This makes it critical that if you do any blocking I/O operations (for example interactions with Hibernate/JPA or JDBC) that you offload those tasks to a separate thread pool that does not block the Event loop.

For example the following configuration will configure the I/O thread pool as a fixed thread pool with 75 threads (similar to what a traditional blocking server such as Tomcat uses in the thread per request model):

Configuring the IO thread pool

Using @ExecuteOn

import io.micronaut.scheduling.TaskExecutors;
import io.micronaut.scheduling.annotation.ExecuteOn;
@Controller("/executeOn/people")
public class PersonController {
    private final PersonService personService;
    PersonController(
            PersonService personService) {
        this.personService = personService;
    }
    @Get("/{name}")
    @ExecuteOn(TaskExecutors.IO) (1)
    Person byName(String name) {
        return personService.findByName(name);
    }

Using @ExecuteOn

Using @ExecuteOn

import io.micronaut.http.annotation.*
import io.micronaut.scheduling.annotation.ExecuteOn
@Controller("/executeOn/people")
class PersonController (private val personService: PersonService) {
    @Get("/{name}")
    @ExecuteOn(TaskExecutors.IO) (1)
    fun byName(name: String): Person {
        return personService.findByName(name)
    }
}

An alternative to the annotation is to use the facility provided by the reactive library you have chosen. RxJava for example features a subscribeOn method which allows you to alter which thread executes user code. For example:

RxJava subscribeOn Example

RxJava subscribeOn Example

import io.micronaut.http.annotation.*
import io.micronaut.scheduling.TaskExecutors
import io.reactivex.*
import io.reactivex.schedulers.Schedulers
import javax.inject.Named
import java.util.concurrent.ExecutorService
    private final Scheduler scheduler
    private final PersonService personService
    PersonController(
            @Named(TaskExecutors.IO) ExecutorService executorService, (1)
            PersonService personService) {
        this.scheduler = Schedulers.from(executorService)
        this.personService = personService
    }
    @Get("/{name}")
    Single<Person> byName(String name) {
        return Single.fromCallable({ -> (2)
            personService.findByName(name)
        }).subscribeOn(scheduler) (3)
    }
}