OutOfMemoryError and high CPU usage when streaming large datasets

I am a full-stack developer working on a Java application using Spring Boot 3.x and Spring WebFlux. I am trying to stream a large dataset (around 500k records) from MongoDB using Server-Sent Events (SSE), but under high load, and the application crashes.

CPU usage spikes to 100%.

Application throws

OutOfMemoryError: Java heap space.

Heap dumps show that internal reactive objects and DTOs are not being garbage collected fast enough.

@RestController
@RequestMapping("/api/data")
public class DataController {
    @Autowired private DataService service;

    @GetMapping(value = "/stream", produces = MediaType.TEXT_EVENT_STREAM_VALUE)
    public Flux<DataDTO> stream() {
        return service.getData();
    }
}

@Service
public class DataService {
    @Autowired private MyRepository repo;

    public Flux<DataDTO> getData() {
        return repo.findAll() // Returns Flux<RawData>
                .map(raw -> new DataDTO(raw.getId(), raw.getName()));
    }
}