Hello. I'm trying to make 3 http requests concurre...

# coroutines

[hmole]

Hello. I'm trying to make 3 http requests concurrently and return single result(like rx zip) with coroutines. Api calls are Deferred<T> with retrofit-coroutines-adapter. But they don't seem to execute in parallel. What am I doing wrong?

[hmole]

message has been deleted

[mp]

Don't call await() immediately.

[mp]

Create all three Deferreds, then await() them when you actually need the value (in your Result ctor)

[hmole]

Do I need to use awaitAll?

[mp]

val worfklows = fsApi.workflows()
val foo = ...
...
Result(workflows.await(), foo.await(), ...)

[mp]

You want to start all the requests before you block on having any of the results available.

[hmole]

That's not helping sadly, here's the log of requests

18:57:06.406 GET 200 1733ms <http://example.com/workflows>
18:57:07.916 GET 200 1479ms <http://example.com/issues/types>
18:57:09.250 GET 200 1326ms <http://example.com/issues/statuses>

[mp]

Then retrofit is probably lazily starting the requests.

[mp]

I seem to remember it doing that when I last used it years ago

[mp]

Configure retrofit to not do that, if possible, or use a more flexible http client.

[mp]

Or, syntactically messy, but might work around this issue... val foo = async { fsApi.workflows().await() }

[hmole]

Thanks, they fire concurrently now. This concludes my experiment with coroutines I think, back go rxjava.🤔

[Jonathan]

And don't use function returning deferred. Write suspending function instead....

[mp]

he can't help that, that's just retrofit doing its thing

[Jonathan]

suspend fun FsApi.loadWorkflows(): List<Workflow> { ... }
suspend fun FsApi.loadTypes(): List<IssueType> { ... }
suspend fun FsApi.loadStatuses(): List<IssueStatus> { ... }

suspend fun loadFsData(): Result = coroutineScope {
	val workflows = async { fsApi.loadWorkflows() }
	val types = async { fsApi.loadTypes() }
	val statuses = async { fsApi.loadStatuses() }
	
	Result(workflows.await(), types.await(), statuses.await())
}

[Jonathan]

@hmole. Coroutines is not about async it is about suspend which makes asynchronous code way much simpler that futures or rxjava 😉

[mp]

indeed. I've used rxjava, and you can certainly be successful with it, but suspending code is typically easier to reason about.

[Jonathan]

retrofit'api provide function which returns deferred?

[mp]

I'm just guessing, but I bet the existing future-based retrofit support just tacks on .asDeferred()

[Jonathan]

In this case the code is still simpler that what we'd get with RxJava:

suspend fun loadFsData(): Result  {
	val workflows = fsApi.workflows().asDeferred()
	val types = fsApi.types().asDeferred()
	val statuses = fsApi.statuses().asDeferred()
	
	return Result(workflows.await(), types.await(), statuses.await())
}

[Jonathan]

Arf, I bet he uses: https://github.com/JakeWharton/retrofit2-kotlin-coroutines-adapter

[Jonathan]

where function return Deferred instead of being suspending function, which is a bit sad.

[Jonathan]

It was probably not possible to do better though

[Jonathan]

suspend fun loadFsData(): Result  {
    val workflows = fsApi.workflows()
    val types = fsApi.types()
    val statuses = fsApi.statuses()
    
    return Result(workflows.await(), types.await(), statuses.await())
}

[dekans]

Maybe this one is better suited:
https://github.com/gildor/kotlin-coroutines-retrofit

[Zach Klippenstein (he/him) [MOD]]

first-class suspend function support is a WIP: https://github.com/square/retrofit/pull/2886