Aggregating requests in WebFlux - spring-webflux

Question

Hi I am trying with Spring WebFlux and reactor.
What I want to achieve is to aggregate some requests into one call to a third party API. And then return the results. My code looks like this:
#RestController
#RequiredArgsConstructor
#RequestMapping(value = "/aggregate", produces = MediaType.APPLICATION_JSON_VALUE)
public class AggregationController {
private final WebClient externalApiClient;
//sink that stores the query params
private Sinks.Many<String> sink = Sinks.many().multicast().onBackpressureBuffer();
#GetMapping
public Mono<Mono<Map<String, BigDecimal>>> aggregate(#RequestParam(value = "pricing", required = false) List<String> countryCodes) {
//store the query params in the sink
countryCodes
.forEach(sink::tryEmitNext);
return Mono.from(
sink.asFlux()
.distinct()
.bufferTimeout(5, Duration.ofSeconds(5))
)
.map(countries -> String.join(",", countries))
.map(concatenatedCountries -> invokeExternalPricingApi(concatenatedCountries))
.doOnSuccess(s -> sink.emitComplete((signalType, emitResult) -> emitResult.isSuccess()));
}
private Mono<Map<String, BigDecimal>> invokeExternalPricingApi(String countryCodes) {
return externalApiClient.get().uri(uriBuilder -> uriBuilder
.path("/pricing")
.queryParam("q", countryCodes)
.build())
.accept(MediaType.APPLICATION_JSON)
.retrieve()
.bodyToMono(new ParameterizedTypeReference<>() {
});
}
}
I have the Sinks.Many<String> sink to accumulate the query params from the callers of the API. When there are 5 items or 5 seconds expired, I want to call the third party API. The issue is that when I do two requests like:
GET localhost:8081/aggregate?pricing=CH,EU
GET localhost:8081/aggregate?pricing=AU,NZ,PT
On one request I get the response and on the other I get null. Also, after the first interaction the system stops working as if the sink was broken.
What am I missing here?