How Does a Web Crawler Work?
The process that web crawlers follow is both systematic and complex. Here’s a breakdown of the main steps involved:
Seed URL Selection: The crawling process begins with a set of initial URLs, often called “seed URLs.” These are typically popular, high-ranking websites or sources deemed valuable by the search engine. The selection of these seed URLs provides the crawler with a starting point.
Fetching Pages: The crawler’s next step is to fetch the content from each URL in the list. It sends an HTTP request to the web server hosting the page, and in response, the server returns the content, which could be HTML, CSS, images, or other multimedia.
Parsing and Analyzing the Content: Once the web crawler retrieves the content, it parses the HTML to extract useful information, such as text, metadata, and URLs to other pages. Links embedded in the content become pathways for the crawler to reach other pages, thus expanding its reach across the internet.
Following Links: Web crawlers follow links from one page to another, gradually mapping out the connections between different pages and sites. This systematic traversal is often referred to as “link traversal.” As the crawler discovers new URLs, they’re added to its “to-do list” or queue, making them available for future crawls.
Content Filtering and Indexing: Not all content is relevant or suitable for indexing. Web crawlers use filters to discard unnecessary or duplicate information, ensuring that only high-quality, valuable data makes it into the search engine’s index. The indexing process is what ultimately enables search engines to retrieve and rank relevant information when a user performs a search query.
Prioritization and Scheduling: Crawlers prioritize URLs based on factors like content relevance, update frequency, and domain authority. High-priority pages, such as news websites or frequently updated blogs, are revisited more often, while lower-priority pages are crawled less frequently.
This structured approach allows web crawlers to systematically explore the internet, gather information, and update the search index in real-time.