spring-integration: how to deliver deferred details as SSE - spring-integration

Question

I have a list of items which I want to retrieve and return as fast as possible.
For each item I also need to retrieve details, they may be returned a few seconds later.
I could of course create two different routes with HTTP gateways and request first the list, then the details. However, I then have to wait until all details have arrived. I want to send back the list immediately and then the details as soon as I get them.
UPDATE
Following Artem Bilan's advice my flow returns a Flux as payload which merges the list of items as a Mono and the processed items as a Flux.
Note that the example below simulates detail processing of the items by calling toUpperCase; my real use case requires routing and outgoing calls to get the details for each item:
#Bean
public IntegrationFlow sseFlow() {
return IntegrationFlows
.from(WebFlux.inboundGateway("/strings/sse")
.requestMapping(m -> m.produces(MediaType.TEXT_EVENT_STREAM_VALUE))
.mappedResponseHeaders("*"))
.enrichHeaders(Collections.singletonMap("aHeader", new String[]{"foo", "bar"}))
.transform("headers.aHeader")
.<String[]>handle((p, h) -> {
return Flux.merge(
Mono.just(p),
Flux.fromArray(p)
.map(t -> {
return t.toUpperCase();
// return detailsResolver.resolveDetail(t);
}));
})
.get();
}
That comes closer to my goal. When I request data from this flow using curl, I get the list of items immediately and the processed items slightly later:
λ curl http://localhost:8080/strings/sse
data:["foo","bar"]
data:FOO
data:BAR
While simply converting the string to uppercase works fine, I have difficulty to make an outgoing call for details using WebFlux.outboundGateway. The detailsResolver in the commented out code above is defined as follows:
#MessagingGateway
public interface DetailsResolver {
#Gateway(requestChannel = "itemDetailsFlow.input")
Object resolveDetail(String item);
}
#Bean
IntegrationFlow itemDetailsFlow() {
return f -> f.handle(WebFlux.<String>outboundGateway(m ->
UriComponentsBuilder.fromUriString("http://localhost:3003/rest/path/")
.path(m.getPayload())
.build()
.toUri())
.httpMethod(HttpMethod.GET)
.expectedResponseType(JsonNode.class)
.replyPayloadToFlux(false));
}
When I comment in the detailsResolver call and comment out t.toUpperCase, the outboundGateway seems to be set up properly (the log says Subscriber present, Demand signaled) but never gets a response (doesn't reach a breakpoint in ExchangeFunctions.exchange#91).
I have ensured that the DetailsResolver itself is working by getting it as a bean from the context and invoking its method - that gives me a JsonNode response.
What can be the reason?

Answer 1

Yes, I wouldn't use toReactivePublsiher() there because you have a context of the current request. You need fluxes per request. I would use something like Flux.merge(Publisher<? extends I>... sources), where the first Flux is for items and the second is for details per item (something like Tuple2).
For this purpose you really can use something like this:
IntegrationFlows
.from(WebFlux.inboundGateway("/sse")
.requestMapping(m -> m.produces(MediaType.TEXT_EVENT_STREAM_VALUE)))
And your downstream flow should produce Flux as a payload for reply.
I have a sample like this in test cases:
#Bean
public IntegrationFlow sseFlow() {
return IntegrationFlows
.from(WebFlux.inboundGateway("/sse")
.requestMapping(m -> m.produces(MediaType.TEXT_EVENT_STREAM_VALUE))
.mappedResponseHeaders("*"))
.enrichHeaders(Collections.singletonMap("aHeader", new String[] { "foo", "bar", "baz" }))
.handle((p, h) -> Flux.fromArray((String[]) h.get("aHeader")))
.get();
}