What Is Web Crawling?
Web crawling is the systematic process of traversing a website — or the entire web — by following hyperlinks from page to page. A crawler (also called a spider or bot) starts from one or more seed URLs, downloads each page, extracts all outbound links, and adds new, unseen URLs to a queue for future fetching.
Search engines like Google and Bing rely on web crawlers to build their indexes. The same technique powers competitive intelligence tools, AI training data pipelines, and site-wide content audits.
Crawling vs. Scraping
These two terms are often used interchangeably but describe different scopes:
| Concept | Scope | Goal |
|---|---|---|
| Web crawling | Many pages, whole sites | Discover and fetch URLs at scale |
| Web scraping | Specific pages or fields | Extract structured data from known URLs |
In practice, most large-scale data collection projects combine both: a crawler discovers URLs, and a scraper extracts content from each one.
How a Web Crawler Works
A typical crawl loop looks like this:
- Seed — load one or more starting URLs into a queue
- Fetch — download the next URL from the queue
- Parse — extract all
<a href>links from the response - Filter — remove already-visited URLs, off-domain links, and paths blocked by
robots.txt - Enqueue — add new URLs to the queue
- Repeat — continue until the queue is empty or a depth/page limit is reached
URL Discovery with KnowledgeSDK
Rather than building your own crawler, KnowledgeSDK's POST /v1/sitemap endpoint returns every discoverable URL on a domain in seconds:
POST /v1/sitemap
Authorization: Bearer knowledgesdk_live_...
{
"url": "https://docs.example.com"
}
The response is a flat list of URLs you can immediately pass to POST /v1/scrape or POST /v1/extract for content extraction — no crawl queue management required.
Key Crawler Concepts
- Crawl depth — how many link hops from the seed URL the crawler will follow
- Crawl frontier — the queue of URLs waiting to be fetched
- URL deduplication — ensuring the same URL is not fetched twice
- Politeness delay — a pause between requests to avoid overloading the server
- Scope control — restricting the crawler to a specific domain, subdomain, or path prefix
Common Use Cases
- Indexing all pages of a documentation site for search
- Discovering product URLs across a large e-commerce catalog
- Monitoring an entire news site for new article URLs
- Building training datasets by collecting pages across many domains
Respecting Crawl Rules
Before crawling any site, check its robots.txt file at https://example.com/robots.txt. This file specifies which paths crawlers may and may not access. Ignoring it is both unethical and, in some jurisdictions, legally risky.