A search engine crawler is a program that automatically scans the web to collect and index information from websites. It helps search engines like Google or Bing discover new pages, update old ones, and understand site content for search rankings. Crawlers operate continuously and follow links between pages.

Without this step, a page will not show in organic search. Crawlers work nonstop to track new pages and changes to old ones. Googlebot and Bingbot are two well-known examples. Other names include web robot, automatic indexer, and spiderbot.

Web crawlers are different from web scrapers, which are used for something else.

A History of How Web Crawlers Evolved

Search engine crawling began in the early 1990s as the internet expanded. Early tools were built to measure web growth and index content. Over time, crawlers evolved into complex systems powering modern search engines.

First web crawlers in the early 1990s

The earliest known crawler was the World Wide Web Wanderer, developed by Matthew Gray in June 1993. It was used to measure the size of the Web and generated an index called Wandex, one of the first search engine indexes.

Another milestone came in December 1993 with JumpStation, created by Jonathon Fletcher. It was the first tool to combine a crawler, indexer, and user-facing search interface, enabling keyword searches across indexed pages.

Rise of full-text search crawlers

In 1994, new crawlers introduced more advanced indexing methods:

• WebCrawler became the first to index the full text of every page it visited.
• Lycos also launched the same year, using ranking methods based on word frequency.
• AltaVista’s crawler, known as Scooter, was released in 1995 and indexed more pages than any crawler before it.

These systems made it possible for users to search not just page titles but any word on any page.

Robots.txt and crawl control

As crawler traffic increased, some site owners grew concerned about server overload. In 1994, Martijn Koster proposed the Robots Exclusion Protocol, which introduced the robots.txt file. This allowed site administrators to define rules for bots, including blocking specific pages or directories from being crawled.

Googlebot and large-scale crawling

Google’s crawler began as part of the BackRub research project at Stanford University in 1996. When Google was founded in 1998, Googlebot became its official crawler. Unlike earlier bots, it ran continuously and scaled with the web’s exponential growth.

By the 2000s, major search engines had adopted distributed crawling systems, spreading crawler tasks across thousands of machines to index billions of pages. However, no crawler could cover the entire Web. A 2009 study showed that even the largest engines indexed only 40 to 70 percent of publicly available pages.

How Do Search Engine Crawlers Work

Search engine crawlers work in steps. They start with a list of pages to visit, fetch content, follow links, and send data to the index. This process runs in a loop, helping the engine stay updated with new and changed pages.

Step-by-step crawling process

Crawling begins with a list called seed URLs. These are starting web addresses picked by the search engine. The crawler fetches each seed page using an HTTP request, then reads its content.

From the downloaded page, it picks up all links (such as <a href=”…”>). These links are added to the crawl frontier, a queue of pages to explore next. The crawler keeps visiting pages from this frontier, collecting more links and growing the list.

This chain continues endlessly. If one page is linked to another, the crawler can find it. This is how search engines discover most public pages on the Web.

Copying page content for indexing

When a page is fetched, its content is copied and sent to the indexing system. The crawler stores the raw HTML and passes it on. The search engine then:

• Splits the text into tokens (words)
• Records headings, title, and metadata
• Notes internal and external links
• Reads special tags like <meta name=”robots”> or canonical tags

This helps match search queries to pages later. The crawler does not analyse deeply — it only collects. Indexing is handled separately. Some engines keep a copy of the page in cache; others discard it after indexing.

Updating the index over time

Crawling is not done once. Pages change. New ones appear. So, crawlers revisit sites often.

Popular or fast-changing pages (like news sites) get checked more often. The engine checks if a page has changed using headers like Last-Modified, or by comparing with the last stored version.

If a page is missing (like 404 errors) or moved, the index updates. If a new page appears, it gets added to the crawl list. This keeps search results fresh.

How crawlers scale for the full Web

The web is too big for one crawler to handle. So, search engines use distributed crawling. That means many crawlers run at the same time on different servers. Each one handles a part of the web, like certain domains or link patterns.

This speeds things up and avoids putting too much load on one website. But even then, crawlers cannot reach everything.

Some pages are hidden — like ones behind login forms or search bars. Others change using JavaScript, which normal crawlers cannot see. Some advanced crawlers now use headless browsers to view such content.

Challenges crawlers face

Search engine crawlers have to deal with problems like:

• Duplicate content (same page with different URLs)
• Infinite loops (like calendar links that go on forever)
• Dynamic content that appears only after user actions
• Parts of the Deep Web that are not linked or accessible

Crawlers are designed to skip such traps using filters and logic. Even so, no crawler indexes every page. They focus on the most important and relevant content.

