System requirements

Functional:

1. Web Crawling:
2. The system should be able to initiate HTTP requests to web pages and download their content.
3. It must support different protocols (HTTP, HTTPS) and handle redirects appropriately.
4. The system should support handling various content types such as HTML, images, and other media files.
5. URL Frontier Management:
6. Maintain a dynamic URL frontier to prioritize and manage the order in which URLs are crawled.
7. Implement a mechanism to add new URLs to the frontier and remove them once crawled.
8. Support for scheduling URLs based on predefined rules or priorities.
9. Robots.txt Compliance:
10. The system must fetch and interpret the robots.txt file for each domain to ensure compliance with crawling rules.
11. Respect rules specified in robots.txt, including crawl-delay and disallow directives.
12. Data Extraction:
13. Extract relevant information from web pages, including text, images, links, metadata, etc.
14. Support for parsing and extracting structured data formats such as HTML, XML, or JSON.
15. Content Deduplication:
16. Implement a mechanism to identify and eliminate duplicate content to minimize redundant crawling.
17. Use hash functions or other techniques to compare and identify duplicate content.
18. Crawl Depth Control:
19. Provide a mechanism to customize the depth of crawling, limiting how many levels deep the crawler should follow links.
20. Support for setting depth limits based on domain or URL patterns.
21. Data Storage:
22. Store crawled data in a structured format (database, file system, etc.).
23. Support for efficient retrieval and update operations for the stored data.
24. Regular Crawling:
25. Schedule and perform regular crawls to ensure data freshness.
26. Support for incremental crawling to update only modified or new content.
27. Prevention of Infinite Loop:
28. Implement mechanisms to prevent the crawler from getting stuck in an infinite loop. Maintain state information to track visited URLs and avoid revisiting them unnecessarily.

Non-Functional:

1. Performance:
2. The system should be able to crawl and download content from a large number of web pages within a reasonable time frame.
3. Efficient handling of parallel requests to optimize throughput.
4. Scalability:
5. The system must be scalable to handle the specified workload of 1 billion web pages per month.
6. Reliability:
7. The crawler should operate reliably under normal and adverse conditions, minimizing downtime.
8. Robustness:
9. The system should gracefully handle errors and unexpected situations to ensure continuous operation.
10. Security:
11. Ensure secure handling of crawled content and avoid potential security vulnerabilities.
12. Implement secure communication (HTTPS) when interacting with web servers.
13. Throttling:
14. Throttling mechanisms should be effective in preventing the crawler from overwhelming servers while maintaining efficient crawling.
15. Compliance:
16. The system must comply with legal and ethical standards for web crawling.
17. Respect privacy and legal restrictions on crawling certain types of content.
18. Monitoring and Logging:
19. Implement comprehensive monitoring and logging to track system performance, errors, and crawling statistics.
20. Provide tools for administrators to analyze and troubleshoot issues.

Capacity estimation

Assumptions

• The system crawls 1 billion web pages per month
• Each web page has 10 media files on average
• An average media file size of 1MB

Storage Required for Web Pages:

Let us assume each web page on average has 100KB of data, so we need

Storage = Number of Pages * Average Page size * 12 months

Storage = 1 billion * 100 KB * 12

Storage = 1,000,000,000 * 1,00,000 * 12 = 1.2 PB

So, we would need 1.2 PB storage for 1 year

Storage required for media files

Since each web page can have 10 media files and each media file is 1 MB.

Storage = Number of Pages * 1 media file size * 10 files * 12 months

Storage = 1 billion * 1 MB * 10 * 12

Storage = 1,000,000,000 * 1,000,000 * 10 * 12 = 120 PB

So, we would need 120 PB storage for 1 year

API design

Below is the list of API's required for the system, although this might not be the exhaustive list, this provides a good starting point.

1. /api/addUrl:
2. Description: This API is used to add a new URL to the crawler's queue for processing.
3. Input: The input will be a JSON object containing the URL to be added.
4. Output: The output will be a response indicating the success or failure of the operation.
5. /api/getNextUrl:
6. Description: This API retrieves the next URL from the crawler's queue for crawling.
7. Input: No input parameters are required.
8. Output: The output is the URL to be crawled next or an indication that the queue is empty.
9. /api/crawlPage:
10. Description: Initiates the crawling process for a given URL.
11. Input: JSON object containing the URL and associated crawl parameters.
12. Output: Response indicating the success or failure of the crawling process.
13. /api/getCrawledData:
14. Description: Retrieves the crawled data for a specific URL.
15. Input: JSON object containing the URL or other identification parameters.
16. Output: The output is the crawled data, including text, images, links, etc., in a structured format.
17. /api/setCrawlDepth:
18. Description: Allows customization of the crawl depth for a specific URL or domain.
19. Input: JSON object containing the URL or domain and the desired crawl depth.
20. Output: Response indicating the success or failure of the operation.
21. /api/updateRobotsTxt:
22. Description: Updates the internal representation of robots.txt for a given domain.
23. Input: JSON object containing the domain and the updated robots.txt content.
24. Output: Response indicating the success or failure of the update.
25. /api/getCrawlStatus:
26. Description: Retrieves the current status of the crawling process.
27. Input: No input parameters are required.
28. Output: The output is a summary of the crawling status, including the number of pages crawled, errors encountered, etc.
29. /api/setThrottle:
30. Description: Allows dynamic adjustment of the crawling speed to prevent overloading servers.
31. Input: JSON object containing the throttle parameters (e.g., delay between requests).
32. Output: Response indicating the success or failure of the operation.
33. /api/checkCrawledStatus:
34. Description: Checks if a given URL has already been crawled.
35. Input: JSON object containing the URL.
36. Output: Response indicating whether the URL has been crawled or not.
37. /api/removeUrl:
38. Description: Removes a specific URL from the crawler's queue.
39. Input: JSON object containing the URL to be removed.
40. Output: Response indicating the success or failure of the removal operation.
41. /api/getMediaForUrl:
42. Description: Retrieves media files (images, videos, etc.) associated with a specific URL.
43. Input: JSON object containing the URL.
44. Output: The output is a list of media files with their respective URLs and metadata.

Database design

Defining the system data model early on will clarify how data will flow among different components of the system. Also you could draw an ER diagram using the diagramming tool to enhance your design...

High-level design

You should identify enough components that are needed to solve the actual problem from end to end. Also remember to draw a block diagram using the diagramming tool to augment your design...

Request flows

Explain how the request flows from end to end in your high level design. Also you could draw a sequence diagram using the diagramming tool to enhance your explanation...

Detailed component design

Dig deeper into 2-3 components and explain in detail how they work. For example, how well does each component scale? Any relevant algorithm or data structure you like to use for a component? Also you could draw a diagram using the diagramming tool to enhance your design...

Trade offs/Tech choices

Explain any trade offs you have made and why you made certain tech choices...

Failure scenarios/bottlenecks

Try to discuss as many failure scenarios/bottlenecks as possible.

Future improvements

What are some future improvements you would make? How would you mitigate the failure scenario(s) you described above?