How Does a Web Crawler Work?

TL;DR

A web crawler works by starting from one or more seed URLs, downloading each page, extracting the links it finds, and adding new links to a queue to visit next. It repeats this loop, tracking already-visited URLs to avoid duplicates, until it runs out of links or hits a limit you set.

The Core Loop

Every crawler, from a 20-line script to a search engine, runs the same cycle:

Seed. Begin with one or more seed URLs.
Fetch. Download the page at the front of the queue.
Parse. Extract the content you want and the links on the page.
Filter and enqueue. Keep links that are in scope and not yet seen, and add them to the queue (the URL frontier).
Repeat until the queue is empty or a limit is reached.

A minimal version makes the loop concrete:

from collections import deque
 
def crawl(seed, same_host, max_pages=500):
    queue, seen, results = deque([seed]), {seed}, []
    while queue and len(results) < max_pages:
        url = queue.popleft()
        html = fetch(url)                 # your fetch + retry logic
        results.append((url, html))
        for link in extract_links(html):  # absolute, in-scope links
            if link not in seen and same_host(link):
                seen.add(link)
                queue.append(link)
    return results

What Separates Toy Crawlers From Real Ones

The loop is simple; running it at scale is not. Production crawlers add:

Politeness. Respecting robots.txt and crawl delay so they do not overload a site. See polite crawling.
Scope control. A defined crawl scope and depth so the crawl does not wander the whole internet.
Deduplication. URL normalization and a seen-set so the same page is not fetched repeatedly.
Ordering. Breadth-first or depth-first traversal depending on the goal.
Resilience. Retries, timeouts, and handling for blocks and errors.

Crawling vs Scraping

Crawling is discovery: finding and traversing URLs. Scraping is extraction: pulling structured data out of a page. Most real jobs do both, and the distinction is covered in crawling vs scraping.

Key Takeaways

A crawler loops over fetch, parse, enqueue, tracking seen URLs to avoid loops.
The loop is trivial; politeness, scope, dedup, and resilience make it production-ready.
Crawling finds pages; scraping extracts data from them.

How ScrapeGraphAI Handles This

ScrapeGraphAI's crawl endpoint runs the full loop for you, with managed politeness, scope and depth controls, deduplication, and per-page extraction, so you describe what to collect instead of building the traversal machinery.