what is the difference between ResponseEntity<Mono> and Mono<ResponseEntity> as a return type of a rest controller

Question

In Spring Webflux, what's the difference between ResponseEntity<Mono> versus Mono<ResponseEntity> as a return type of a rest controller?

When is the most appropriate to what?

Following up on this question, let's say I need to return a list, or let's say several elements of Foo, there are many examples of returning Flux. Would it make sense to return ResponseEntity<Flux> or Flux<ResponseEntity>?

When I'm looking for this question, I found the same question posted here: https://github.com/spring-projects/spring-framework/issues/22614, but no answer, I searched spring docs, but find no info.

Thanks for the help.

Answer 1

Here are the various options you can make with a ResponseEntity return value:

• ResponseEntity<Mono<T>> or ResponseEntity<Flux<T>> -- this makes the response status and headers known immediately while the body is provided asynchronously at a later point. Whether the body is Mono or Flux depends on how many values the response has.
• Mono<ResponseEntity<T>> -- this provides all three -- response status, headers, and body, asynchronously at a later point. IT allows the response status and headers to vary depending on the outcome of asynchronous request handling.
• Mono<ResponseEntity<Mono<T>>> or Mono<ResponseEntity<Flux<T>>> are also possible but less common. They provide the response status and headers asynchronously first and then the response body, also asynchronously at a second point later.