Is Spring Boot blocking on database operations?

Question

I am building a backend server with Spring Boot and MongoDB, basically according to this tutorial https://spring.io/guides/gs/accessing-mongodb-data-rest/ .

All my repositories are annotated with @RepositoryRestResource. The controllers for each REST repository are annotated with @RestController.

My question is, are the database operations in a REST application in Spring Boot blocking or non-blocking? That is, do the threads that handle one specific HTTP request (eg GET ) just idle when waiting for a response from the MongoDB, or do they resume to some thread pool in the meanwhile?

I have been looking up the documentation, but I could not seem to find a specification for the thread model in this case. I have also been looking at tutorials for "Making an asynchronous application in Spring Boot", but it didn't seem like they quite covered this specific issue.

Answer 1

The web layer can be either blocking on non-blocking , based on the stack you are using.

If you are using a blocking stack for your web layer (for example, one based on spring-boot-starter-web ), DeferredResult can be used to make sure you don't run out of request handling threads.

See the snippet below for sample usage:

@RestController
public class EntityController {

    private ExecutorService executor;

    @GetMapping
    public DeferredResult<Entity> getEntity(/**params**/) {
        DeferredResult<Entity> result = new DeferredResult<>();
        // request thread freed and returned to the request handling thread pool
        executor.execute(() -> output.setResult(entity)
        return result;
    }

}

For a complete example on DeferredResult , see this tutorial