This document discusses scalable web application architectures. It begins by defining scalability and explaining the objectives of scalable systems, including handling traffic and data growth while maintaining system maintainability. There are two main types of architectures discussed: network-based architectures and application-based architectures. Network-based architectures focus on load balancing and distributing traffic across servers, while application-based architectures separate an application into tiers or layers, with the most common being three-tier architectures using a model-view-controller (MVC) pattern. The document provides an overview of common scalability patterns including caching, databases, and file storage solutions.