What is Web Crawler?
A web crawler, also known as a spider or bot, is an automated program that systematically browses the internet to collect data from websites.
The primary function of a web crawler is to index web pages for search engines like Google or Bing, allowing them to retrieve relevant information when users search. By visiting web pages and following links, web crawlers can traverse the entire internet, creating a vast index of web content.
How Web Crawlers Work?
1. Seed URLs: The web crawler starts with a list of initial URLs called seeds. It visits these URLs and retrieves their content.
2. Link Extraction: The crawler scans each webpage for hyperlinks. It adds these new links to its queue to be crawled later, effectively mapping the web by continuously discovering new pages.
3. Content Analysis: After retrieving a webpage, the crawler parses the content, focusing on keywords, metadata, or other relevant data. This helps in categorizing and indexing the pages for future searches.
4. Indexing: The analyzed data is stored in an index, a large database that allows search engines to quickly pull relevant information when users input search queries.
5. Respecting Robots.txt: Web crawlers adhere to a file called robots.txt, which websites use to tell crawlers which pages or sections should be excluded from crawling. This ensures that sensitive or unnecessary data isn’t indexed.
What are the Types of Web Crawlers?
1. Search Engine Crawlers: These crawlers are used by search engines like Google and Bing to index the entire web, making search results more accurate and relevant.
2. Focused Crawlers: These crawlers collect data related to a specific topic or niche. For example, a focused crawler might only index news articles or product listings.
3. Incremental Crawlers: Instead of visiting all pages, these crawlers revisit previously indexed pages to check for updates or new content, ensuring that the index remains up to date.
Why Are Web Crawlers Called ‘Spiders’?
Web crawlers are often called spiders because of the way they “crawl” through the web by following links from one page to another, similar to how a spider moves across its web. Just as a spider creates and navigates a complex web, web crawlers navigate the interconnected links across websites.
Difference Between Web Crawling and Web Scraping?
Web Crawling is the process of systematically browsing the web to gather information for indexing purposes. The goal is to index as much of the web as possible.
Web Scraping, on the other hand, is a more targeted approach to extracting specific data from websites, often for purposes like data mining or analysis. While crawlers aim to index entire sites, scrapers extract specific pieces of information, such as prices, reviews, or product listings.
Web crawlers, or spiders, are essential tools that help search engines and other web services index the vast amounts of content available on the internet. They work by following links and gathering content, building up a massive index that makes searching the web efficient and fast. The difference between web crawling and scraping lies in the broader, systematic approach of crawling versus the targeted extraction of specific data in scraping.