Add reactive mongodb sample

[rajadilipkolli]

adds spring-boot-data-mongdb-responsive sample using new spring data mongodb reactive starter

[mp911de]

I'm not sure about the example.

The code shows reactive API usage within a synchronous piece of code (CommandLineRunner) and heavily makes use of .block() methods. I think an example might make more sense in a different context, such as WebFlux or inserting data asynchronously (Flux.interval(…)).

[rajadilipkolli]

@mp911de agreed that block shouldn't be used. what is the reactive equivalent of CommandLineRunner ?

I have used this in my WebFlux project and copy pasted from there,hence it makes sense there but not here.

[rajadilipkolli]

@mp911de Can you please review my latest changes and let me know if I need to improve on anything in this PR

[snicoll]

I am not sure we should add a reactive/non reactive variant of each spring data sample, especially considering how the setup is similar. It feels to me that a Spring Data sample is a better place for it and perhaps such sample already exists.

Let's see what the rest of the team thinks.

[mp911de]

We have a few samples in our Spring Data Examples repository. We aren't entirely happy with our current samples because the majority of environments (CommandLineRunner, JUnit tests) follows an imperative programming model which is a limiting factor. An end-to-end reactive flow isn't possible without the appropriate runtime environment thus we are forced to use blocking calls or test facilities (such as StepVerifier).

A reactive sample would make sense in the context of WebFlux, but then it's no longer a pure MongoDB example.

[bclozel]

I agree with @snicoll, there's no point in adding all the possible variants out there, especially when the setup is so similar.

Now about the CommandLineRunner, you definitely have a point.
I've seen a lot of code samples out there just calling Flux.subscribe(Consumer) — this won't guarantee that the processing of that Flux is done before the application is shut down/the webapp is started.

We could create something like:

@FunctionalInterface
public interface ReactiveCommandLineRunner {
  Mono<Void> run(String... args);
}

At processing time, we would need to concat all of them (to guarantee a processing order) and block, like:

firstMono.concatWith(secondMono).then().block();

What do you think?

[snicoll]

@bclozel thanks for the reminder. Can we move the CommandLineRunner conversation to a dedicated issue? In the meantime, we're going to close this one as the Spring Data samples are already covering this.