Finally, a production-ready reactive system built on top of Project Reactor and reactive Spring Framework should expose all vital operational metrics. For that purpose, the Spring ecosystem offers a few options with different granularities for application monitoring.

First of all, Project Reactor itself has built-in metrics. It offers the Flux#metrics() method that enables the tracking of different events within a reactive stream. However, besides the manually registered monitoring points, an ordinary web application should track a lot of internal processes. It should also somehow report its operational metrics. For that purpose, the Spring Framework ecosystem offers the updated Spring Boot Actuator module that enables the primary metrics for application monitoring and troubleshooting. The new generation of Spring Actuator provides complete integration with WebFlux and uses its asynchronous, nonblocking programming model in order to expose metric endpoints efficiently.

The final option for monitoring and tracking the application is provided by the Spring Cloud Sleuth module, which gives out-of-the-box distributed tracing. A noticeable improvement here is the support for reactive programming with Project Reactor, so all reactive workflows within the application are correctly traced.

To summarize, as well as reactive improvements in the core framework, the Spring ecosystem takes care of production-ready features and enables detailed application monitoring, even for reactive solutions. All of these aspects are covered in Chapter 10, And, Finally, Release It!