Designing a Web Crawler: A Comprehensive Guide

Kacper Bąk
7 min readMar 29, 2023

Indexing robots, also known as robots or spiders, have become essential tools for search engines, data mining, and other applications that require the collection and indexing of large amounts of web content. In this article, I will discuss design principles and considerations for building an efficient, scalable, and robust indexing robot.

Understanding the Problem and Establishing Design Scope

The first step in designing a web crawler is to understand the problem and establish the design scope. The goal of a web crawler is to download web pages from a set of URLs, extract URLs from those pages, and follow those links to collect new content. The basic algorithm seems simple, but designing a web crawler that can scale to billions of web pages requires careful planning and implementation.

Before jumping into the design, it is important to ask questions to understand the requirements and clarify assumptions. Some of the questions to ask include:

  1. What is the main purpose of the web crawler? Is it for search engine indexing, data mining, or something else?
  2. How many web pages does the web crawler need to collect per month?
  3. What content types are included? Is it HTML only or other content types such as PDFs and images as well?
  4. Should the web crawler consider newly added or edited web pages?
  5. How do we handle web pages with…