🔹 What is a Crawler?
A crawler (aka spider or bot) is a program that automatically browses the web to discover, read, and index content.
It’s used by search engines like Google, Bing, etc., to build and update their search index.
🔹 What Does a Crawler Do?
-
Starts with a list of known URLs
-
Fetches the content of each URL
-
Parses the page to find:
-
Content (text, images, metadata)
-
Links to other pages
-
-
Adds new links to its queue
-
Repeats the process on newly discovered pages
This is how search engines explore and “understand” the web.
🔹 What Does a Crawler Look For?
-
Page content (text, keywords)
-
Meta tags (
<title>,<meta name="description">) -
Canonical URLs (to avoid duplicate content)
-
Robots rules (
robots.txt,meta robots) -
Sitemap files (
sitemap.xml) -
Page load speed and mobile-friendliness
🔹 Key Files That Control Crawlers:
1. robots.txt
A special file at the root of your website that tells crawlers what not to access.
User-agent: *
Disallow: /admin2. sitemap.xml
Lists all your site’s URLs, so crawlers can find content more efficiently.
🔹 Types of Crawlers:
| Type | Example |
|---|---|
| Search Engine Crawler | Googlebot, Bingbot |
| SEO Audit Tools | AhrefsBot, Screaming Frog |
| Scrapers | Bots that extract data for other uses (legal or illegal) |
| Internal Crawlers | Used by apps for content indexing or link checking |
🔹 Crawler Challenges:
-
Handling JavaScript-heavy SPAs (bots may not see JS-rendered content)
-
Duplicate content from multiple URLs pointing to the same page
-
Respecting
robots.txtand crawl rate limits -
Avoiding overloading the server
🔹 Crawler vs Indexer:
-
Crawler: Fetches and discovers content
-
Indexer: Analyzes, ranks, and stores that content in the search database