Is there a way to split/buffer a ByteReadChannel? My use case is downloading a file from a http URL ...

# ktor

[Quy D X Nguyen]

Is there a way to split/buffer a ByteReadChannel? My use case is downloading a file from a http URL on GET request and then streaming it to the client as as well as a local file.

[e5l]

Hi @Quy D X Nguyen, it looks like: https://api.ktor.io/1.3.2/io.ktor.util/split.html

[Quy D X Nguyen]

So this code does not respond, but the commented line works fine.

get("/") {
	call.respondBytesWriter(ContentType.defaultForFileExtension(".jpg")) {
		client.get<HttpStatement>("<https://chandra.harvard.edu/photo/2017/arp299/arp299_4k.jpg>").execute { response: HttpResponse ->
			//response.receive<ByteReadChannel>().joinTo(this, false)
			val (channelResponse, channelFile) = response.receive<ByteReadChannel>().split(this@embeddedServer)
			
			channelFile.joinTo(File("image").writeChannel(), true)
			channelResponse.joinTo(this, false)
		}
	}
}

[e5l]

Could you move one of your consumers to launch { } ? The channels should be consumed simultaneously.

[Quy D X Nguyen]

I'm not too familiar with coroutines, so I don't know where you want this launch{} call

[Quy D X Nguyen]

I edited my sample with me also consuming the channelFile, but this still does not work.

[Quy D X Nguyen]

Does ktor have a coroutinecontext for blocking IO?

[Quy D X Nguyen]

Apparently, the default <http://Dispatchers.IO|Dispatchers.IO> context for writeChannel() does not play well with split(). Do you know why?

val fileSave = newFixedThreadPoolContext(8, "fileSave")
...
                        channelFile.joinTo(File("image").writeChannel(fileSave), true)

This works,

[Quy D X Nguyen]

It appears that the split() gums everything up, but they work individually