Kotlin Coroutines run synchronously in spite of launch method call

Question

I have a list of employees and I want to hit the API for each of them. In synchronous mode it takes a lot of time and I want to improve the performance with the coroutines. This is what I've done so far:

fun perform() = runBlocking {
    employeesSource.getEmployees()
            .map { launch { populateWithLeaveBalanceReports(it) } }
            .joinAll()
}

suspend fun populateWithLeaveBalanceReports(employee: EmployeeModel) {
        println("Start ${COUTNER}")
        val receivedReports = reportsSource.getReports(employee.employeeId) // call to a very slow API
        receivedReports { employee.addLeaveBalanceReport(it) }
        println("Finish ${COUTNER++}")
    }

When I try to run this, the code is being run synchronously and in the console I see the following output:

Start 0
Finish 0
Start 1
Finish 1
Start 2
Finish 2

which means that the calls are being done sequentially. If I replace this whole code in the populateWithLeaveBalanceReports function with delay(1000), it will work asynchronously:

Start 0
Start 0
Start 0
Finish 0
Finish 1
Finish 2

What am I doing wrong? Any ideas??

Marko Topolnik · Accepted Answer · 2019-10-22 11:39:51Z

3

Coroutines don't magically turn your blocking network API into non-blocking. Use launch(Dispatchers.IO) { ... } to run blocking tasks in an elastic thread pool. Just note that this doesn't do much more than the plain-old executorService.submit(blockingTask). It's a bit more convenient because it uses a pre-constructed global thread pool.

answered Oct 22, 2019 at 11:39

Marko Topolnik

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2 Comments

Roma Pochanin Over a year ago

Thank you for the explanation! When I use this approach, SecurityContextHolder.getContext().authentication is equal null inside the populateWithLeaveBalanceReports method. I assume it's because SecurityContext is not shared between threads? I'm having troubles calling the API as my token is stored inside the authentication object

Marko Topolnik Over a year ago

Yes, it's a thread-local holder. It wasn't obvious from your question that you're running this inside a managed Spring app. I'm not sure what's the policy in that context on launching arbitrary threads. But I think you can work around the missing token by retrieving it into a local variable before you enter the launch block.

Vitaly Roslov · Accepted Answer · 2019-10-22 12:06:40Z

0

These lines might be using a blocking code - the code that relies on blocking threads to wait for task completion.

val receivedReports = reportsSource.getReports(employee.employeeId) receivedReports { employee.addLeaveBalanceReport(it) }

It is likely that you are using non asynchronous http client or jdbc driver under the hood of reportsSource.getReports call .

If so, you should either

rewrite the code of reportsSource.getReports so it would not rely on any blocking code. This is the new / non-blocking / challenging way
use a threadPool executor to distribute executions manually instead of using coroutines. This is the old / simple way.

edited Oct 22, 2019 at 12:06

answered Oct 22, 2019 at 11:59

Vitaly Roslov

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2 Comments

Roma Pochanin Over a year ago

Yes, indeed, I'm using webflux's WebClient in order to hit the API. But I'm using it in non-reactive way with the block() method. Which http client can be used instead? Fuel library maybe??

Marko Topolnik Over a year ago

Why don't you continue using WebClient but start using it in a non-blocking way? Spring has built-in support for Kotlin coroutines.

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