Hi guys, wanted to understand the general practive of releasing resources when a Ktor Server shutsdo...

# ktor

[Mohammed Toufeeq Ahamed]

Hi guys, wanted to understand the general practive of releasing resources when a Ktor Server shutsdown, say like closing a database driver and connections or completing all the running requests, came across #arrows implementation of SuspendApp and Resources concept. But want to know how everyone else does ?

[simon.vergauwen]

I am the author behind SuspendApp, and Resource so 😅

But afaik the alternative is to manually manage closing resources using application.enviroment.monitor(ApplicationStopping),
or addShutdownHook { }.

Afaik Ktor-koin doesn't automatically take into account AutoCloseable like Spring does, so when using koin-ktor you still need to manually take care of this.

I'm curious if there are other options, SuspendApp and Resource are more general than just Ktor though. You can also use it in CLI applications, for producing/consuming records from Kafka for example.

[Mohammed Toufeeq Ahamed]

Understood, but, in a multi-module app where database is very far away from main module where SuspendApp in is setting up the resources, it would really hard to get access to database. Say the structure of app is as below, there is no way, I can get access to Database Module in App Module.

[simon.vergauwen]

That is where Resource is really great 😅 It's a typed (compile-time) version of what Spring does with runtime introspection / reflection.
To be honest, I rather call it ResourceScope, and I see it as CoroutineScope. That's also how I explain it in my talk,

https://youtu.be/A69_t_oEP_E?feature=shared▾

.

I also have an example here, https://github.com/nomisRev/ktor-arrow-example/blob/main/src/main/kotlin/io/github/nomisrev/env/Dependencies.kt.

So in every module you can expose suspend fun ResourceScope.dependency(): A for "structured resources" and then you can call these functions, similarly how you do fun CoroutineScope.concurrency(): A for structured concurrency.

I've considered splitting ResourceScope completely from Arrow, so you don't have any additional dependencies coming in. So you can just use SuspendApp and Resource without anything else from Arrow on the classpath. Would that lower the threshold for people to use it? 🤔

[Mohammed Toufeeq Ahamed]

Got you, I didn't wanted to import anything from Service Module to App Module. I will give it a try

[simon.vergauwen]

You need to somehow through the config, right?

[simon.vergauwen]

Even if you were using Spring

[Mohammed Toufeeq Ahamed]

Its a Ktor App, Config is imported into both modules, not the other way round, I will try to restructure

[simon.vergauwen]

Ow, sorry I missread your diagram. Yes, but I was referring to Spring because I though you wanted to do something similar to @AutoWired where in App module you don't touch any code from Service module.

Somewhere there is some code constructing service and repository because those are dependencies of UseCase, right? Which are probably hiding the dependencies?

[Mohammed Toufeeq Ahamed]

Yeah correct, each module is dependent on its lower module. I will try to play with Config Module and change its role to handle resources to be passed to SuspendApp.
Koin also has hooks provided for Application Shutdown like

environment.monitor.subscribe(KoinApplicationStarted) {
        <http://log.info|log.info>("Koin started.")
}

But again these are dependent on Application's context

[simon.vergauwen]

Yes, I think that's pretty much the same as application.enviroment.monitor(ApplicationStopping).

[simon.vergauwen]

Also, that technique doesn't really work great with streaming events for me. https://github.com/arrow-kt/suspendapp/tree/main#suspendapp-with-kafka

I mean it's possible but a bit strange.