Design Multithreaded Web Crawler
Last Updated: February 4, 2026
A web crawler (also called a spider or bot) is a program that systematically browses the web by following links from page to page. It starts with a set of seed URLs, fetches each page, extracts links from the content, and adds new URLs to a queue for future processing.
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Why do we need it?
- Search engine indexing: Google, Bing, and other search engines use crawlers to discover and index web pages.
- SEO analysis: Tools analyze websites to find broken links, missing metadata, or structural issues.
- Data collection: Research projects and analytics platforms gather data from public websites.
- Web archiving: Services like the Wayback Machine preserve historical snapshots of websites.
We'll design a multithreaded crawler that handles the core concurrency challenges: coordinating multiple workers, avoiding duplicate URLs, and respecting per-domain rate limits. Let's start by defining exactly what we need to build.
1. Problem Definition
Design a multithreaded web crawler that crawls URLs concurrently while preventing duplicate crawls and respecting per-domain politeness constraints.
At first glance, the requirement sounds simple: fetch pages and follow links. But once multiple worker threads compete for URLs from a shared queue, the problem becomes a real concurrency challenge.
Consider what happens when two workers both check if a URL has been visited, see that it hasn't, and both add it to the crawl queue. The same page gets crawled twice, wasting bandwidth and potentially annoying the target server. Or imagine five workers all picking URLs from the same domain simultaneously, overwhelming that server with requests and getting your crawler blocked.
- URL deduplication: Never crawl the same URL twice. If a URL has been seen before (either already crawled or currently in the queue), it should be ignored.
- Concurrent crawling: Multiple worker threads fetch pages in parallel to maximize throughput. A single-threaded crawler would be far too slow for any real-world use case.
- Per-domain politeness: Limit concurrent requests to the same domain. Hitting a server with too many simultaneous requests is rude at best and gets you blocked at worst.
- Graceful shutdown: When signaled to stop, workers should complete their current work and exit cleanly rather than terminating mid-crawl.
In short, the system must guarantee that each URL is crawled exactly once, workers operate efficiently in parallel, and no single domain is overwhelmed with requests.