Quarkus using eventloop thread for Kotllin suspend functions

[mpienaar-t]

Hello Friends!

I've been using Quarkus with Kotlin in an application that is destined for production, with the quarkus-kotlin extension installed, and I noticed that I was getting a warning sometimes about the eventloop thread being blocked for more than 2 seconds.
This was of course a concern to me. My understanding, after reading https://quarkus.io/blog/resteasy-reactive-smart-dispatch/ was that resource endpoints should automatically offload the work to a worker or IO thread, depending on circumstances mentioned in the document. I created a new app using https://quarkus.io/guides/kotlin and experimented a bit. I noticed that if your endpoint function is a suspend function, then if you print out the thread's name it is an eventloop thread.
eg.

    @GET
    @Produces(MediaType.TEXT_PLAIN)
    suspend fun hello(): String {
        println("Thread Name: ${Thread.currentThread().name}") // Prints out "Thread Name: vert.x-eventloop-thread-0"
        return "Hello from RESTEasy Reactive"
    }

If you remove the suspend keyword, then the thread's name is an executor thread.
eg.

    @GET
    @Produces(MediaType.TEXT_PLAIN)
    fun hello(): String {
        println("Thread Name: ${Thread.currentThread().name}") // Prints out "Thread Name: executor-thread-0"
        return "Hello from RESTEasy Reactive"
    }

Is this behaviour expected? I am guessing that this is the cause of the occasional "eventloop thread being blocked" warning.

Thanks so much for your help.

[quarkus-bot[bot]]

/cc @evanchooly (kotlin), @geoand (kotlin)

[geoand]

Hi,

Yes, what you are seeing with the thread names is the expected behavior

[geoand]

You need to avoid performing any long running, operations in those methods

[mpienaar-t]

If this endpoint makes a database/api call or something that could take long - where would you do it? Is there a reference example for how to handle these situations?

[geoand]

In that case you probably either want to not have the function be a suspend function or use Kotlin's runBlocking

[mpienaar-t]

Ok thanks so much. I really appreciate your quick responses.

[geoand]

You are very welcome.