Transform
Description
Reactor ships with several operators that can be used to transform data.
Practice
In the first place, we will capitalize a String. Since this is a simple 1-1 transformation
with no expected latency, we can use the map operator with a lambda transforming a T
into a U.
We can use exactly the same code on a Flux, applying the mapping to each element as it
becomes available.
Now imagine that we have to call a webservice to capitalize our String. This new call can
have latency so we cannot use the synchronous map anymore. Instead, we want to represent
the asynchronous call as a Flux or Mono, and use a different operator: flatMap.
flatMap takes a transformation Function that returns a Publisher<U> instead of a U.
This publisher represents the asynchronous transformation to apply to each element. If we
were using it with map, we'd obtain a stream of Flux<Publisher<U>>. Not very useful.
But flatMap on the other hand knows how to deal with these inner publishers: it will
subscribe to them then merge all of them into a single global output, a much more useful
Flux<U>. Note that if values from inner publishers arrive at different times, they can
interleave in the resulting Flux.
