was well articulated to manage different speeds.
Additional Considerations
- Address the need for a full table scan in the database.
- Discuss the priority of URLs in the queue based on update time and scheduled crawling.
- Consider the use of a Bloom filter for managing seed URLs and deduplication.
Key Takeaways
- Focus on high-level requirements: latency, scalability, durability, and reliability.
- Understand the user journey from MVP to scaling for a larger user base.
- Clarify requirements and iterate through the design process step-by-step.
- Recognize the differences between L4 and L5 candidates in terms of problem-solving and identifying key features.
Conclusion
The interview highlighted the importance of a structured approach to system design, emphasizing the need for clear requirements, effective communication, and a thorough understanding of technical components. The candidate demonstrated a solid grasp of the challenges involved in building a distributed crawler and provided thoughtful solutions to potential issues.