Crawlers: How Search Engines Index Websites

What is a crawler?

A crawler, also known as a bot or spider, is an automated program that tirelessly traverses the vastness of the Internet and visits websites. There, it reads content, follows links, and moves from page to page. The information it collects is then processed by the search engine and used for indexing.

Only what the crawler can reach and capture during its journey can subsequently be processed and indexed by the search engine. Content that is technically difficult to access, blocked, or cannot be rendered correctly by the crawler may be difficult or impossible for the search engine to capture.

Where did the crawler originate?

One of the first web crawlers was developed in the early 1990s. It bore the romantic name “World Wide Web Wanderer.” The crawler was given the name “spider” because it moved across the web like a spider. Over time, crawlers have become increasingly powerful and today crawl enormous numbers of pages.

Why is the crawler important?

Before a page’s content can appear in a search engine’s results, it must first be crawled, then processed, and finally indexed. It’s important for businesses to understand how to guide the bot through their own site so that the content can be effectively crawled from a technical standpoint.

Advantages

Businesses do not need to actively submit their sites to be found.
You can design your website so that important content is reliably crawled while less important content is excluded.

Challenges

Crawlers have their limitations. They do not always fully capture content generated by complex browser technology. Furthermore, search engines have only limited resources available for crawling a website, which—especially for large sites—can result in not all pages being visited with equal frequency.

Furthermore, technical errors such as blocking can prevent the crawler from indexing entire sections.

Example

A company presents a product’s key features in an elaborate graphic. While this looks attractive, it is ultimately just an image. The text within the graphic is not as reliably available to the search engine as evaluable page content as real HTML text is.

Once the content appears as actual text on the page, the crawler captures it during a subsequent visit and provides the information for indexing. This improves the foundation for the page to be found when relevant search terms are used.