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What are web crawlers and how do they work? The complete guide to the robots that index the web

Can you imagine searching a library the size of the Library of Congress and getting perfect results in a split second? Thanks to the humble web crawler, we essentially get to do this on the internet every single day.

Web crawlers are the largely invisible programs that constantly traverse the endless depths of the world wide web. Without these robotic librarians methodically cataloging web pages, search engines like Google would never be able to deliver the right information at lightning speed.

In this comprehensive guide, you‘ll discover everything you need to know about these unsung heroes of the digital age. What are web crawlers? How do they work their magic? And can you even build your own? Let‘s find out.

A (very) brief history of web crawlers

The web crawled before it walked. Automated web spiders first started roaming primitive websites in the early 1990s, when the web was in its infancy.

As the amount of information being published online exploded, it quickly became apparent that some type of indexing system would be needed to keep pace.

Search engines like Altavista, Ask Jeeves and Northern Light began deploying primitive crawlers to start cataloging the web‘s pages. Google arrived in 1998 with an especially ambitious vision for organizing the world‘s information.

Today, Google‘s army of highly sophisticated crawlers explore over 20 billion web pages per day, feeding Google‘s ever-expanding search index.

The scale of today‘s web crawling endeavors boggles the mind:

  • Google‘s index contains hundreds of billions of web pages
  • Bing crawls about 15 billion pages per day
  • Google accounts for over 90% of all web traffic

Clearly, web crawling has come a very long way in just 30 short years! The technology continues advancing rapidly, allowing search engines to deliver incredibly relevant content at blistering speeds.

What exactly is a web crawler?

Now that we‘ve glimpsed the scale of today‘s web indexing efforts, how do web crawlers actually work? What magic happens under the hood?

In simple terms, a web crawler is an automated script that browses the web in a methodical, automated manner. Starting from a list of "seed" URLs, the scripts recursively visit links contained within each page to crawl the entire website.

As they explore, crawlers grab page content and ship data back to a central repository. This allows the pages to be indexed for fast retrieval later on.

It‘s easiest to think of web crawlers as the librarians of the internet. Just as librarians catalog every book in their library, web crawlers catalog every page on the websites they visit. This makes finding information on the boundless web possible.

Broadly, web crawlers serve two main functions:

Web Indexing

Web crawlers that index the entire internet for search engines are focused on breadth over depth. Their goal is to crawl as much of the publicly available web as possible, indexing page content along the way.

Also called web spiders or spider bots, these crawlers allow services like Google and Bing to instantly deliver relevant search results.

Web Scraping

Beyond search, web crawlers are also used for more niche web scraping or data extraction tasks. These scrapers target specific sites or topics, extracting narrow data like product listings or news articles.

While their end goals differ, scrapers employ the same crawling techniques under the hood to systematically traverse websites.

There are infinite varieties of highly customized web crawlers suited for specific use cases. But at their core, they all work the same basic way.

How do web crawlers work their magic?

The web crawling process boils down to a simple repetitive loop:

  1. Fetch a page
  2. Extract links and content
  3. Add links to the crawl queue
  4. Repeat

This allows crawlers to incrementally explore websites and build an index. Let‘s look at the steps in more detail:

respecting robots.txt

Before accessing a site, crawlers check for a robots.txt file that provides rules about what can be accessed. This file might look like:

User-agent: *
Disallow: /private-pages/ 
Crawl-delay: 10

This tells all crawlers they can‘t access /private-pages/ and to wait 10 seconds between requests.


Crawlers start with a list of "seed" entry URLs to visit first. These initial pages serve as jumping off points into a website.

As the bot visits each page, it extracts all the hyperlinks using markup parsers. These newly discovered URLs get added to the request queue.

Page Analysis

In addition to links, the crawler extracts other page content like text, images, scripts, etc. This data gets processed and stored for later use.

Queue Management

The crawler maintains a queue of URLs it needs to crawl next. It prioritizes which pages to visit and avoids duplicates.

By repeating this loop indefinitely, web crawlers can explore vast swaths of the web. Of course, the scale and complexity quickly increases for large sites. Next we‘ll look at Google‘s epic web crawling operation.

How Google crawls the entire web

Google operates by far the largest web crawler fleet ever created. The Googlebot crawls over 20 billion pages across the web each day!

As Googlebot visits each page, it analyzes the content, extracts links, and sends data back to Google‘s indexing system. This powers Google‘s legendary search capabilities.