What Rules and Algorithms Do Crawlers Follow

Search engine crawlers do not scan the web at random. They follow clear rules to decide what to crawl, when to return, how to behave on a website, and how to divide work across servers. These are called crawling policies.

Selection policy: deciding what to crawl

Crawlers cannot scan the full Web, so they choose which pages matter most. This policy helps sort URLs by priority.

• Link-based importance: Pages with more inbound links (especially from high-authority domains) are usually crawled first.
• Homepage preference: Root URLs (like example.com) are often favoured over deep or obscure links.
• PageRank and signals: Some crawlers use early forms of PageRank or custom scoring to find valuable content faster.
• Query relevance: Modern engines also check if a page fits what users search for, using NLP and past search behaviour.

Re-visit policy: deciding how often to crawl again

Websites change over time. The re-visit policy tells crawlers how often to return to a page.

• Change frequency: News sites or blogs are checked often. Static pages are revisited less.
• Historical patterns: Crawlers remember how often a page changed in the past to estimate future updates.
• Sitemap hints: Tags like <lastmod> in XML sitemaps help suggest when a page was last updated.
• HTTP headers: Fields like Last-Modified allow fast checks for updates without downloading the full page.

Politeness policy: avoiding harm to websites

Crawlers follow rules to avoid putting too much pressure on any single site.

• Delay between requests: A crawler waits a few seconds between page fetches from the same domain.
• Connection limits: It avoids opening too many connections at once with a single server.
• Backoff on errors: If a site slows down or throws 5xx errors, crawlers reduce their pace.
• Site control options: Webmasters can request crawl limits through Google Search Console, while engines like Bing respect the crawl-delay rule in robots.txt.

Parallelization policy: handling large-scale crawling

To scan billions of pages, search engines use many crawler bots at once. This policy helps split the work.

• Domain-based partitioning: URLs are grouped by domain or hostname. Each crawler handles a specific group.
• Separate queues: Each crawler keeps its own URL list (or crawl frontier) to prevent overlap.
• Duplicate checks: Systems make sure two crawlers do not scan the same page twice.
• Adaptive scaling: Sites with more useful or popular content may be assigned more threads or resources.

Crawl scheduling: setting the right order

The order of page visits affects both speed and value. Search engines combine different strategies:

• Breadth-first: Useful for covering top-level pages from many sites.
• Depth-first: Helps gather complete data from large, important sites.
• Hybrid mix: Most engines now use blends, switching as needed based on site size and page type.

To avoid crawler traps, crawlers use:

• URL normalization: Removes unnecessary parameters or duplicates.
• Duplicate content detection: Helps skip pages with nearly identical content.
• Crawl budget enforcement: Sets a cap on how many pages from one site will be crawled in a given period.

How to Use Robots.txt to Control Crawlers

Before visiting a website, most search engine crawlers check a simple file called robots.txt. This file, stored at the root of a site, tells bots what they can and cannot crawl. It plays a key role in crawl control and polite web behaviour.

What is robots.txt

The robots.txt file is part of the Robots Exclusion Protocol (REP). It uses plain text to set rules for crawlers, based on the bot’s User-Agent (e.g. Googlebot, Bingbot). Every major crawler checks this file before fetching content.

Examples of basic rules:

• Disallow: /admin – blocks all bots from crawling URLs under /admin
• Allow: /public-page.html – lets bots crawl a specific page, even if others are blocked

Crawlers follow these instructions voluntarily. While they could ignore the file, reputable search engine bots respect it.

Key directives and common extensions

The file supports a few main commands:

• User-agent: Targets rules to a specific bot
• Disallow: Blocks certain paths from being crawled
• Allow: Overrides a disallow rule on a specific path
• Sitemap: Provides the XML sitemap link (supported by Google and others)
• Crawl-delay: Tells bots how many seconds to wait between requests (used by Bing, not Google)

The Robots Exclusion Protocol was introduced in 1994 and officially became an internet standard in 2022 (RFC 9309).

Page-level crawling controls

Apart from robots.txt, websites can control bots using meta tags and HTTP headers.

Common options include:

• <meta name=”robots” content=”noindex”>: Prevents the page from being indexed
• <meta name=”robots” content=”nofollow”>: Tells the bot not to follow any links on that page
• X-Robots-Tag (HTTP header): Used to apply noindex or nofollow to non-HTML files

These are stronger signals than robots.txt, as they affect indexing, not just crawling.

