Probably a basic question: But we want to launch a...

# ktor

[Anders Sveen]

Probably a basic question: But we want to launch a "background" task from KTor route. We can't get KTor to return the payload before everything completes. Even if we wrap the slow part (no need for the result) and do

launch(<http://Dispatchers.IO|Dispatchers.IO>) { ... }

It does return 200 fast, but wait with the payload till after launch completes. Any pointers on how to achieve this? 🙂

[Aleksei Tirman [JB]]

So do you want to wait for a background job completion before responding?

[Anders Sveen]

No, want to respond without waiting 🙂

[Anders Sveen]

The other stuff we need to create the actual payload is already done 🙂

[Aleksei Tirman [JB]]

The launch call should work in that case

[Aleksei Tirman [JB]]

Please share a code example

[Anders Sveen]

post("company") {
    val company = companyService.create(call.receive<Company>())
    println("Before")
    launch(<http://Dispatchers.IO|Dispatchers.IO>) {
        companyService.slowUpdateThing()
        println("Inside")
    }
    println("After")
    call.respond(HttpStatus.OK_200, company.id)
}

We get the sequence of:
• Before
• After
• 200OK Headers
• Inside
• Actual response payload

[Aleksei Tirman [JB]]

The server in the following code responds immediately and prints Inside after some time:

fun main() {
    embeddedServer(Netty, port = 3333) {
        install(ContentNegotiation) {
            json()
        }

        routing {
            post("company") {
                val company = call.receive<Company>()
                println("Before")
                launch(<http://Dispatchers.IO|Dispatchers.IO>) {
                    repeat(10) {
                        delay(500)
                    }
                    println("Inside")
                }
                println("After")
                call.respondText { company.x.toString() }
            }
        }
    }.start()
}

@Serializable
data class Company(val x: Int)

[Aleksei Tirman [JB]]

Maybe the problem is that companyService is used in both places

[Anders Sveen]

Hjm, maybe. I'll have to dig down. Thanks, knowing that launch should work helps. 🙂

[Trevor Stone]

You need to launch from a different coroutine scope

[Trevor Stone]

(not best practice) but you can try replace with GlobalScope.launch and you should see the results you're expecting

[Trevor Stone]

The correct thing to do would be to define a coroutine scope for all of these background tasks, probably with a SupervisorJob and then launch from that scope

[Trevor Stone]

that way you can define error handling and even cancel the scope if it is beneficial

[Jacopo Franzoi]

Hello, not sure it’s completely related, but I was sharing a similar use case, with our proposed solution, and waiting for some feedback.. but didn’t received much so far.. but in case it fits your needs, we published a lightweight library for this. Hope this helps!

https://kotlinlang.slack.com/archives/C0A974TJ9/p1635367313063000

[Anders Sveen]

Thanks for the feedback guys. Turned out as you say that we had to do CoroutineScope(Dispatchers.IO).launch { .. }. Exactly why I am not sure, but it works. Some day I will understand CoRoutines. 😜