1. The Architecture: Undertow is a lightweight web server that is built around a non-blocking I/O model, allowing it to efficiently handle thousands of concurrent connections with very low resource consumption. On the other hand, Nginx is also designed to handle a large number of simultaneous connections, but it uses an event-driven architecture that relies on asynchronous and event-driven processing, allowing it to efficiently serve static files and act as a reverse proxy for load balancing and caching.
2. Configuration and Extensibility: Undertow uses a configuration file in XML format to define its settings and deployments, making it customizable and easy to manage. Nginx, on the other hand, uses a simple and readable configuration file written in its own scripting language that provides flexibility and extensibility to customize its behavior and integrate with other systems.
3. SSL/TLS Support: Both Undertow and Nginx provide support for SSL/TLS encryption protocols to secure web communications. However, Nginx has built-in capabilities to handle SSL/TLS termination and certificate management, making it more suitable for handling HTTPS traffic without the need for additional components or configurations.
4. HTTP/2 Support: Nginx has better native support for the HTTP/2 protocol, which brings significant performance improvements over the older HTTP/1.1 protocol. By contrast, Undertow requires additional configuration and modules to support HTTP/2, which may require more effort and expertise to set up and optimize.
5. Embedded Mode: Undertow can be embedded within Java applications, allowing developers to create lightweight and self-contained web servers as part of their application deployments. On the other hand, Nginx is typically used as a standalone web server or a reverse proxy and does not have built-in capabilities for embedding within applications.
6. Operating System Support: Nginx is a cross-platform web server that can run on a wide range of operating systems, including Windows, Linux, macOS, and various Unix flavors. Meanwhile, Undertow is primarily tailored for Java applications and is supported on Java Virtual Machines (JVMs) running on different platforms.

In summary, Undertow is a lightweight Java-based web server that excels in terms of performance and resource efficiency, while Nginx offers a versatile and highly customizable solution with better support for SSL/TLS, HTTP/2, and operating system diversity.