Where would one find the documentation for Reactor Core (not an API javadoc), something similar to Reactor Kafka Docs, which one could read. Does it exist? How is Reactor's documentation structured, its dedicated page doesn't seem to contain reference docs for all the modules except Reactor Kafka, only APIs. Is that by design?
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While and until there's no official documentation on Reactor 3 core yet, as per @Simon Baslé's answer, I found the following resources to be of great help in learning Project Reactor:
Dave Syer's reactive series part I, part II & part III with code
Reactive Stream's official page with its API description and explanation of goal, design and scope, as well as the Project Reactor's own javadoc with its excellent visual diagrams
Developing Reactive applications with Reactive Streams and Java 8 presentation at Devoxx by Brian Clozel, Sébastien Deleuze with its speakerdeck, video, 1st and 2nd parts of live coding
Spring 5 documentation has some good resources on Reactive Web Applications
Perhaps they'll be useful for someone else getting to know Reactive Project as well.
It's not by design per se, but the reference guides don't exist for Reactor 3 core yet, it is our next focus. Reactor-Kafka development better handled this and produced documentation immediately, but it is very focused on the Kafka use-case.