A crawler—often called a "web crawler" or a "spider"—is an automated bot made and configured to inspect web pages for a specific purpose. These bots discover URLs of interest and follow links ("crawl") from page to page while scanning their contents. Generally, these crawlers continually monitor for page and linking updates across the web versus performing targeted actions (but not always).
While many crawlers are relatively easy to build using a few lines of code, others can be pretty sophisticated. There are exceptions to every rule, but a crawler made by an individual enthusiast or data analyst (to name a common example) will generally be more simplistic than equivalents from large companies such as Google or OpenAI.
A lot has changed since Google first started crawling the web in 1998. While crawler variety was somewhat limited overall and restricted to an index of 25 million unique URLs, Google alone maintains an index of well over 1 trillion unique URLs today. You can bet that other crawlers and their creators have a lot to gain from inspecting such a staggering number of websites.
What makes crawlers useful?
The most well-known crawler that often comes to mind is Googlebot, which Google uses to discover, index, and ultimately rank web pages on their search engine results page (SERP). Without the ability to understand each website's robots.txt file and site map, it'd be much harder for search engines to serve users relevant information.
While Googlebot (and most legitimate crawlers) will follow the restrictions and guidance found in robots.txt, some bots are designed to suck up as much information as possible—sometimes without specifically checking for restricted paths. This is one reason why you might configure a load balancer to restrict such bots.
Crawlers come in many different flavors and can perform additional actions:
Search engine optimization (SEO) – By understanding how common search engine bots operate or through in-house testing, teams can see how page structure, content, and overall performance impact discoverability.
Scraping – Taking a more targeted approach to crawling, scraper bots excel at collecting specific information from various websites and structuring it within a database.
Website testing – By creating crawlers with novel functions or those emulating common bots functions across the web, teams can ensure their sites are in working order.
Data mining – Sharing similarities with scrapers, data mining crawlers take things a step further by assigning meaning to data and any patterns present throughout.
Large language model (LLM) training – Companies such as OpenAI and Claude inspect and gather content, then train their AI models to better recognize chat prompts. This allows ChatGPT users (for example) to ask the application questions and receive contextual, human-like responses back.
Since crawlers are so prevalent, they account for a significant portion of overall web traffic. They individually consume backend resources—such as bandwidth, memory, or CPU—just like any regular user. Bad crawlers can cause performance degradation if left unchecked. They can also introduce privacy concerns depending on how they gather (often copyrighted) information.
As a result, many companies with web properties might want to consider controlling crawler activity through blocking or throttling. However, a fear of being excluded from search indexes, general apathy, lack of awareness, or the desire to help companies like OpenAI train their models may lead many sites to openly welcome crawlers. Business goals and overall preferences should influence these decisions.
How does HAProxy handle crawlers?
HAProxy Enterprise ships with the HAProxy Enterprise Bot Management Module for those who wish to decide which crawlers are allowed to access their sites (and how frequently). Crawlers are a subset within the automated bots category. HAProxy gives organizations the tools to detect and block scraping, abuse, and unverified or impersonated entities without disrupting human traffic.
To learn more about crawler and bot management in HAProxy, check out our How to Reliably Block AI Crawlers Using HAProxy Enterprise or Bot Protection With HAProxy blog posts.
