I need to lookup something by id asynchronously an...

# coroutines

[spand]

I need to lookup something by id asynchronously and limit it to one lookup per id. Would something like this be the way to do it?

val cache = ConcurrentHashMap<Int, Deferred<String>>()

suspend fun getById(id: Int) : String {
    return cache.computeIfAbsent(id) {
        async { asyncAction(id) }
    }.await()
}

suspend fun asyncAction(id: Int): String = TODO()

[dave08]

Why not an actor?

[withoutclass]

Also, why use async only to immediately await? may as well just use withContext

[Daniel Tam]

Is actor better only because it doesn't block? ConcurrentHashMap is less boilerplate imo, and reads are lock free anyway

[bj0]

actor isnt' really different, it only helps organize things conceptually, but it's more than you need for this case

[bj0]

suspend fun getById(id: Int) = withContext(CommonPool) { asyncAction(id) }

[bj0]

all you need

[Daniel Tam]

I don't think that satisfies the initial use case. Also why withContext rather than simply running asyncAction ?

[bj0]

based on his first post I assumed he wanted it done on a background thread, but maybe not. what wouldn't it satisfy?

[withoutclass]

val cache = ConcurrentHashMap<Int, String>()

suspend fun getById(id: Int) : String {
    return cache.computeIfAbsent(id) {
        withContext(IO_DISPATCHER) { asyncAction(id) }
    }
}

suspend fun asyncAction(id: Int): String = "Some String"

[withoutclass]

This is probably closer to what you want

[Daniel Tam]

I mean it wouldn't satisfy the actual thing he wanted to do with the whole computeIfAbsent

[withoutclass]

looks to me like it will compute the string if it’s missing to me, so either I fundamentally misunderstand compute if absent or I misunderstand the use case

[Daniel Tam]

sorry, I meant @bj0

[bj0]

I didn't notice the cache, was focused on the coroutine stuff and assumed the logic was inside asyncAction

[spand]

@withoutclass does that compile? The compute call blocks the calling thread and it’s closure is not inline so I avoid that with the async call

[spand]

The asyncAction is io though so I guess that needs another dispatcher. Thx for that :-)

[uli]

The price is, awaiting the deferred on every cache lookup which should be pretty fast if it is already resolved. I'd say fine if it suits your use case

[withoutclass]

Unsure if it compiles I just did a quick and dirty change