Definition
Web Crawlers
Automated programs that systematically browse the internet to collect information from web pages.
They are also known as bots, spiders, or web robots.Note
Web crawlers are essential for search engines, as they gather data to build and update search indexes.
How Web Crawlers Work
Starting with Seed URLs
- Crawlers begin with a list of seed URLs, which are the initial web addresses to visit.
- These URLs are often well-known or frequently updated websites.
A crawler might start with popular news sites or directories as seed URLs.
Fetching Web Pages
- The crawler sends HTTP requests to the seed URLs to retrieve the web pages.
- The server responds with the HTML content of the page.
This process is similar to how a web browser loads a page, but crawlers do it automatically and at scale.
Parsing and Extracting Data
- The crawler parses the HTML content to extract useful information, such as text, metadata, and links to other pages.
- This data is stored in a database for further processing.
The crawler might extract the page title, headings, and keywords to help search engines understand the content.
Following Links
- Crawlers identify hyperlinks within the HTML content and add them to a list of URLs to visit next.
- This process allows the crawler to navigate the web, discovering new pages.
Crawlers prioritize which links to follow based on factors like page importance, update frequency, and relevance.
