The document discusses middleware and microservices architectures. Middleware makes it easier for developers to implement communication and input/output between applications and systems. It gained popularity in the 1980s as a way to connect newer apps to older legacy systems. Microservices architectures break applications into smaller, independent pieces that can be updated independently. While this improves scalability and maintainability, it also increases complexity and the security surface. The document provides examples of middleware and microservices architectures used by companies like Netflix and Twitter.