Fastify is a web framework focused on providing excellent performance by utilizing highly efficient JavaScript runtime engines. On the other hand, FeathersJS is a web framework specifically designed for building real-time applications, providing a set of functionalities out-of-the-box for rapid development.

1. Routing and Middleware: Fastify has a built-in router that supports both traditional routing and prefix-based routing. It also has an extensible middleware system that allows developers to add custom middleware to the request/response cycle. FeathersJS, on the other hand, uses a service-based approach for routing, where each service has its own set of CRUD operations and can be easily customized with hooks. It also provides hooks for handling middleware-like functionality at the service level.

2. Real-time Functionality: FeathersJS has native support for real-time communication through WebSockets and offers real-time APIs out-of-the-box. It uses Socket.IO to establish real-time connections, allowing bidirectional event-based communication between clients and servers. Fastify, on the other hand, is primarily focused on building fast and lightweight APIs, and does not have native support for real-time functionality. However, it can be integrated with third-party libraries like Socket.IO to achieve real-time capabilities.

3. Database Integration: Fastify doesn't provide any built-in database integration. It leaves the choice of database to the developer and provides a wide set of plugins for integrating with various databases such as MySQL, MongoDB, and PostgreSQL. Alternatively, FeathersJS comes with a powerful built-in ORM (Object-Relational Mapping) called Sequelize, which supports multiple databases and provides an intuitive way to interact with them. It also provides adapters for other popular databases like MongoDB and NeDB.

4. Scalability and Performance: Fastify is designed to be highly scalable and performant, utilizing various optimization techniques like schema validation, request/response serialization, and asynchronous programming. It is built on top of Node.js and leverages its non-blocking I/O model. FeathersJS, although it can provide good performance, is not specifically optimized for high scalability and performance like Fastify. It focuses more on providing a rich set of features for building real-time applications.

5. Authentication and Authorization: FeathersJS provides authentication and authorization out-of-the-box using its built-in feathers-authentication module, which supports various strategies like local, OAuth, and JWT authentication. It also provides role-based access control through the feathers-permissions module. Fastify, on the other hand, does not have built-in support for authentication and authorization. Developers need to use third-party libraries or implement custom solutions for these functionalities.

6. Community and Ecosystem: Fastify has gained popularity in the Node.js community due to its excellent performance and extensive plugin ecosystem. It has a large and active community, which contributes to its continuous improvement and provides support through forums and GitHub issues. FeathersJS also has a decent community, but it is relatively smaller compared to Fastify. It has a set of official plugins and various community-contributed plugins that extend its functionality.

In Summary, Fastify is a lightweight and highly performant web framework focused on building APIs, while FeathersJS is a feature-rich web framework specifically designed for real-time applications with native support for real-time communication.