Crawler, web crawler, web spider, or web bot is a program or automated script used by search engines to index documents online. The primary purpose of a crawler is to visit web pages, analyze their content, and collect information to create an index that users can search and retrieve when they perform search queries.
How does a crawler typically work?
- The crawler starts from a set of known web addresses called “seed URLs.” (These seed URLs can be provided manually or obtained from the search engine’s index)
- It initiates HTTP requests to the seed URLs and downloads the corresponding web pages. (It may also follow links within these pages to discover new URLs to crawl.)
- Once a web page is downloaded, the crawler parses its content, extracting components such as text, images, and metadata.
- The relevant information extracted from the web page is added to the search engine’s index.
- The crawler continues fetching and parsing pages while following links on the pages it encounters. (This allows it to traverse a significant portion of the internet.)
- Popular search engines like Google crawl the web continuously to keep their index updated. They revisit previously crawled pages to detect changes and update the index accordingly.
Key features of the crawler
- Respect for Robots.txt
- Duplicate Content Handling
- Crawl Frequency Control
Benefits of using crawlers
- Enable users to find relevant information quickly
- Enable search engines to provide fast and relevant search results to users’ queries
- Reduce the need for manual intervention
- Ensure comprehensive coverage of web pages and resources
- Enhanced user experience
- Provide data for advanced search results ranking
- Support various services (search engines, website monitoring applications, data mining apps, content aggregation apps.
Crawlers play a crucial role in enabling search engines to create and maintain comprehensive indexes of the vast amount of information available on the internet. When a user performs a search query, the search engine can quickly retrieve relevant results from its index, facilitating efficient information retrieval.