What are Web Crawlers? Pros and Cons – Vacouf

If you want to optimize your website for SEO (Search Engine Optimization) or you were just curious about how the search engine works, you might have heard the name web crawler bounce off in your research. If you’re thinking that it is some sort of “thing” that crawls over the internet, then you’re almost right. But let us make you absolutely right. Keep on reading to find out what web crawlers are and what are their advantages as well as disadvantages.

What is a web crawler?

A web crawler is a bot designed to browse the internet and copy the web pages for indexing purposes. Web crawlers are also known as spiderbot, spider or simply crawler. They are used by search engines to analyze and index the information available on the internet. Now, when a user demands that information, it is easier for the search engines to display it because it is already indexed in the servers. A web crawler can also be used for data-driven programming or web scraping needs. 

Web crawlers are a type of software that collects, analyzes and indexes web content. They are used to create search engines, web directories and more. Web crawlers collect information from websites, analyze it in order to find relevant content for the target audience, and then index this content so that it can be searched later on. They also provide other tools such as social media monitoring tools and anti-spam services.

It is estimated that there are 1.18 billion websites on the internet out of which 83% are inactive. According to Google, its Google Search index data is already more than 100,000,000 gigabytes in size, and this data is of hundreds of thousands of webpages – the 17% active web pages. However, it is still less than how much data the whole internet contains because the whole internet isn’t accessible by the web crawlers. Specifically, the deep or dark web can’t be searched by these crawlers due to some reasons but Google is constantly trying to reach these websites and list them. Moreover, a web crawler also works according to some rules that prohibit it from visiting certain sites from indexing in the search engine. 

Common web crawlers

There isn’t a specific number about how many crawlers are out there on the internet because anyone can design and deploy one. However, there are some common ones that are legal. These are:

  • Googlebot for Google search engine
  • Bingbot for Bing search engine
  • Baiduspider for Baidu search engine
  • Yahoo! Slurp for Yahoo!
  • Applebot for Apple products like Siri and others
  • Yandexbot for Yandex search engine
  • DuckDuckBot for DuckDuckGo search engine
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Moreover, a website can host an in-house crawler for its own purposes. As mentioned earlier, anyone can design and deploy their bot on the internet to perform various functions. There is no limit as to how the bots can be used, which is the reason why you see those reCaptcha’s asking if you’re a human or a robot. They just want to protect their websites from unwanted bots.

How does a web crawler work?

With so much data flowing in the internet, it is virtually impossible to read and index all of it. That is the reason why web crawlers use certain algorithms to crawl over these sites. This is how a web crawler works:

  1. Web crawlers start crawling from a known list of URLs or sitemaps. They copy these websites as they crawl through them.
  2. If they encounter other hyperlinks in a webpage, they add it to their crawling list to visit later.
  3. Due to the reason that this process can go on infinitely, they adopt some policies. For example, the websites which are cited more than the others are preferred as they are supposed to contain authentic information. 
  4. Also, developers can define a robots.txt file for these web bots. This file is stored on the internet and the crawlers are supposed to follow the instructions written in them.

Robots.txt, also known as the robots exclusion standard or robots exclusion protocol, is a set of instructions provided by the site developer for the web crawlers. When these crawlers visit your site, they will follow the rules in the robots.txt file. Sometimes, there are such web pages where you don’t want anyone to look other than the required people; you can rule out in the robots.txt file that this particular webpage shouldn’t be crawled. Also, there are some malicious crawlers out there that can slow down your website; you can’t ban these crawlers using the robots.txt file.


Sitemaps are XML files that contain all the URLs related to a particular web domain. The developer might add additional information like when this particular web page was last edited and update its metadata. Sitemaps can be very useful to list contents that are very different from the other contents available on your website. 

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If both robots.txt and sitemap are used in conjunction, they will yield an optimized SEO performance which will help your site to list better in the browser Search Engines. 

Pros of web crawlers

1. Easy to gather data

Sometimes you just want to collect all the information of a website. Manually going through each site and then copying the information is a tedious task. However, if a web crawler is used for this purpose, the task becomes easier. Crawlers can easily and efficiently extract data from a webpage and store it for future use.

2. Increased site-traffic

Web Crawlers are responsible for finding authentic sources of information on the internet and then listing them accordingly. If you wish your site to be indexed higher on the servers, you need to provide a proper sitemap and robots.txt file, and also you have to display quality content. If your site gets listed, then it will result in increased traffic which means increased income.

3. Keep track of user activity

We all want to provide our users and viewers with the most interactive and high-quality content. To achieve that, it is important that we first record and analyze how users are spending time on our websites. This way, it will be easier for us to find bad spots and correct them hence increasing the user comfort. If a user feels good after visiting your site, it is already a win-win condition for you.

Before you track a user’s activity, you should notify them about it and ask for their permission because some people don’t like to be monitored. It is your duty as a web admin to keep in mind your user’s privacy and work according to it.

4. Keep track of important information

Here, important information is an umbrella term for any data that a certain company might find useful. For example, keeping up with the industrial trends and noticing the moves of rival companies are important tasks in the business field. To achieve that, web crawlers can be designed to keep track of this information and store it for future use.

Moreover, a developer can program these crawlers to analyze this data and prepare meaningful charts and graphs that convey the required information.

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Cons of a web crawler

1. Malicious Bots

With great advantages, comes a great disadvantage. And this is probably the biggest disadvantage there is of web crawlers. Just like there are good crawler bots, there are bad crawlers too. These bad crawlers want to scrape your site data and use it for malicious purposes. These evil purposes can cause a lot of damage to your website and you don’t want that.

2. Data Breach

Evil crawlers might scrape your sensitive data stored over the internet and then make it publicly available or sell it. This will lead to a data breach in your system and not only that, the user’s trust in you will also degrade. 

3. Too much traffic

Whether it is due to your listing in the Search Engines or malicious bots trying to overload your server, if your site gets too much traffic and it isn’t designed for that, bad things might happen. For example, an increase in traffic might result in decreased bandwidth which will ruin the user experience further degrading your site. The repair process is lengthy and costly which will create a financial imbalance in your life.

4. Prohibited pages

A web crawler isn’t supposed to list private web pages such as login pages, data reports for your company and other kinds of secrets. This can be easily prevented by using the robots.txt file and sitemap. But malicious bots don’t follow these sitemaps and they might try to copy this secret information. To prevent that, you should consider using a firewall on your private files stored over the internet.


The conclusion is that if good web crawlers crawl on your site that it is a win-win situation. But if the bad ones try to copy your information then you should be aware of that and plan to counter that issue. 

Web crawlers are an essential part of Search Engine Optimization as they are responsible for listing the websites on the servers. If you are able to clearly communicate with these crawler bots and you pass all their policies, then there is a high chance that you might get listed higher in the chain. This will result in more visitors on your website which is something we all want. 

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