Combining kotlin flow results - kotlin

Question

I'm wandering if there is a clean way to launch a series of flows in Kotlin and then, after their resolution, perform further operations based on whether they succeeded or not
For example's sake I need to read all integers from a DB (returning them into a flow), check if they are even or odd against an external API (also returning a flow), and then remove the odd ones from the DB
In code it would be something like this
fun findEven() {
db.readIntegers()
.map { listOfInt ->
listOfInt.asFlow()
.flatMapMerge { singleInt ->
httpClient.apiCallToCheckForOddity(singleInt)
.catch {
// API failure when number is even
}
.map {
// API success when number is odd
db.remove(singleInt).collect()
}
}.collect()
}.collect()
}
But the problem I see with this code is the access to the DB deleting entries done in parallel, and I think a better solution would be to run all API calls and somewhere collect all that failed and all that succeeded, so to be able to do a bulk insertion in the DB only once instead of having multiple coroutines do that on their own

Answer 1

In my opinion, it's kind of an anti-pattern to produce side effects in map, filter, etc. A side effect like removing items from a database should be a separate step (collect in the case of a Flow, and forEach in the case of a List) for clarity.
The nested flow is also kind of convoluted, since you can directly modify the list as a List.
I think you can do it like this, assuming the API can only check one item at a time.
suspend fun findEven() {
db.readIntegers()
.map { listOfInt ->
listOfInt.filter { singleInt ->
runCatching {
httpClient.apiCallToCheckForOddity(singleInt)
}.isSuccess
}
}
.collect { listOfOddInt ->
db.removeAll(listOfOddInt)
}
}
Parallel version, if the API call returns the parameter. (By the way, Kotlin APIs should not throw exceptions on non-programmer errors).
suspend fun findEven() {
db.readIntegers()
.map { listOfInt ->
coroutineScope {
listOfInt.map { singleInt ->
async {
runCatching {
httpClient.apiCallToCheckForOddity(singleInt)
}
}
}.awaitAll()
.mapNotNull(Result<Int>::getOrNull)
}
}
.collect { listOfOddInt ->
db.removeAll(listOfOddInt)
}
}