Let‘s explore some mind-boggling facts about Google‘s web crawling infrastructure:

  • Google‘s index contains over 200 billion web pages
  • There are likely 100+ Google data centers around the world
  • Googlebot issues over 1 trillion URLs requests per day
  • Google accounts for over 90% of global internet traffic

When you search Google, it checks its vast index for pages matching your query based on:

  • Keyword usage on page
  • Page topic and focus
  • Quality and uniqueness of content
  • Freshness of content
  • Link authority and relevance
  • Hundreds of other ranking signals

By constantly re-crawling the web, Google can deliver the most useful, up-to-date results in an instant. Their web crawler fleet works tirelessly behind the scenes to make this possible.

The relationship between crawling and SEO

SEO stands for Search Engine Optimization – improving your web pages to rank better in search engines.

One of the main goals of SEO is making your website easy for search engine crawlers to digest. The better optimized a page is for crawling, the better chance it has to rank.

Here are 5 essential SEO tips focused on web crawler optimization:

Crawlers struggle with dead links, so they directly hurt your rankings. Redirect or remove broken links.

Make site architecture crawl-friendly

Structure your site so crawlers can easily navigate between pages. This improves indexation.

Optimize page speed

Crawlers visit millions of pages per day. Faster loading sites tend to get crawled more frequently.

Create unique, useful content

Avoid thin or duplicate content. Offer something new and valuable to readers.

Enhance on-page SEO

Use target keywords appropriately throughout your content so crawlers understand relevance.

There are many factors that determine search rankings. But optimizing for the machines that index the web is a key piece of the puzzle.

How web crawlers differ from web scrapers

While often used interchangeably, web crawlers and scrapers refer to related but distinct technologies.

  • Web crawlers focus on comprehensively indexing the entire open web. They gather broad swaths of data.

  • Web scrapers extract specific data from targeted sites. They are more specialized and focused.

For example, an ecommerce price monitoring scraper would crawl product listings, extracting prices, images, specs and more. This data could be used to track price changes.

While scrapers employ similar crawling techniques as indexers, their end purpose differs. Web scrapers gather structured data; web crawlers create searchable web indexes.

Building your own simple web crawler

To build a web crawler, you‘ll need knowledge of a programming language like Python or JavaScript. Open source scraper libraries make development easier.

Here is a simple crawler architecture:

  1. Seed URLs – The starting URLs crawl from.
  2. Frontier – Manages the request queue.
  3. Page Fetcher – Downloads page content .
  4. Page Parser – Extracts data from pages.
  5. Data Store – Stores extracted information.
  6. URL Filters – Removes certain URLs.
  7. URL Seen – Checks for duplicate URLs.

And here is a partial Python crawler script:

import scrapy

class MySpider(scrapy.Spider):

  name = ‘basic-spider‘
  allowed_urls = [‘‘]
  start_urls = [‘‘]

  def parse(self, response):
    for link in response.css(‘a::attr(href)‘):
        yield scrapy.Request(link.extract())

    text = response.css(‘p::text‘).extract() 
    yield {‘text‘: text}

This uses Scrapy to initialize the crawler, start crawling from a single URL, extract links to crawl next, and grab paragraph text from each page.

With the building blocks above, you can develop a functional crawler for any site. Scaling up with things like proxies, throttling, and AI/ML is where it gets exponentially more complex!

10 tips for responsible web crawling

Here are a few tips to keep in mind when building or deploying crawlers:

  1. Use proxies – Rotate different IP addresses to distribute traffic and avoid blocks.

  2. Vary user agents – Mimic different browsers‘ user agents to avoid detection.

  3. Respect robots.txt – Avoid crawling any pages blocked in the robots.txt file.

  4. Limit crawl speed – Excessive crawling can overload target sites and get you banned.

  5. Check for duplicate content – Avoid re-crawling pages you‘ve already indexed.

  6. Watch out for honeypots – Some sites deploy fake pages to identify scrapers.

  7. Scrape data ethically – Only gather data from sites that permit it.

  8. Use crawl delay – Insert pauses between requests to avoid flooding servers.

  9. Separate crawling from parsing – Crawl first, parse pages for data later.

  10. Consult sitemaps – Sitemaps help crawlers index sites more efficiently.

Wield your web crawling skills responsibly! With great power comes great responsibility.

Let‘s recap

Web crawlers are the critical but under-appreciated robots that index the endless depths of the web. By recursively crawling links, they allow search engines like Google to deliver incredibly relevant results in milliseconds.

Whether you want to optimize for search, perform web scraping, or create custom web crawlers, understanding how these bots work their magic is invaluable. Web crawlers are an essential tool for unleashing the power of the world‘s information.

So next time you magically find exactly what you need on Google, take a moment to thank the web crawlers that made it possible! The untold army of digital librarians cataloging the web behind the scenes are true unsung heroes of the Information Age.

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