Hi! How much does it make sense to do all the work...

# ktor

[walla]

Hi! How much does it make sense to do all the work with databases inside withContext(Dispatchers.IO) block? I mean backend application.

[gildor]

It makes sense to avoid blocking of Main or other threads that shouldn’t be blocked

[gildor]

I usually write suspend wrapper for DB or similar IO use cases to avoid potentially error-prone wrapping on call site (where you easily may forget to wrap to IO)

[walla]

Thanks

[Jorge Bo]

Hi! Regarding this topic. Is it ok to write the following kind of code in Ktor? Or Does Ktor webclient takes care of not blocking main thread?

override suspend fun find(searchAttributes: ProviderApi.SearchAttributes): List<ProviderApi.ItineraryDTO> =
    withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
        <http://httpClient.post|httpClient.post>("$serverRootUri$RESOURCE_URI") {
            contentType(ContentType.parse("application/json"))
            body = searchAttributes
        }
    }

[gildor]

You don't need Dispatcher.IO for suspend functions, in this case it does nothing, Ktor has own dispatcher for all requests and it already non blocking

[Jorge Bo]

cool

[gildor]

More general rule that there is no reason to wrap suspend calls to dispatchers, suspend functions shouldn't block threads, and if some suspend function does block it's a bug of this function

[Jorge Bo]

yep, i have read about that, but wasn’t sure as i’m new to Ktor. And the only thread that i saw in logs was eventLoopGroupProxy-4-1. 👍

[gildor]

Ktor has own dispatcher + it utilizes dispatcher of HTTP client which used by different Ktor engines
Some http clients are non blocking such as Netty, so they do not block threads anyway

[gildor]

Thread which you see is probably built in dispatcher used to dispatch client code coroutines, but under the hood uses the engine to execute actual http request

[Jorge Bo]

thanks, for the info it was very helpful

[gildor]

As I see it's actually Netty thread

[gildor]

So this is why you see only one, because it's non-blocking http server

[Jorge Bo]

just one more questions, perhaps you could give me an advice. Is it ok to write this kind of code in Ktor to execute parallel requests?

override suspend fun find(searchAttributes: ProviderApi.SearchAttributes): List<ProviderApi.ItineraryDTO> {
    return coroutineScope {
        providers.map {
            async {
                it.find(searchAttributes)
            }
        }.awaitAll()
            .flatten()
    }
}
it.find(searchAttributes) => use Ktor webclient(netty)

[gildor]

Yep, it's completely fine