Design a Web Crawler
What is a Web Crawler?
A web crawler, also known as a spider or bot, is a system that automatically browses the internet to discover and collect web pages. The collected data is typically stored and indexed for use in applications such as search engines, analytics, or archiving.
For example, Google Search relies heavily on web crawlers to continuously fetch and update its index of billions of pages.
In recent years, they’ve also become essential for training large language models (LLMs) by collecting massive amounts of publicly available text data from across the internet.
At its core, crawling seems simple:
- Start with a list of known URLs (called seed URLs)
- Fetch each page
- Extract hyperlinks
- Add new URLs to the list
- Repeat
However, designing a crawler that can operate at internet scale, processing billions or even trillions of pages, is anything but simple. It introduces several complex engineering challenges like:
- How do we prioritize which pages to crawl first?
- How do we ensure we don’t overload the target servers?
- How do we avoid redundant crawling of the same URL or content?
- How do we split the work across hundreds or thousands of crawler nodes?
In this article, we’ll walk through the end-to-end design of a scalable, distributed web crawler.
Let’s begin by clarifying the requirements.