`ByteReadChannel.copyTo(WritableByteChannel)` is documented to only work with blocking NIO channels

# ktor

[uli]

ByteReadChannel.copyTo(WritableByteChannel) is documented to only work with blocking NIO channels

[Deactivated User]

Where is that used? Kotlinx-io has some classes similar in name, but not from the java package. And those are marked as suspend

[uli]

I used it in my code in order to write the ByteReadChannel created via HttpResponse.content to a file channel

[Deactivated User]

let me check

[uli]

val networkChannel = httpResponse.content
FileChannel.open(tmpPath,
                StandardOpenOption.WRITE,
                StandardOpenOption.TRUNCATE_EXISTING).use { targetChannel ->
            networkChannel.copyTo(targetChannel)
        }

[uli]

And I wonder if this is simply the wrong approach

[uli]

copyTo comes from kotlinx.coroutines.experimental.io.jvm.nio. WritingKt

[Deactivated User]

what you want is to copy a ByteReadChannel from kotlinx-io into a https://docs.oracle.com/javase/7/docs/api/java/nio/channels/AsynchronousFileChannel.html ?

[uli]

I guess, that would do the trick, yes

[Deactivated User]

It end using a thread pool in any case

[uli]

I also suppose HttpResponse.content creates a non-blocking channel. is this true?

[Deactivated User]

thought they might change in the future

[Deactivated User]

yeah, sockets should be fully asynchronous

[uli]

files are not?

[Deactivated User]

An AsynchronousFileChannel is associated with a thread pool to which tasks are submitted to handle I/O events and dispatch to completion handlers that consume the results of I/O operations on the channel. The completion handler for an I/O operation initiated on a channel is guaranteed to be invoked by one of the threads in the thread pool (This ensures that the completion handler is run by a thread with the expected identity). Where an I/O operation completes immediately, and the initiating thread is itself a thread in the thread pool, then the completion handler may be invoked directly by the initiating thread. When an AsynchronousFileChannel is created without specifying a thread pool then the channel is associated with a system-dependent default thread pool that may be shared with other channels. The default thread pool is configured by the system properties defined by the AsynchronousChannelGroup class.

[uli]

but the thread pool only handles 'io events'. does not sound blocking to me.

[Deactivated User]

well your main thread not, but you can also dispatch the copy while using a FileChannel

[uli]

I mean, your quote says associated with a thread pool to which tasks are submitted to handle I/O events and `This ensures that the completion handler is run by a thread with the expected identity``
both sound non-blocking to me. It does not read as if the actual IO would be performed in the thread pool.

[Deactivated User]

aha

[Deactivated User]

well, what you can do is to create a copyTo function working with AsynchronousFileChannel

[Deactivated User]

by reading chunks from the ByteReadChannel, and writing to the AsynchronousFileChannel

[Deactivated User]

you can create a suspend extension method for handling AsynchronousFileChannel methods using completion handlers

[Deactivated User]

I can help you if you have troubles doing that, and put a sample in the ktor-samples repo

[uli]

I was just wondering if something along those lines was already there as ktor's idea is non-blocking coroutines .

[uli]

Shouldn't that go in the ktor client lib?
EDIT: into kotlinx-io

[Deactivated User]

maybe eventually, but I think there is no work on filesystem yet for kotlinx-io (unless I missed something)

[uli]

thanks for your support. I'll see if i find the time and get back to you.

[uli]

it is probably not filesystem related. My first impression is, an implementation on `AsynchronousChannel`would also be possible

[Deactivated User]

Yeah, for sure, I also think it is possible. But maybe not done yet (I have still to check in detail). If that’s the case, the first step is to create a working snippet, then a PR 🙂. I usually put those in ktor-samples, and change the code once they are available in the main codebase (so people can use that functionality in the meantime)

[uli]

Ok. Thanks

[cy]

Note that AsynchronousChannel is running a blocking operations on a separate pool in fact. This is because files are always blocking due to underlying native implementation except Windows.

[rocketraman]

Wait, Windows does something better??