Apache Tomcat vs Undertow: What are the differences?

Apache Tomcat and Undertow are both popular web servers and servlet containers used for hosting Java web applications. Let's explore the key differences between them.

1. Architecture: Apache Tomcat is a monolithic application server that includes a built-in web server, whereas Undertow is a lightweight web server commonly used as a servlet container. Tomcat provides a complete Java EE application server environment, while Undertow focuses on delivering high-performance HTTP and WebSocket services.

2. Embedded Capabilities: Undertow has stronger support for embedding within other Java applications. It provides a flexible API for programmatic configuration and direct integration, making it an ideal choice for microservices architectures and embedding within non-traditional server deployments. Tomcat, on the other hand, is primarily designed for standalone use and may require more effort to embed within other applications.

3. Performance: Undertow is known for its high performance and low resource usage. It is built with an asynchronous, non-blocking architecture that enables it to handle a large number of concurrent connections efficiently, making it suitable for high-performance applications. Tomcat, while also capable of handling a significant number of connections, may have a slightly lower performance when compared to Undertow.

4. Ease of Configuration: Tomcat provides a rich set of configuration options through XML-based configuration files, allowing fine-grained control over various server aspects. Undertow, on the other hand, offers a simpler and more lightweight configuration approach through Java code or properties files. This can be advantageous for developers who prefer programmatic configuration or desire a streamlined configuration process.

5. Ecosystem Support: Apache Tomcat has been around for a longer time and has a larger community, making it more mature and widely adopted. It has an extensive ecosystem with numerous plugins, libraries, and documentation available. Undertow, although gaining popularity, may have a smaller ecosystem and fewer resources for support and guidance.

6. Servlet Specification Compliance: Both Apache Tomcat and Undertow are compliant with the Java Servlet specification. However, Tomcat has a more extensive history and is typically quicker to implement new versions of the specification. This means that Tomcat may provide better compatibility with applications that rely on the latest servlet features or may have more specific servlet-related requirements.

In summary, Apache Tomcat is a feature-rich, widely adopted application server that provides a complete Java EE environment, while Undertow is a lightweight and high-performance web server suitable for embedding and microservices architectures. Undertow offers better performance, ease of configuration through code, and is well-suited for high-concurrency scenarios. However, Tomcat has a larger ecosystem, better community support, and may be more suitable for applications with complex Java EE requirements or specific servlet-related needs.