I am attempting to perform parallel API calls with kotlin Ho

Question

I am attempting to perform parallel API calls with kotlin However the second api call is cancelled when the first call s await method is called How do i force all methods to finish executing before returning from the method The code snippet ```suspend fun isFinished token String Boolean return coroutineScope val a = async updateProfile token = token val b = async updateStatus token = token a await amp amp b await ``` Methods updateProfile token String and updateStatus token String are the api methods

Joffrey · Answer

The code looks correct If the first `await` cancels the other `async` it probably means the first call failed with an exception You should check that

Joe Altidore · Answer

There is no exception If i swap the position of the awaits then b await fails

Joe Altidore · Answer

It also works fine when i use runBlocking

Joffrey · Answer

How do you tell `b await ` fails when you swap Is that not an exception that is thrown

Joffrey · Answer

If it works fine when using `runBlocking` it could be that both calls are using something that is not thread safe Did you check that It would be nice if you could share enough code to reproduce the problem

uli · Answer

Another point to look at is boolean short circuit If a returns false b is never awaited Currently not sure if your coroutineScope block is enough to compensate that

Joe Altidore · Answer

< Joffrey> I get an JobCancellationException on the endpoint That s how I know it fails

Joe Altidore · Answer

However It finally worked when I did used the approach below ```suspend fun isFinished token String Boolean val a = CoroutineScope async updateProfile token = token val b = CoroutineScope async updateStatus token = token val result = CoroutineScope async a await amp amp b await return result await ``` But I feel there could be a better way

uli · Answer

I guess it s the boolean short circuit I guess your coroutineScope block handgs waiting for b to be awaited until it is cancelled by some outer mechanism

uli · Answer

Try extracting a await and b await to variables and then apply amp amp to the variables and everything should work

Joffrey · Answer

< Joe Altidore> please don t do this You re creating scopes that you never cancel and just spawn coroutines without structured concurrency which is usually a bad sign Also you re creating 3 scopes why Did you figure out what was wrong in your initial code Don t try random stuff without understanding what s wrong in the correct code otherwise you might just be hiding a deeper problem

louiscad · Answer

Also creating scopes on the fly can lead to coroutines being garbage collected mid way which is extremely hard to diagnose If one of the coroutines gets cancelled it s because they get cancelled either from within or from outside though it seems it s from within in you case < Joe Altidore> Try replacing your `updateXxx` functions with `delay ` calls instead you ll see it works perfectly with your original snippet

Joe Altidore · Answer

< Joffrey> I was very skeptical of the approach too The `updateProfile ` and `updateStatus ` methods are recursive methods and I am suspecting that to be the reason

Joe Altidore · Answer

< louiscad> I will love to try that but have limited knowledge on the approach you ve suggested More help will be appreciated

louiscad · Answer

< Joe Altidore> You use this instead ```suspend fun isFinished token String Boolean return coroutineScope val a = async delay 3 seconds true val b = async delay 2 seconds true a await amp amp b await ``` You ll see that it finishes successfully

louiscad · Answer

BTW at the line where you have the `await ` calls the parenthses around the calls are useless I edited my snippet to reflect that

louiscad · Answer

Recursive functions are rareley a good idea unless you re doing what is sometimes called sports programming which is different from writing production code that needs to be reliable and efficient

Joe Altidore · Answer

I guess I should state the use case after all I have an REST API service that returns a list of items I need to retrieve the items and store them in a local db However the service implements pagination hence the use of recursion

louiscad · Answer

Why would recursion be needed for pagination

Joe Altidore · Answer

I need to ensure that all items are queried successfully before returning a result

Joffrey · Answer

Still pagination really is inherently iterative it feels strange to use recursion for that

Joe Altidore · Answer

let me share one of the methods

Joe Altidore · Answer

```private suspend fun getExpense offset Int token String Response val expense = api syncExpense offset token return if expense is Resource Success val res = expense data as ExpenseDto Data if res rows isNotEmpty expenseDao addExpenses res rows map it toExpenseEntity if res rows size == LIMIT return getExpense offset + LIMIT token res else expense as Resource Failure data ```

uli · Answer

< louiscad> what happens if your mocked workloads return false instead

louiscad · Answer

< uli> It waits until both are done anyway just like in the original version

louiscad · Answer

< Joe Altidore> Your code can lead to a very very awful UX Today I was in the train just like yesterday where I spent around 9 hours there In such a situation network keeps dropping packets from time to time because of the speed which means a long chain like yours is almost guaranteed to break because of a single failure Instead just treat your data as paged saving one thing after another or doing multiple calls concurrently for multiple pages so if the network is stable for a few seconds it s all done

louiscad · Answer

An alternative is to change the backend so you don t have to do many network calls and can do a single big one but not too big of course

Joe Altidore · Answer

I wrote the backend service with ktor and it has the potential of becoming large since its a data synchronisation service

louiscad · Answer

What do you want to achieve on the client side I don t quite get the use case with such a vague function signature `suspend fun isFinished token String Boolean`

Joe Altidore · Answer

Well upon login I try to sync the user data which is obtained via different service in this case get Expense and get Funds services which both implements pagination