How does Uni<T> works in Quarkus

I am trying to understand the behavior of Uni in Quarkus framework, after checking out their official tutorial getting started with async guide. In the service method I have made the following changes

package org.acme.getting.started.async;

import javax.enterprise.context.ApplicationScoped;

import io.smallrye.mutiny.Uni;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;


@ApplicationScoped
public class GreetingService {

    ExecutorService executor = Executors.newFixedThreadPool(10, r -> {
        return new Thread(r, "CUSTOM_TASK_EXECUTION_THREAD");
    });


    public Uni<String> greeting(String name) {
        System.out.println("greeting Executing on Thread "+Thread.currentThread().getName());
        return Uni.createFrom().item(ioSimulation(name))
                .emitOn(executor);//Infrastructure.getDefaultExecutor()
    }

    public String ioSimulation(String param){
        System.out.println("ioSimulation Executing on Thread "+Thread.currentThread().getName());
        try {
            Thread.sleep(8000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return "hello "+param;

    }

}

Then I have tested the /greeting/{name} resource, the execution was not async at all, in fact it executed all related methods within the same thread and in synchronous manner.

then what would be the difference between

@GET
    @Produces(MediaType.TEXT_PLAIN)
    @Path("/greeting/{name}")
    public Uni<String> greeting(@PathParam String name) {
    }

and

@GET
    @Produces(MediaType.TEXT_PLAIN)
    @Path("/greeting/{name}")
    public String greeting(@PathParam String name) {
    }

and how is it asynchronous? please help me to understand it.

The problem is in Uni.createFrom().item(ioSimulation(name)). You synchronously call ioSimulation before the Uni is created.

You should at least use the Supplier variant: Uni.createFrom().item(() -> ioSimulation(name)). That should help.