Ways to log traffic in Ktor?

Question

I'm running a ktor server as the primary api for an application I'm working on and one of the endpoints delivers big files to some clients. I would like to implement some form of logging of how many bytes of the file were sent in a given request and have not been able to figure out an accurate way of doing so.

I hope I haven't missed anything totally obvious ^^

I have for example tried to extend the LocalFileContent class and added a coroutine job to log the traffic like this

    private fun listenChannel(channel: ByteReadChannel) {
        scope.launch {
            while (!channel.isClosedForRead)
                delay(500L)
            addTraffic(device, channel.totalBytesRead)
            println("Adding ${channel.totalBytesRead} bytes of traffic to the buffer.")
            listenJob.complete()
        }
    }

Which kind of works but sometimes ends up with wildly unrealistic numbers and even more so when running behind a reverse proxy for some reason.

Answer 1

You can write a custom plugin to log how big the to-be-responded body is in bytes.

fun Application.installLogger() {
    sendPipeline.intercept(ApplicationSendPipeline.After) { body ->
        val content = when (body) {
            is OutgoingContent.ReadChannelContent -> {
                object : OutgoingContent.ReadChannelContent() {
                    override val contentLength = body.contentLength
                    override val contentType = body.contentType
                    override fun readFrom(): ByteReadChannel {
                        return GlobalScope.writer {
                            val copied = body.readFrom().copyTo(channel)
                            println("Returned $copied bytes")
                        }.channel
                    }
                }
            }

            is OutgoingContent.ByteArrayContent -> {
                println("Returned ${body.bytes().size} bytes")
                body
            }

            else -> body
        }

        proceedWith(content)
    }
}

fun main() {
    embeddedServer(Netty, port = 3000) {
        install(PartialContent)
        installLogger()
        
        routing {
            get("/file") {
                call.respondFile(File("file.jpg"))
            }
        }
    }.start(wait = true)
}