Is there a way to cap the concurrency with coroutines? when I run a lot of

async { ... http request ... }

at some point I am hitting connection refused, because the other end can not keep up.

If you are using

suspend

-based http-client (Ktor or any adapter around existing async clients), then there is no built-in solution and limiting threads count won’t help. https://github.com/Kotlin/kotlinx.coroutines/issues/460 may help, feel free to vote for it. As ad-hoc solution you can use suspend-based Semaphore (it was posted in a thread above)