A brief history of web crawlers

Web crawlers have a long and interesting history. Early web crawlers collected statistics about the web. In addition to collecting statistics about the web and indexing the applications for search engines, modern crawlers can be used to perform accessibility and vulnerability checks on the application. Quick expansion of the web, and the complexity added to web applications have made the process of crawling a very challenging one. Throughout the history of web crawling many researchers and industrial groups addressed different issues and challenges that web crawlers face. Different solutions have been proposed to reduce the time and cost of crawling. Performing an exhaustive crawl is a challenging question. Additionally, capturing the model of a modern web application and extracting data from it automatically is another open question. What follows is a brief history of different techniques and algorithms used from the early days of crawling up to the recent days. We introduce criteria to evaluate the relative performance and objective of web crawlers. Based on these criteria we plot the evolution of web crawlers.