Basic question Am I able to create a `Channel` launch a coro

Question

Basic question Am I able to create a `Channel` launch a coroutine in `CoroutineContext` A to send some data through the channel and then launch a coroutine in `CoroutineContext` B in order to receive that data I threw together a basic example and it looks like you must send receive to from a channel in the same `CoroutineContext` If so why is this the case

kingsley · Answer

gt Am I able to Yes gt it looks like you must send receive to from a channel in the same `CoroutineContext` No It doesn t have to be the same coroutine context You can go through the documentation on channels It shows several examples of this <https kotlinlang org docs reference coroutines channels html>

Robert Menke · Answer

Thanks for clarifying So I ve put together a really dumb example ``` fun main runBlocking produce consume val channel = Channel lt String gt suspend fun produce coroutineScope launch println Producing channel send Hello world println Done producing suspend fun consume coroutineScope launch println Consuming val value = channel receive println value ``` My understanding is that the `coroutineScope` builder creates a new scope that inherits the context of its parent which in this case is blocking I don t understand why `channel send` is blocks the thread in this case though `Done producing` is never printed

Robert Menke · Answer

If I reorganize my example to the following it works as expected ``` fun main runBlocking produce consume val channel = Channel lt String gt fun CoroutineScope produce launch println Producing channel send Hello world println Done producing fun CoroutineScope consume launch println Consuming val value = channel receive println value ```

Robert Menke · Answer

So simply put Why does this not work ``` fun main runBlocking produce consume val channel = Channel lt String gt suspend fun produce coroutineScope launch println Producing channel send Hello world println Done producing suspend fun consume coroutineScope launch println Consuming val value = channel receive println value ``` but this does ``` fun main runBlocking produce consume val channel = Channel lt String gt fun CoroutineScope produce launch println Producing channel send Hello world println Done producing fun CoroutineScope consume launch println Consuming val value = channel receive println value ```

streetsofboston · Answer

Because `coroutineScope ` only resumes returns after all its child coroutines have finished In your example the `courtineScope` calls `launch`creating a child coroutine that never finishes It never finishes because this child coroutine calls `channeld send` on a RENDEZVOUS channel the `send` call will never resume return

streetsofboston · Answer

The `channel send` never resumes because the `channel receive` is never called

streetsofboston · Answer

The `channel receive` is never called because the `consume` function is never called This is because `produce` never resumes returns in your first code sample

kingsley · Answer

In your first example `produce` and `consume` are 2 suspending methods and they both follow structured concurrency which is good But this means they will execute sequencially `consume` will only get called after `produce` has completed But this will never happen because produce will get suspended waiting for a consumer that will never come The second example is indeed the right way to go about this since each method is executed concurrently so produce and consume are able to communicate with each other If you really want 1 to work though you could set a buffer value for the channel but that doesn t really scale if you want implement a proper channel based setup

Robert Menke · Answer

Gotcha thank you < streetsofboston> and < kingsley> I m really more interested in understanding the underlying scoping mechanic here I ve typically gone with the approach in the 2nd example but I couldn t explain to my coworkers why example 1 didn t work

Robert Menke · Answer

gt Because `coroutineScope ` only resumes returns after all its child coroutines have finished being the key reason thumbsup

bdawg.io · Answer

I would highly recommend watching Roman s talk at HydraConf about Structured Concurrency which goes into the design paradigm and rationale for `coroutineScope` to best understand how you would pick one over the other <https youtu be Mj5P47F6nJg>