I'm not sure if this is a coroutines question or a...

# coroutines

[xenomachina]

I'm not sure if this is a coroutines question or a ktor question...

We were using ktor's EngineMain, but needed to do some even earlier initialization (place some things in MDC globally), so we switched to using the embedded server.

Our main function ended up looking something like:

fun main() {
    val config = getConfig()
    runBlocking() {
        setupGlobalMDC(config)
        withContext(MDCContext()) {
            embeddedServer(Jetty, port = config.port) {

                // The stuff in here used to be in the function
                // referenced by ktor.application.modules
                val backgroundJob = launch { backgroundWorker() }
                serverSetup(config, backgroundJob)

            }.start(wait = true)
        }
    }
}

This mostly worked, except it turned out that backgroundJob didn't seem to be running. It's a loop that listens on a channel, and somehow it did not seem to be doing anything even when messages were sent to its channel. It worked fine before we switched to using the embedded server.

On a hunch, I tried replacing runBlocking() with runBlocking(<http://Dispatchers.IO|Dispatchers.IO>). Now it works, but I don't really understand why it didn't work in the first place.

Does anyone know why <http://Dispatchers.IO|Dispatchers.IO> is required here?

[Dmitry Khalanskiy [JB]]

I don't know much about Ktor, but I know about the behavior of `runBlocking`: by default, it uses a single-threaded coroutine dispatcher, so if something in the server was hogging the thread, there is no opportunity for the background job to execute. If you specify a multi-threaded dispatcher explicitly, the background job can execute independently.

[Dmitry Khalanskiy [JB]]

By the way, you don't have to specify the dispatcher for the whole runBlocking, you can also pass it to launch. Then only that job will execute on another dispatcher.

[CLOVIS]

Hi! This is not related to Ktor, nor Coroutines… None of the Ktor functions you are calling here are suspend , so you shouldn't need runBlocking at all. My guess is serverSetup is suspend, but you didn't send its code so I can't tell.

[xenomachina]

(I just realized that this message has been in my drafts for over a month. Sorry for the late reply, I thought I'd already sent it.)

Yes, serverSetup is suspend . Additionally, embeddedServer is a extension function on CoroutineScope . In the real code the launch is actually inside that function.

I think your answer gave me a better idea about what's going on, and why using <http://Dispatchers.IO|Dispatchers.IO> "fixes" it, though it also makes me think that's not really the correct way to accomplish this.

I looked in ktor's EngineMain, and saw that it seems to use construct a dispatcher out of Jetty's thread pool. Unfortunately, there doesn't seem to be any way to mimic this when using embeddedServer. However, I saw that embeddedServer has an overload that uses GlobalScope, so I tried switching to that, and it works as desired.

Thank you both!