Throttle or Limit Kotlin CoRoutine count

I am trying to hit an http service from my coroutine. I might have to hit the service a million times. I prefer to do it in parallel as those are independent of each other, at the same time I don't want to DOS that service. I want to throttle my coroutine (some kind of back pressure)

I know I can batch the requests to acceptable concurrent number of requests. But, that's too boilerplate I think. Is there any http library out there handles this in an idiomatic way

Solution

Option 1:

OK HTTP with retrofit can limit number of requests:

Dispatcher dispatcher = new Dispatcher();
dispatcher.setMaxRequests(100);
dispatcher.setMaxRequestsPerHost(10);
OkHttpClient client = new OkHttpClient.Builder()
    .dispatcher(dispatcher)
    .build();

You can see the example here: OkHttpClient limit number of connections?
There is an adapter for coroutines: https://github.com/JakeWharton/retrofit2-kotlin-coroutines-adapter
So both together will give you what you need.

Option 2:

Use Retrofit with AsyncHttpClient which also has an adapter: https://github.com/AsyncHttpClient/async-http-client/tree/master/extras/retrofit2

Then limit the resources like this:

AsyncHttpClient http = asyncHttpClient(config()
    .setMaxConnections(500)
    .setMaxConnectionsPerHost(200)
    .setPooledConnectionIdleTimeout(100)
    .setConnectionTtl(500)
);

That example is from the wiki: https://github.com/AsyncHttpClient/async-http-client/wiki/Connection-pooling

Option 3:

Use one of the above clients (or any other one) without retrofit. Then wrap the callback yourself or find a lib that already did that (exists for many callback types): https://github.com/Kotlin/kotlin-coroutines/blob/master/kotlin-coroutines-informal.md#wrapping-callbacks