Im using java 17 and I would like to know an efficient way to make calls in parallel using reactive programming.

In the example shown below I have 3 calls to external services. Each one of them fills an object that will be sent in the last call: serviceFour.callServiceFour(resultMap).

@Service
public class ExternalServiceImpl {

    private final ServiceOne serviceOne;
    private final ServiceTwo serviceTwo;
    private final ServiceThree serviceThree;
    private final ServiceFour serviceFour;

    public Mono<String> makeSequentialCalls() {

        Map<String, String> resultMap = new LinkedHashMap<>();

        return serviceOne.callServiceOne()
                .doOnNext(response -> resultMap.put("service1", response))
                .flatMap(response -> serviceTwo.callServiceTwo())
                .doOnNext(response -> resultMap.put("service2", response))
                .flatMap(response -> serviceThree.callServiceThree())
                .doOnNext(response -> resultMap.put("service3", response))
                // Send the response
                .flatMap(response -> serviceFour.callServiceFour(resultMap));
    }
}

The idea is that calls to services 1, 2 and 3 are made in parallel and that finally when they finish, service 4 is called to send the complete response.

Thx.