Spring Web and Spring Reactive Web (also known as Spring WebFlux) are both dependencies in Spring Boot that are used for creating web applications, but they have different underlying architectures and use cases1234.

Spring Web is well-suited for traditional web applications that require synchronous communication3. It is based on thread pools4. This means that for each client request, a thread is allocated until the response is ready. This model works well when the application is I/O bound, meaning the time to process a request is largely spent waiting for I/O operations (like querying a database) to complete.

Spring Reactive Web (Spring WebFlux), on the other hand, is a reactive web framework that is built on top of Reactive Streams3. It is designed to handle non-blocking I/O, where a thread is not blocked while waiting for a response from a database or another service3. This means that the thread can be used to serve other client requests while waiting for the I/O operation to complete. This model is efficient in handling a large number of concurrent connections with fewer resources5. It’s based on an event-loop mechanism4.

Here are some key differences:

• Programming Model: Spring Web follows a synchronous and blocking model, while Spring WebFlux follows an asynchronous and non-blocking model13.
• Concurrency Model: Spring Web is based on a thread-per-request model, while Spring WebFlux uses an event loop model4.
• Backpressure: Spring WebFlux supports backpressure, which is a mechanism to ensure producers do not overwhelm consumers5.
• Database Access: With Spring WebFlux, you can use reactive SQL connectors like R2DBC for non-blocking database access1.
• Learning Curve: Writing reactive applications with Spring WebFlux requires understanding the reactive paradigm, which can be more challenging compared to the imperative style of programming in Spring Web1.

It’s important to note that the choice between Spring Web and Spring WebFlux depends on the specific needs of your application. If your application has high I/O and you want to handle a large number of concurrent connections efficiently, Spring WebFlux might be a good choice. However, if your application is CPU-bound and doesn’t require handling a large number of concurrent connections, Spring Web might be sufficient134.