Handling of blocked and indexed pages

It is possible for a blocked URL to be indexed. If a page is blocked by robots.txt but linked from elsewhere, search engines like Google may still index the URL (without content or a snippet). To fully keep it out of results, a noindex tag is needed.

Verifying crawler identity

Most bots say who they are using the User-Agent field. But not all are honest. Some malicious bots fake their identity.

To verify real crawlers:

• Site owners use reverse DNS lookups to check the bot’s IP
• Search engines like Google publish their crawler IP ranges for validation
• A full reverse-plus-forward DNS check confirms if the request comes from the actual bot network

Only genuine crawlers respect rules and avoid scraping blocked content.

How search engine crawlers find new pages

Crawlers usually find pages by following links. When one page links to another, the crawler can move between them like stepping stones. But not every page is easy to reach. So, search engines also use other ways to find important content.

XML sitemaps help find hidden pages

Website owners can create a file called an XML sitemap. It lists the main pages they want crawlers to visit. This helps when:

• Pages are deep inside the site
• Some pages do not have many links
• JavaScript makes it hard for crawlers to see content

Sitemaps are placed in the robots.txt file or submitted using tools like Google Search Console. Search engines read these sitemaps regularly to keep their index up to date.

Manual submissions in webmaster tools

Sometimes, site owners want a new page to appear in search quickly. They can use manual tools to request this. Google and Bing both allow this through their webmaster portals.

• Google has the URL Inspection Tool
• Bing has its own submission panel

These tools let owners tell the search engine, “Please look at this page.” But they only allow a few requests per day.

IndexNow and push-based updates

Some search engines like Bing use a method called IndexNow. It works like a ping — the website sends a signal when something changes. This tells the crawler to fetch the new or updated page right away. It saves time and avoids waiting for the next crawl cycle.

Feeds for news and special content

For fast-moving content like news, search engines use RSS or Atom feeds. These are files that list new articles. Crawlers check these feeds to discover:

• News stories
• Job postings
• Event updates

Even after finding the URL in a feed, the search engine still sends its crawler to fetch the actual content.

How crawlers handle media, scripts, and documents

Search engine crawlers were once designed only for simple HTML pages. But today’s web includes images, videos, scripts, and PDFs. Modern crawlers have adapted to handle this richer content, though with some limits.

Media files like images, video, and audio

Crawlers can find and fetch media files when they are linked inside a page. For example, when a crawler sees an <img> or <video> tag, it can download that file. But crawlers cannot read the content inside images or audio the way they read plain text.

To understand media files, search engines use:

• Alt text, captions, and nearby words
• The file name and its URL
• Extra metadata stored with the file

Googlebot may also use computer vision to guess what an image shows. But this guess is limited. Images and videos can show up in places like Google Images or rich results, but they are indexed with only basic information.

JavaScript and dynamic web content

Many websites today show content using JavaScript. This means the content does not load in the first HTML file. Instead, the page changes after scripts run.

To handle this, Googlebot uses a tool called the Web Rendering Service (WRS). It works in two parts:

1. First, it fetches the raw HTML.
2. Then it renders the page, just like a browser, to run scripts and load all content.

Once this is done, the crawler collects new links and readable text. Bingbot also supports JavaScript using a newer Edge engine. But many smaller search engines still struggle with dynamic content.

For important content, SEO best practice is to use server-side rendering or dynamic rendering, so crawlers can see the main content in the HTML itself.

PDF files and other non-HTML documents

Search engine crawlers can also read and index PDFs, Word documents, and other formats.

• Text in these files is turned into plain text if possible.
• If the file is an image scan, it might not be readable.
• Links inside PDFs may or may not be followed by the crawler.

These files are added to the index like regular pages, but they lack HTML structure. The search engine reads them as single blocks of content.

Embedded resources and crawl load

When a crawler visits an HTML page, it may also fetch files like:

• CSS stylesheets
• JavaScript files
• Fonts, icons, or tracking scripts

All of these count toward the site’s crawl budget. If a page has too many resources, it might slow down how often search engines can reach that site’s main content. To help crawlers, websites should keep things simple and make key content easy to access in plain HTML.

How Crawled Pages Get Interpreted and Indexed

Indexing is the step that follows crawling. While crawlers collect web pages, indexing helps search engines understand them. It sorts content, extracts facts, and prepares each page so that it appears correctly in search results.

