Returning multiple Monos in Spring Webflux

Question

I'm trying to play about with SpringBoot 2.0 and the new reactive webFlux library. I want to know how I can return the results of two calls made via the none blocking WebClient to the caller of my Springboot API. The code I have is:

@RequestMapping("/search")
public CombinedResults perfomSearch(@RequestParam final String searchTerm) {
    Mono<SearchResponse> fasMono = searchService.getSearchResults(searchTerm, "fh");
    Mono<SearchResponse> esMono = searchService.getSearchResults(searchTerm, "es");
    CombinedResults combinedResults = new CombinedResults(fasMono, esMono);
    return combinedResults;

}

The CombinedResult object is just a POJO:

public class CombinedResults {

private Mono<SearchResponse> fasSearchResponse;

private Mono<SearchResponse> esSearchResponse;

public CombinedResults(final Mono<SearchResponse> fasSearchResponse, final Mono<SearchResponse> esSearchResponse) {
    this.fasSearchResponse = fasSearchResponse;
    this.esSearchResponse = esSearchResponse;
}

public Mono<SearchResponse> getFasSearchResponse() {
    return fasSearchResponse;
}

public void setFasSearchResponse(final Mono<SearchResponse> fasSearchResponse) {
    this.fasSearchResponse = fasSearchResponse;
}

public Mono<SearchResponse> getEsSearchResponse() {
    return esSearchResponse;
}

public void setEsSearchResponse(final Mono<SearchResponse> esSearchResponse) {
    this.esSearchResponse = esSearchResponse;
}

However if I call this the response I get back is

{
  "fasSearchResponse": {
    "scanAvailable": true
  },
  "esSearchResponse": {
    "scanAvailable": true
  }
}

Rather than the contents of the SearchResponse objects. I feel I might be missing a fundamental point of how this is supposed to work! My thought was that because the WebClient is none blocking I can fire of the two calls to the web services and then combine them without the need for completable futures etc?

Answer 1

Spring WebFlux doesn't support nested reactive types. You should instead have something like this:

Mono<SearchResponse> fasMono = searchService.getSearchResults(searchTerm, "fh");
Mono<SearchResponse> esMono = searchService.getSearchResults(searchTerm, "es");
Mono<CombinedResults> results = fasMono.zipWith(esMono, 
    (fas, es) -> {return new CombinedResults(fas, es);});

Answer 2

I think that you should return a Mono of an object that represents the model responded by this action. Suppose that CombinedResults is your model. This class should be something like:

public class CombinedResults {

    private SearchResponse fasSearchResponse;

    private SearchResponse esSearchResponse;

    public CombinedResults(final SearchResponse fasSearchResponse, final SearchResponse esSearchResponse) {
        this.fasSearchResponse = fasSearchResponse;
        this.esSearchResponse = esSearchResponse;
    }

    //... getters AND/OR setters
}

And, on your controller you do something like this:

@RequestMapping("/search")
public Mono<CombinedResults> perfomSearch(@RequestParam final String searchTerm) {
    Mono<SearchResponse> fasMono = searchService.getSearchResults(searchTerm, "fh");
    Mono<SearchResponse> esMono = searchService.getSearchResults(searchTerm, "es");
    Mono<CombinedResults> combinedResults = 
        fasMono
          .flatMap(fh -> esMono.map(es -> new CombinedResults(fh, es)));
    return combinedResults;
}

In that way you are returning a Mono object holding what you wanted as a response. The chain of operations fasMono.flatMap with esMono.map builds the CombinedResults when both Mono emit items. This combination is rather common when trying to join two Monos into one. I think you could also use zip operator to join the Monos. All of this is unrelated to WebClient. If your getSearchResults performs only async-nonblocking operations than everything is async-nonblocking.