Hi there. Kotlin/coroutines newbie here. I have a...

# coroutines

[Gus]

Hi there. Kotlin/coroutines newbie here.

I have a function that opens a WebSocket connection to a server. The underlying protocol is very simple: The server sends a binary message (tagged with a unique id) and the client acknowledges it's safely stored the message by replying with a text message representing the message's unique id. I'm trying to model this with coroutines, but without leaking any details about the underlying implementation (e.g., the use of WebSockets). I basically want the user of my function to do something like this:

retrieveMessages().map {
    File("the path").writeBytes(it.message)
    it.ack()
}

It's crucial that the ACK message be only sent to the server if and when the the incoming message is safely stored. That's why the ACK is explicit.

I started to model this with a Flow<*> but I'm finding it rather cumbersome because I'd have to track all the unacknowledged messages so I won't close the connection too soon, which I think is a code smell since flows are supposed to be cold streams and therefore the underlying resource(s) should only be active while the flow is active. So for that reason I'm starting to question whether I should be using flows (or something like it) here, compared to just getting a callback to process each incoming message and sending the ACK to the server when the callback completes without errors; e.g.:

retrieveMessages { message -> File("the path").writeBytes(message) }

Thoughts?

[travis]

Your last code snippet looks reasonable.

For a WebSocket, assuming you're controlling the connection separate from the data stream, then it would be hot.

You can have hot-flows (e.g. broadcastChannel.asFlow()). You could send to the BroadcastChannel upon receipt of a message. Then after the send call completes reply with the ack.

Just be aware that if you don't have any subscribers of the BroadcastChannel then the message will be lost (same would also be true of a listener/callback style if you don't have any listeners registered).

[Gus]

Thanks Travis!