What indexing does with a crawled page

When a crawler brings in a page, the indexer reads its text, picks out words, and records where they appear. This creates an inverted index, which helps match search terms to pages. The system also checks the structure of the page, including titles, headings, and meta tags. It may detect if a page looks harmful or low-quality and decide whether to keep it in the index.

Natural language processing for deeper understanding

Modern search engines use Natural Language Processing (NLP) to go beyond exact words. NLP helps the engine understand the meaning behind the text. For example, if a page says “Alice Smith won the 2025 Nobel Prize,” the system may recognise Alice Smith as a person, link her to the Nobel Prize, and store this fact in its records. This kind of understanding helps match searches with the correct information, even when users don’t type exact phrases.

Building the Knowledge Graph

Many of the facts found during indexing are added to a Knowledge Graph. This is a structured map of people, places, organisations, and other known entities. The graph connects these entities to events, awards, products, and more. It allows search engines to show facts in panels alongside normal search results. Crawled pages help build this graph, especially when they include useful, reliable information.

Using structured data for better accuracy

Website owners can include structured data on their pages using formats like JSON-LD or microdata. This makes it easier for search engines to understand the page. For example, a recipe page can show ingredients and cook time using code that is meant for machines. A product page can show price and stock status. When crawlers find this data, it helps the indexer add correct details to both the regular index and the Knowledge Graph.

Sorting and classifying content types

Indexing also includes sorting pages by type. A page may be a news article, a forum post, or a product listing. These labels help decide how the page is ranked and where it appears. Search engines also look for spam, fake content, or harmful signals. If a page fails quality checks, it might be ignored or given low ranking.

Why indexing matters

Without indexing, crawled pages are just stored text. Indexing turns those pages into organised knowledge. It connects keywords, facts, and meaning so that users can get better answers when they search. This step is key to making modern search engines work.

Examples of search engine crawlers

Many search engines use their own crawlers to explore websites and collect information. Each bot has a unique role, user-agent, and crawling pattern. These bots follow crawl rules, discover new pages, and help build searchable indexes.

Googlebot

Googlebot is the main crawler used by Google Search. It works across both desktop and mobile content.

• It follows robots.txt rules, adapts to site speed, and respects structured data.
• Googlebot has different types, including ones for images, videos, and Google News.
• Its user-agent string includes: Googlebot/2.1 and links to Google’s official bot documentation.

Bingbot and Yahoo Slurp

Bingbot powers Microsoft Bing and also supports Yahoo Search through shared infrastructure.

• It obeys robots.txt rules and supports the crawl-delay directive.
• Yahoo Slurp was Yahoo’s original bot but now relies on Bing’s systems.
• Crawl behaviour can be adjusted by site owners through Bing Webmaster Tools.

Baiduspider

Baiduspider is the crawler for Baidu, the most-used search engine in China.

• It mainly focuses on Chinese websites but also crawls international content.
• The bot identifies itself as Baiduspider in its user-agent string.
• It obeys robots.txt rules and follows standard crawling protocols.

YandexBot

YandexBot is used by Yandex, a major search engine in Russia.

• It supports the Host directive in robots.txt, useful for multi-domain setups.
• YandexBot includes separate crawlers for mobile, images, and videos.
• Site owners can manage its behaviour through Yandex Webmaster tools.

DuckDuckBot

DuckDuckBot is the crawler used by DuckDuckGo, which is known for privacy-focused search.

• It crawls selected pages and supplements data from Bing and other partners.
• The bot respects robots.txt rules and supports IndexNow for fast updates.
• Its crawl rate is lower and more targeted than larger search engines.

Amazonbot

Amazonbot is used by Amazon to support services like Alexa and internal site search.

• It gathers general web content that can help answer spoken queries.
• The user-agent includes Amazonbot and follows crawl rules respectfully.
• While not tied to a public search engine, it uses common crawling methods.

Other notable crawlers

Various search engines and web platforms around the world operate their own bots.

• Sogou Spider is used by Sogou in China.
• Exabot is run by the French engine Exalead.
• Common Crawl and Heritrix collect public web data for research and archiving.

Site owners can check server logs to see these bots and control them using robots.txt or meta tags. Most major crawlers publish their user-agent details to help webmasters verify bot identity and manage access.

Web Crawlers vs. Web Scrapers

Web crawlers and web scrapers are both automated bots that read websites, but their goals are different. Crawlers explore and index content for search engines. Scrapers extract specific data for business, research, or other uses.

