Create a web scraper.

This question asks you to demonstrate your understanding of creating a web crawling program from the ground up. A web crawler, commonly referred to as a spider, is a software application utilized by search engines to navigate the internet and index web pages. It must be capable of retrieving web pages, parsing the information, and following hyperlinks to other pages, all while effectively managing the amount of data. This question assesses your skills in System Design and your comprehension of the internet's foundational architecture. When addressing this question, consider the following key points: A list of starting URLs, often called 'seeds', is required. The crawler must fetch web pages from these URLs. The content of the pages needs to be parsed, and links to additional pages must be extracted. A clear policy is needed to determine the order in which to visit the next pages. It is essential to adhere to the guidelines set by websites, such as the Robots Exclusion Protocol.

When designing an efficient web crawler, several key components must be taken into account: URL Frontier : This is the data structure that holds all the URLs that have been retrieved but not yet accessed. A Queue data structure could be utilized to ensure that all new URLs are eventually processed. HTTP Fetcher : This component's role is to obtain web pages from the URLs provided by the URL Frontier. It involves sending a GET request to the server hosting the URL to fetch the HTML page. Extractor : After receiving the HTML page, we need to extract all the links contained within it. We can utilize regular expressions or HTML parsing libraries for this extraction process. Duplicate URL Eliminator : This component prevents the crawler from accessing the same page multiple times, which helps conserve resources. Downloader & Archiver : Another important function of our crawler is to store relevant data for further analysis. This could involve saving the HTML pages locally or archiving only the pertinent data after processing. Controller : This part integrates all other components, overseeing the entire system's operation. It supplies URLs from the Frontier to the HTTP Fetcher, manages requests and responses, updates the Frontier with new URLs from the Extractor, and monitors the Downloader. To adhere to the guidelines set by web administrators, it is essential to comply with the "robots.txt" file, which outlines any restrictions.

Creating a web crawler consists of various components, each serving a distinct function: URL Handler : This component manages the initial seed URLs, ensuring they are correctly formatted and checked for duplicates to avoid unnecessary traffic. Downloader : This part retrieves content from the URLs managed by the URL handler. It operates using HTTP or HTTPS protocols and addresses any network-related challenges. Analyzer : The content obtained is forwarded to the analyzer, which extracts links and returns them to the URL handler for additional crawling. It may also involve extracting other information such as social media links, contact information, or any other specifics outlined in the requirements. Data Storage : The analyzed content needs to be stored, which can be in a file system or a database. Given the large volume of data, it might also be essential to consider employing distributed databases or storage solutions. Scheduler : This module manages the overall crawling process, determining which URL to crawl next, handling requests and responses, and ensuring that the web crawler does not overwhelm the network or storage systems. Additionally, an important aspect of web crawler design is adhering to the rules of the websites being crawled, which includes following the "robots.txt" file, respecting the "Crawl-delay" directive, and verifying whether the website permits search engines to index its pages.