What are Web Crawlers and How do They Work? – hackernoon.com


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@gabijafateGabija Fatenaite

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Web crawlers, also known as spiders, are used by many websites and companies. As an example, Google uses several of them too. In their case, they use crawlers primarily to find and index web pages.

In a business setting, a web crawler can gather important information to gain a competitive advantage in the market, or to catch fraudsters and their malicious acts. But before going into business use cases, let me first explain the terminology.

What is a web crawler?

A web crawler is a bot (AKA crawling agent, spider bot, web crawling software, website spider, or a search engine bot) that goes through websites and collects information. In other words, the spider bot crawls through websites and search engines searching for information.

To give you an example, let’s go back to Google. Their search engine bot would work something like this: Google spider’s main purpose is to update the content of search engines and index web pages of other websites. When the said spider crawls a certain page, it will gather the page’s data for later processing and evaluation. 

Once the page is evaluated, a search engine can index the page appropriately. This is why when you type in a certain keyword into a search bar, you will see the most relevant (according to the search engine) web page. 

How do web crawlers work?

Web crawlers are provided with a list of URLs to crawl. What the crawler does is it goes through the provided URLs, and then finds more URLs to crawl within the pages. This could become an endless process of course, and that is why all crawlers need a set of rules (what pages should they crawl, when should they crawl, etc.) Web crawlers can:

  • Discover readable and reachable URLs
  • Explore a list of seeds or URLs to identify new links and add them to the list
  • Index all identified links 
  • Keep all indexed links up to date

What’s more, a web crawler can be used by companies that need to gather data for business purposes. In this case, a web crawler is usually accompanied by a web scraper that downloads, or scrapes, required information.

For business cases, web crawlers and scrapers have to use proxies. Well, they don’t have to, but it is very much encouraged to. Without proxies, data gathering operations would be difficult due to high block-rates.

Crawling challenges

As with any business operation, there comes a set of challenges one should overcome. Crawling is no exception. The main challenges a crawler can face include:

  • Crawling requires a lot of resources – this includes building an infrastructure, creating a storage system for gathered data, employing developers, etc.)
  • Overcoming anti-bot measures – even though bots, including crawler bots, are not malicious, they will be flagged as such, and therefore blocked. 
  • Data validation and cleaning – gathering vast amounts of data means a lot of duplicates and unnecessary information. A solution for data cleaning will be needed.

One of the popular challenges are bot detection and blocks. This can be avoided by implementing proxies as a solution.

Popular proxy types

To prevent getting blocked, it is important to have a pool of proxies on hand, and rotate proxy IPs to avoid detection. The most popular proxy types for crawling are residential and datacenter proxies. Both types allow the user to access content that might be unavailable or geo-restricted, ensure anonymity, and reduce IP address blocks. The wider the set of locations and the pool of IPs your proxy provider has, the better.

The main difference between residential and datacenter proxies is their origin. Residential proxies are IP addresses supported by an Internet Service Provider. They are genuine IP addresses. Datacenter proxies come from data centers, and they are not affiliated with an ISP. Usually, datacenter proxies are used for infrastructure, such as servers, web hosting, and so on.

Popular business use cases 

Web crawling can be used to power your business, gain a competitive advantage, or steer away from fraud. Here are a few most popular business use cases that use proxies:

  • Market research
  • Brand protection
  • Ad verification
  • Data aggregation
  • Pricing intelligence
  • SEO monitoring
  • Risk management
  • E-commerce and retail
  • Social media monitoring


Web crawlers are quickly becoming a necessity in the modern business landscape. They are perfect tools for staying ahead of the competition and getting to know what they are and how they work is a good beginning. 


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