Running Tasks in different thread in Spring Webflux Annotated controller - spring-boot

Question

I have a spring Webflux Annotated controller as below,
#RestController
public class TestBlockingController {
Logger log = LoggerFactory.getLogger(this.getClass().getName());
#GetMapping()
public Mono<String> blockForXSeconds(#RequestParam("block-seconds") Integer blockSeconds) {
return getStringMono();
}
private Mono<String> getStringMono() {
Integer blockSeconds = 5;
String type = new String();
try {
if (blockSeconds % 2 == 0) {
Thread.sleep(blockSeconds * 1000);
type = "EVEN";
} else {
Thread.sleep(blockSeconds * 1000);
type = "ODD";
}
} catch (Exception e) {
log.info("Got Exception");
}
log.info("Type of block-seconds: " + blockSeconds);
return Mono.just(type);
}
}
How do I make getStringMono run in a different thread than Netty server threads. The problem I am facing is that as I am running in server thread I am getting basically less throughput (2 requests per second). How do I go about making running getStringMono in a separate thread.

Answer 1

You can use subscribeOn operator to delegate the task to a different threadpool:
Mono.defer(() -> getStringMono()).subscribeOn(Schedulers.elastic());
Although, you have to note that this type of blocking should be avoided in a reactive application at any cost. If possible, use a client which supports non-blocking IO and returns a promise type (Mono, CompletableFuture, etc.). If you just want to have an artificial delay, then use Mono.delay instead.

Answer 2

You can use Mono.defer() method.
The method signature is as:
public static <T> Mono<T> defer(Supplier<? extends Mono<? extends T>> supplier)
Your Rest API should look like this.
#GetMapping()
public Mono<String> blockForXSeconds(#RequestParam("block-seconds") Integer blockSeconds) {
return Mono.defer(() -> getStringMono());
}
The defer operator is there to make this source lazy, re-evaluating the content of the lambda each time there is a new subscriber. This will increase your API throughput.
Here you can view the detailed analysis.