Published: 21/05/2021

Web Crawler

What is a web crawler?

A web crawler, also known as a spider or spiderbot, is an Internet bot that systematically browses the World Wide Web.

What is site crawling?

It is when web crawlers are used to gather information from the internet. For example, a search engine may index content found on websites and provide that content in response to queries submitted by users.

What are the main use cases of web crawlers?


The main goal of web crawling is to provide an up-to-date directory and database of all available sites on the Web. For example, Google’s crawlers visit billions of pages per day, examining each page for links to others that have not yet been discovered to create an up-to-date list of available websites. The frequency with which Google visits a particular site can vary from minutes for large sites, to years for very small ones.

Data Mining

Web crawling can also be used in data mining, which is the process of extracting information from large volumes of data, usually to identify useful patterns, knowledge or insights about hidden relationships within a dataset.

Site Health

Web crawlers also help webmasters find broken hyperlinks on their pages and fix them, or identify when the content on one page no longer matches that of a linked page on an external website.


Web crawlers may also be employed for security reasons, for example, to verify links which appear online but do not lead anywhere, or links that appear manipulated with the intent to mislead users about their destination and purpose.

What is the difference between web crawling and web scraping?

Put simply, web scraping concerns extracting data from various sites, but crawling involves discovering URLs or links on the web. Therefore, chronologically, you need to crawl first, then scrape the data from these pages.

How does a web crawler work?

A web crawler starts with a list of URLs to visit, called the spider’s start page. The spider visits each URL in sequence. It looks at what it finds and does one or more of these activities:

  • Copies links from that page into its starting point (the spider’s start page)
  • Follow those links recursively until all pages have been visited
  • Adds any new pages found along the way that aren’t already in its database.
  • Extracts information from those pages, typically including text and hyperlinks, according to a set of instructions encoded within them

Good web crawlers vs bad web crawlers

  • Good web crawlers go through all of your website’s pages to find out what is new or updated with no malicious intent; they don’t steal any data from you and do not violate privacy policies.
  • Bad web crawlers continuously visit your website because they aim to get private personal information about its visitors without having permission. These areas of interest should be closely monitored for security purposes because if these intruders find something sensitive like credit card numbers it could lead to seriously compromising someone’s identity or financial status.

How to block web crawlers and prevent them from entering your site

  • Utilize the Robots.txt exclusion protocol (REP) to prevent search engines from crawling your site and indexing it on their results pages
  • Replace content on public-facing websites that you wish not to be accessed by spiders with a 403 Forbidden error message when detected
  • Block domain names of known scrapers and downloader agents such as Googlebot, Baiduspider, BingBot, etc. using an IP filtering agent
  • Utilize advanced bot management tools like Netacea to distinguish the type of crawler and control its access to your website.

Should web crawler bots always be allowed to access web properties?

Web crawlers have specific policies that determine which pages they visit, the order of their visits, and how often they recheck for updates.

Page importance is key. Web crawlers don't aim to cover the entire Internet. However, they prioritise certain pages, such as those with high-quality content with a variety of backlinks. Pages that contain authoritative information will be crawled more easily.

Regular page revisits are essential. As web content evolves, gets removed, or relocated, web crawlers will return periodically to ensure that the most current content is indexed.

Crawlers consult the robots.txt protocol before exploring a page, which means that they set rules for bots. It is important to note that different search engines incorporate these factors into their algorithms, therefore crawler behaviours vary. Ultimately, however, the goal remains the same: downloading and indexing webpage content.

How do web crawlers affect SEO?

Crawlers store information from a website, therefore, if the information is missing on a page, it won’t be crawled and will impact your position in the search engine ranking positions.

Hence, it is important to consider that all the information is easily accessible so you can improve your SEO performance. This means you can drive traffic easily to your website.

A list of 10 web crawlers

There are a variety of web crawlers and these include:

  1. Googlebot: Googlebot is Google’s web crawler for Google’s search engine.
  2. Bingbot: Bingbot was created in 2010 by Microsoft. Bingbot scans and indexes URLs to offer a relevant search engine.
  3. Yandex Bot: Yandex Bot is a crawler for the Russian search engine, Yandex.
  4. Apple Bot: The Apple Bot crawls and indexes webpages for Siri and Spotlight Suggestions.
  5. DuckDuck Bot: The DuckDuckBot is the web crawler for DuckDuckGo.
  6. Baidu Spider: In China, Baidu is the leading search engine, and the Baidu Spider is its crawler.
  7. Sogou Spider: Sogou is another Chinese search engine. Sogu Spider reportedly was the first search engine with 10 billion Chinese pages indexed!
  8. Exabot: Exalead was made in 2000 in France. Exabot provides search platforms for consumer and enterprise clients.
  9. Swiftbot: Swiftype is a custom search engine for a website.
  10. Slurp Bot: Slurp Bot is the Yahoo crawler which indexes pages for Yahoo.

Web crawling best practices

Minimize impact

You can reduce the impact on target websites by:

  • Scraping during off-peak hours.
  • Limiting concurrent requests.
  • Adding delays between requests.

Review Robots.txt

Before web scraping, review the website's robots.txt file so you can understand the guidelines on crawling and scraping permissions.

Cache data

Cache pages to prevent unnecessary requests and improve efficiency.

Ensure you have considered the following legal issues:

  • You have permission to scrape behind login walls.
  • You avoid copyright violations for copyrighted content.
  • You are preventing overload that may lead to legal issues.
  • You are adhering to GDPR for personally identifiable information (PII).

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