How can I use `IO` with Spring-boot for asynchronous servlets, just like returning a `CompletableFut...

# arrow

[Gopal S Akshintala]

How can I use IO with Spring-boot for asynchronous servlets, just like returning a CompletableFuture from a REST Controller in Spring-boot?

[pakoito]

the question is too general and I'm not familiar with Spring

[pakoito]

could you please add some code to the question? maybe I can get an idea

[Bob Glamm]

async Spring Boot uses Reactor, which IIRC has controller method responses of Mono<A> or Flux<A> (single A or stream of A responses, respectively)

[Gopal S Akshintala]

@Bob Glamm I am not speaking about async non-blocking

[Bob Glamm]

How can I use IO with Spring-boot for asynchronous servlets

[Bob Glamm]

Which part of that is not speaking about async non-blocking

[Gopal S Akshintala]

Something that look like this

@PostMapping("/add-book")
    public CompletableFuture<CatalogueBookResponse> addBookToCatalogue(@Valid @RequestBody AddBookToCatalogueRequest addBookToCatalogueRequest) {

[Bob Glamm]

oh you just need the Future bridge, not the Reactor bridge

[Gopal S Akshintala]

yes

[Gopal S Akshintala]

I can think of

CompletableFuture
                .supplyAsync(() -> IO.unsafeRunSync)

[Gopal S Akshintala]

in return

[Gopal S Akshintala]

at the controller

[Gopal S Akshintala]

I think Spring understands CompletableFuture and this is the only way I can think of converting IO to CompletableFuture at the controller

[Bob Glamm]

That's probably the cleanest approach

[Bob Glamm]

unless there is a way to make a global response converter from IO<A> -> CompletableFuture<A>

[Bob Glamm]

then all your controllers could just return IO

[Gopal S Akshintala]

Yes, that way my question, if there is any such way from the arrow library

[Bob Glamm]

That would be something exposed by Spring Boot, not Arrow

[Gopal S Akshintala]

hmm ya

[Bob Glamm]

HttpMessageConverter, maybe, but I'm not sure if more than one of those get called per controller

[Bob Glamm]

e.g. cleanest is IO<A> -> CompletableFuture<A> -> some external representation of A, like JSON

[Bob Glamm]

ideally that is two converters, but I do not know if Spring will apply two converters in succession

[Bob Glamm]

but there must be some filter or response-type handling mechanism.. hmm.

[Gopal S Akshintala]

Hm I shall look into it, thanks @Bob Glamm

[stojan]

Afik you could also go with Coroutines support. IO has a suspended method to convert to Coroutines

[Bob Glamm]

Now that I think about it, an extension function on IO that converts it to CompletableFuture would likely be preferable over a filter or interceptor or converter that converts IO -> CompletableFuture

[Hiosdra]

We at work, for returning scala Future created own return value handler like:

class ScalaFutureHandler(timeoutMillis: Long)(implicit ec: ExecutionContext)
    extends AsyncHandlerMethodReturnValueHandler {

And overriten 3 methods:

isAsyncReturnValue
supportsReturnType
handleReturnValue

This allowed us to return Future[A]

[Hiosdra]

Maybe you can do something like this, but for IO?