Java Spring Boot WebFlux: guide with Reactive Streams Backpressure Handling
I've spent hours debugging this and Can someone help me understand I'm sure I'm missing something obvious here, but I'm developing a reactive web application using Spring Boot 2.5.4 with WebFlux and I've run into unexpected behavior with backpressure when streaming data from a large database result set. My controller method is designed to handle a stream of data, but I'm seeing a `java.lang.IllegalStateException: Request already cancelled` behavior intermittently. Here's a snippet of the code where I handle the streaming: ```java @GetMapping("/items") public Flux<Item> getItems() { return itemService.getAllItems(); } ``` And in my service, I'm using the following method to fetch items: ```java public Flux<Item> getAllItems() { return Flux.defer(() -> Flux.fromIterable(itemRepository.findAll())); } ``` The `findAll()` method returns a `List<Item>`, which is being converted into a `Flux<Item>`. I suspect this might not be the best practice for handling large datasets because it loads all items into memory at once. Additionally, I've noticed that if the client stops consuming the data before it's fully sent, the server throws the above exception. I tried switching to `itemRepository.findAll().stream()` to create a `Stream<Item>` and then using `Flux.fromStream()` like this: ```java public Flux<Item> getAllItems() { return Flux.fromStream(itemRepository.findAll().stream()); } ``` However, I still encounter backpressure issues, and the `IllegalStateException` continues if the client disconnects early. I've also added `.subscribeOn(Schedulers.boundedElastic())` to the Flux, hoping to manage the demand better, but it hasn't resolved the question. Could someone provide insights into the proper way to handle large datasets with backpressure in a reactive context using Spring WebFlux? Are there better approaches or patterns I should be considering? Any help would be greatly appreciated! For context: I'm using Java on CentOS. What's the correct way to implement this? What am I doing wrong? This is part of a larger web app I'm building. I'd be grateful for any help. Any ideas how to fix this?