What dispatcher to use for blocking IO, with Netty?

[glasser]

I'm a little confused about what the current recommendation for doing blocking IO is, both in Kotlin Coroutines as a whole, and in ktor. I've read Kotlin/kotlinx.coroutines#261 and a bunch of issues that link to it. I assume it's withContext(Dispatchers.IO), but that doesn't actually seem to be in any documentation for either Kotlin Coroutines or Ktor!

One specific confusion I have — my understanding is that the cool thing about Dispatchers.IO (vs just Executors.newFixedThreadPool().asCoroutineDispatcher()) is that individual threads aren't permanently assigned to Dispatchers.Default vs Dispatchers.IO, and withContext(Dispatchers.IO) can often just mean adjusting the thread assignment counts without a full context switch. (I've also discovered that with the internal API (Dispatchers.Default as ExperimentalCoroutineDispatcher).blocking(N) I can create my own Dispatchers.IO-like dispatchers so that I can limit particular types of IO (sql, etc) to a specific number of threads in that thread pool.)

However, I also note that in the Netty engine (which we are using kind of by default, porting our app from Netty-based Ratpack), the dispatcher used for ApplicationCalls isn't Dispatchers.Default at all, but a Netty-specific dispatcher that just sends the coroutine directly to Netty.

I assume I still need to use some alternative dispatcher for blocking code, right? Netty's event loop also wants to not block?

But in this case is there any real point with Ktor/Netty to using Dispatchers.IO instead of Executors.newFixedThreadPool().asCoroutineDispatcher()? If the advantage of Dispatchers.IO over Executors.newFixedThreadPool().asCoroutineDispatcher() is that it can reuse a thread from Dispatchers.Default but we're not using Dispatchers.Default, should I even bother? I suppose I do still get to share threads between multiple dispatchers made with .blocking().

[glasser]

Or should we just use CIO? :)

[glasser]

Although, heh, looks like CIO uses the same code as Dispatchers.Default/IO but its own version? And doesn't expose them in an easy way to make your own separate blocking subpool either?

[e5l]

You can use Dispatchers.IO. It's a preferred way right know.