# Block an asynchronous call from a foreign API

**URL:** https://discuss.kotlinlang.org/t/block-an-asynchronous-call-from-a-foreign-api/17575
**Category:** Web Development
**Created:** [May 10, 2020, 9:12am UTC](https://discuss.kotlinlang.org/t/block-an-asynchronous-call-from-a-foreign-api/17575 "2020-05-10T09:12:24Z")
**Posts on this page:** 1
**Page:** 1

<div class="post-metadata">

### Author: ![jmg.duarte](https://avatars.discourse-cdn.com/v4/letter/j/22d042/32.png) [@jmg.duarte](https://discuss.kotlinlang.org/u/jmg.duarte)
#### Post date: [May 10, 2020, 9:12am UTC](https://discuss.kotlinlang.org/t/block-an-asynchronous-call-from-a-foreign-api/17575/1 "2020-05-10T09:12:24Z")

</div>

Hello everyone!

I’m currently developing an HTTP server with Ktor backed by the [BFT-SMaRt library](https://github.com/bft-smart/library).

The BFT-SMaRt library is a Byzantine Fault Tolerant State Machine Replication library, our usage is to replicate system operations across nodes.

This library has an async call [invokeAsynchRequest](http://bft-smart.github.io/library/javadoc/bftsmart/tom/AsynchServiceProxy.html#invokeAsynchRequest(byte%5B%5D,%20bftsmart.communication.client.ReplyListener,%20bftsmart.tom.core.messages.TOMMessageType)), I need to call this method in a Ktor Route, however since it is asynchronous it gets called, goes on with it’s life and returns.  
I want to somehow block the method.

The method receives a [Listener](http://bft-smart.github.io/library/javadoc/bftsmart/communication/client/ReplyListener.html) that gets invoked whenever a reply to a request arrives, whenever we have enough we want the Ktor Route to return.

I have the following Listener:

```kotlin
class Listener(val proxy: AsynchServiceProxy, val handler: (List<TOMMessage>) -> Unit) : ReplyListener {
    val quorumSize = 4
    val replies = mutableListOf<TOMMessage>()

    override fun replyReceived(ctx: RequestContext, message: TOMMessage) {
        replies.add(message)
        LOGGER.debug("${message}")
        if (replies.size > quorumSize) {
            LOGGER.debug("handler size > quorum ${replies}")
            handler(replies)
            proxy.cleanAsynchRequest(ctx.operationId)
        }
    }

    override fun reset() {
        LOGGER.debug("reset called")
        replies.clear()
    }
}

```

It receives an `handler: (List<TOMMessage>) -> Unit` that gets called whenever I have sufficient messages, this handler should be the one signaling when the Route request is ready to return.

In the HTTP side I have this

```auto
invokeAsynchRequest(
    msg, 
    Listener(asyncServiceProxy) {
        call.respond(HttpStatusCode.OK)
    },
    TOMMessageType.ORDERED_REQUEST
)

```

This however doesn’t work because `call.respond` needs to be called from a coroutine context.

I know the question might not be fully clear but any help would be appreciated and I’ll be glad to clarify any detail!  
Thank you everyone in advance!