Key Differences Between Crawlers and Scrapers

Aspect	Web Crawler	Web Scraper
Main goal	Discover and index as many pages as possible for a search engine	Extract selected data like prices, reviews, or emails
Usage	Helps users search the web by showing relevant results	Used by individuals or companies for data gathering or analysis
Scope	Broad; crawls whole websites by following links	Narrow; targets specific pages or patterns
Politeness	Follows robots.txt, respects site speed and crawl limits	Often ignores crawl rules; may overload servers
Techniques	Follows links recursively; builds a site map or link graph	Targets set URLs; uses selectors to pull data (e.g., price or product name)
Output format	Indexes pages for search, often with metadata and link info	Stores data in spreadsheets, databases, or local files
Ethics	Designed to be a “good citizen” of the web	Some are ethical; many are not; some power mirror or illegal content
Examples	Googlebot, Bingbot, Baiduspider	Price tracker bots, review collectors, form-fill bots

Impact of search engine crawlers on SEO and website practices

Search engine crawling plays a direct role in how websites appear in search results. If a page is not crawled, it cannot be indexed. If it is not indexed, it cannot rank. Understanding how crawlers work is essential for good SEO.

Crawlability and internal linking

A site must be crawlable to show up in search. Pages hidden behind login forms, broken links, or JavaScript-heavy structures without proper rendering often go unseen. If important pages are not linked anywhere or missing from the sitemap, crawlers will not reach them.

Good internal linking helps bots move through the site and discover all key content. Clean URL structures and the use of canonical tags prevent confusion and wasted crawl effort. Duplicate pages, endless filters, or poorly designed navigation may block access or lead to inefficient crawling.

Robots directives and indexing control

The robots.txt file and meta robots tags help control what gets crawled or indexed. Mistakes here can block valuable content from showing in search. For example, blocking the entire /blog/ folder in robots.txt will stop Googlebot from reaching those pages at all.

A noindex tag removes a page from results but still allows crawling. A nofollow hint may stop bots from following certain links. These tools should be used with care. Only block or exclude pages that truly do not add value, like login forms, internal admin panels, or duplicate pages.

Crawl budget and large websites

For most websites, crawl budget is not a concern. But very large sites with thousands of URLs need to guide the crawler. Search engines define crawl budget as the mix of crawl rate (how many pages per second) and crawl demand (how important or fresh the content is).

Low-value URLs, like infinite calendar pages or filter-generated duplicates, consume budget and reduce crawl efficiency. These should be blocked with robots.txt, tagged with noindex, or removed entirely. The goal is to help crawlers focus on the most useful pages.

Server health matters too. A fast, stable site allows more pages to be crawled. Frequent 5xx errors or slow response times may lead to reduced crawl activity.

Freshness, updates, and sitemap signals

Publishing new content does not guarantee immediate visibility. Crawlers visit based on known patterns and demand. Adding pages to a sitemap with <lastmod>, submitting URLs through Search Console, or gaining external links can help bring crawlers faster.

SEO changes like title updates or new keywords do not affect search rankings until the page is re-crawled and re-indexed. For time-sensitive content like job listings or live events, APIs like Google Indexing API can notify crawlers instantly.

Crawler simulation and site audits

SEO tools like Screaming Frog or Lumar mimic crawler behaviour. They show how a search engine might see the site, highlight broken links, orphaned pages, and technical errors. These audits reveal real crawling issues and help fix them before they affect performance.

For example, if a section is only reachable by form input, it will not be crawled. If a page lacks proper links, it may stay invisible. These tools ensure that all content meant for search can actually be found.

Mobile-first crawling

Google now uses mobile-first indexing for most websites. This means Googlebot crawls and stores the mobile version of the page. If the mobile layout lacks text or links that appear on desktop, that content may be missed.

Websites must make sure their mobile view is complete. Responsive design solves this in most cases. But if the site uses a separate mobile URL, both versions must be crawlable and properly linked using <link rel=”alternate”> and <link rel=”canonical”>.

Ethics and crawler access control

Sites must show the same content to crawlers as they do to users. This is called non-cloaking. Serving different content to Googlebot than to visitors is against guidelines and may lead to penalties.

Not all bots are search engines. Some are scrapers or spam bots. Website owners can block such unwanted bots using server-level rules or robots.txt. However, blocking major crawlers like Googlebot or Bingbot will remove those pages from search results.