Web Crawlers: How do They Work?

Web crawlers are programs that crawl and index content on the internet, and they are essential to the functioning of search engines. In this article, we take a closer look at how web crawlers work and what that means for SEO.

A web crawler is also called a spider because it uses hyperlinks to move like a spider through its web. In the process, it collects information and uses it to create an index. The first web crawler, which started operating in 1993 under the beautiful name World Wide Web Wanderer, worked according to this principle. Web crawlers are best known as search engine crawlers. However, they can also be used for other functions. 

How web crawlers work

Web crawlers are bots: they automatically perform predefined, repetitive tasks. Depending on the underlying code, they evaluate hashtags and keywords, among other things, and index URLs as well as content. They can also use various tools to compare data or call up links.

Diagram showing how Google processes search queries.

Types of web crawlers

The best known are the crawlers of search engines, the so-called searchbots. These include the Googlebot, which exists in several versions. The main task of searchbots is to index content on the internet and make it available to users via search results.

In other words, it is crawlers that produce search results, and only the pages that they crawl will appear in search results.

In addition, crawlers are used, among other things, for:

  • Carrying out data mining, e.g. collecting addresses
  • Conducting web analysis
  • Comparing data on products for comparison portals
  • Collating news
  • Finding faulty content

Important: Crawlers are not always scrapers. While web crawlers are primarily used to read, analyse and index information, scrapers extract data from websites, for example for timetables, but also in the context of copyright infringements.

Crawlers and SEO

There are ways to influence how searchbots crawl your pages. For example, you can make sure that the web crawler finds important content and does not crawl or index certain content. Both can also have a positive impact on your ranking.

There are also ways to have a favourable impact on the crawl budget. This is the number of subpages that Google can and “wants” to crawl per URL. In this context, you also have to consider crawl optimisation or crawl budget optimisation. Through such optimisation, you create conditions for the budget to be sufficient for all URLs.

Please note: Google itself has pointed out in the past that the crawl budget is sufficient in most cases without any problems. As a rule, all owners of small or medium-sized websites do not need to worry about this.

Tips for crawl optimisation

To make the web crawler’s job easier and to optimise the crawl budget, consider the following:

  • The web crawler prefers a flat page architecture with short paths
  • Optimise internal linking
  • Use robots.txt to prevent the web crawler from crawling unimportant pages
  • Make sure you provide the crawler with an XML sitemap
  • Track the crawling on your pages. This is the only way to know what can be improved.

Tip: Despite a corresponding entry in the robots.txt file, it may be the case that the page in question is still indexed by Google. If you want to prevent indexing, you can use a no-index command.

Steve Paine