Web Crawler: What It Is and How to Optimize a Website for It

A web crawler is a tool that explores websites and gathers information for subsequent processing by a search engine. Other terms used include spiderbot and spider.

How web crawlers work

In simple terms, a website crawler is a program that works behind the scenes of search engines like Google and Yandex, looking for pages for them.

A search robot is like an internet researcher. It constantly browses websites, collects information, and adds it to the search engine's database.

Search engines use special algorithms to show us the most important and relevant results. If a robot cannot scan a page, the search engine will decide that it is irrelevant. As a result, it will be ranked lower in the search results.

The process is similar to how a librarian adds new books to a catalog. If the librarian doesn’t know about a book, it won’t appear in the catalog, and it will be hard for people to find it.

How crawlers process a resource

A search engine robot perceives a website quite differently than we do. Instead of images and text visible to us, it looks at technical details such as the page title, server response, IP address, and others.

The "spider" evaluates many criteria, including: HTTP status code, web server type, timestamp in GMT format, MIME content type, byte size, presence of Keep-Alive, address, redirection response code, server IP address, cookies set, and link structure.

By the way, speaking of link structure — how to build it correctly and why it’s important? Read more in the article «Link Building: How It Can Help Your Website.”

For a page to appear in search results, it must first be found by a robot. Typically, crawlers discover new sections of a website by following links from sections they already know. For example, if a "spider" regularly checks a glossary, it will notice new posts and add them to its database.

If the website contains a special file with a site map (sitemap.xml), the search robot always reviews it first. This document tells the crawler what to check on the site (and what should not be crawled).

Site maps can be created through special services. For example, mysitemapgenerator.com

If you want the search "spider" to definitely check a specific section on your website, add the site to the database of a special tool. For example, in Yandex.Webmaster or Google Search Console, there is a feature where you can specify the exact address of the page you want to be indexed.

After the robot accesses the page, it scans it. It reads all the text, examines the HTML code, and finds all the links.

Once the robot finishes examining the page, it sends all the data to the server. There, all unnecessary elements are removed from the collected information, and it is organized in a specific order. Then, the data is sent to a special database known as an index. Although indexing is handled by a different program, it is often also referred to as a search robot.

Search engines process new websites at different speeds. Yandex adds fresh pages to the results after a few days, while Google can do it within just a couple of hours. If the site is brand new and search engines don't know anything about it yet, full site indexing will take much longer — often months.

Search robots don’t just visit a website once. They constantly monitor changes on it. If a page has been deleted or moved, the crawler will inform the search engine. How often robots check the site depends on the size of the website, the number of visitors, and how frequently fresh information appears on the site.

Common issues with web crawlers

Below, we’ll discuss the difficulties that can arise with web crawlers:

It takes a long time. If the site is large and complex, with a huge number of pages and sections, the search robot will require a lot of time to fully index it. This is especially true for websites with a confusing structure and insufficient internal links between sections. In this case, the process of full indexing can take months. Additionally, errors in the website code and the presence of duplicate pages also slow down indexing and negatively impact its results. This will lead to some sections of the site not appearing in the results or ranking in lower positions.

How to ensure your site always ranks at the top of search results? Read the article "SEO Optimization: What It Is and How It Works."

It overloads the website. Search crawlers that constantly visit the site create a load on the server. This happens because the robots mimic the actions of real users. If there are too many "spiders," the server may become overwhelmed, rendering the site inaccessible. Popular search engines typically avoid overloading websites, but if a lot of new pages are added to the site at once, the load increases significantly. In this case, you should either manually limit the number of visits by crawlers or configure the server to send "spiders" an overload signal (code 429). This code tells them to reduce the frequency of requests.
It can be dangerous. If the site owner doesn’t restrict access to certain pages, the search "spider" will find and index them. Due to privacy setting errors or the absence of no-index rules, materials that should not be published may appear online. For example, client data could become accessible through search engines.

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Sometimes crawlers fail to index pages. This issue can arise for several reasons — we’ll explain them below.

Why web crawlers can't see pages

Below, we’ll discuss some of the most common reasons a page isn’t indexed and how to address these issues.

1. The page is invisible. Sometimes, search crawlers fail to find certain parts of a website because they are simply hidden. This can happen if:

No other pages link to it.
You’ve intentionally blocked search engines from indexing this section using special tags or the robots.txt file.

Learn more about tags in the article "Tags: What They Are and How They Improve Website Rankings."

The page is not listed in the sitemap
Your site takes too long to load.

What to do:

Add links to the desired section from other parts of your site.
Include the page in the sitemap.
Improve loading speed. For example, compress images using tools like TinyPNG or ILoveImg, or convert images to WebP format.

2. Server error. It’s crucial that your server can handle the load from web crawlers scanning the site. If the server response time is too slow or errors occur, crawlers won’t be able to explore your site.

What to do:

Check server errors in the indexing report in Google Search Console or using a tool like Screaming Frog.

3. Your site is too large. A website with a vast number of pages requires more time for scanning. As a result, web crawlers may fail to cover all your sections.

What to do:

Fix all broken links and remove unnecessary redirects.
Eliminate duplicate pages to avoid confusing search engines.

How to optimize your site for web crawlers

Below, we’ll explore in detail how to make your site more readable for robots.

The server should be fast. When web crawlers scan your site, the server must not slow down. Use Google Search Console to check your server's speed. Ideally, it should respond in under 0.3 seconds.

Add more internal links between pages. Search engines will navigate your site more effectively, and users will also find it easier to move from one page to another. Ensure that interlinking is relevant and natural, avoiding spammy-looking links. Ideally, your homepage should link to other key sections of your site, and those sections should connect to each other. The faster a crawler finds your most valuable content, the better.

Remove duplicate content. Search engines aim to provide users with useful information. By clearing your site of low-quality duplicate content, you make it easier for crawlers to locate and index your valuable pages. This improves your chances of ranking higher in search results and attracting more visitors. Check whether you have identical tags across different pages of your site. You can easily do this using the crawl statistics report in Google Search Console.

Regularly check for broken links. Broken links not only frustrate visitors but also hinder crawlers. Imagine navigating a map filled with incorrect directions — it would slow your journey significantly. Similarly, broken links confuse web crawlers and make it harder for them to assess your site properly.

Use robots.txt. This file allows you to give search engines instructions on which sections of your site can and cannot be indexed. Located in the root directory of your site, this text file helps manage server load and prevent overload. Search engines generally follow the rules specified in this document.

This is how robots.txt yandex.ru looks like

Make sure to check all redirects on your site. Redirects are necessary to guide visitors to relevant pages, but improperly configured redirects can confuse web crawlers and negatively affect your visibility in search results.

Conclusion

A web crawler is a software agent that explores resources. It analyzes the content of site sections to identify keywords and assess how pages are interconnected. The information gathered is used by search engines to build an index, making it easier to find relevant pages in response to user queries.

To ensure your site ranks high in search results, it is essential to optimize it according to search engine algorithm requirements.