Anatomy of a Web Crawler
A web crawler, also known as a spider or bot, is a program that automatically traverses the web and indexes web pages and other online resources. Here is a brief overview of the anatomy of a web crawler:
Seed URLs: The first step in the crawling process is to identify seed URLs, which are the starting points for the crawler to begin indexing web pages. Seed URLs can be provided manually or generated automatically.
URL Frontier: The URL frontier is the list of URLs that the crawler has discovered but not yet visited. The frontier is typically managed by a queue data structure that ensures that each URL is visited only once.
URL Normalization: Before visiting a URL, the crawler will normalize the URL by removing unnecessary parameters, resolving relative links, and converting the URL to a canonical form.
Web Page Download: The crawler will download the web page associated with a given URL. This involves sending a request to the web server and receiving a response containing the HTML code of the web page.
HTML Parsing: The crawler will then parse the HTML code of the web page to extract information such as links, metadata, and text content. This process involves using regular expressions and other techniques to identify patterns in the HTML code.
URL Extraction: The crawler will extract all the links from the web page and add them to the URL frontier for further processing.
Data Storage: The information extracted from the web page is stored in a database or other data store for later retrieval and analysis. This information may include the URL, page title, description, keywords, and other metadata.
Politeness: To avoid overloading web servers and causing performance issues, crawlers must adhere to certain rules of politeness, such as limiting the number of requests per second and respecting the robots.txt file that indicates which pages should not be crawled.
Recrawl Strategy: Crawlers typically implement a recrawl strategy to revisit web pages periodically and update their index with any changes or updates. The recrawl frequency can vary depending on the importance and frequency of updates to the web page.
Overall, the anatomy of a web crawler is complex, involving multiple components and processes that work together to efficiently index and organize web pages and other online resources.