How to return execution to initial scheduler

[s-spindler]

Hi all,

I have a question regarding the scheduling of background tasks.
On some requests that I proxy I would like to start an asynchronous task in the background.
The request should continue without waiting for the background task.

So far I have not found a way of continuing the request on the default scheduler after publishing the background task on my custom scheduler:

override fun filter(exchange: ServerWebExchange, chain: GatewayFilterChain): Mono<Void> {
    return Mono.just("bla")
            .publishOn(someCustomScheduler)
            .doOnNext {
                // do something expensive here
                println(it)
            }
            // how to return to default scheduler here?
            .flatMap {
                chain.filter(exchange)
            }
}

I can't find a way of getting a handle on the default scheduler. Is it possible to get it somehow?
Or is my approach wrong anyway?

[spencergibb]

@simonbasle or @smaldini can you answer? I'm not actually sure.

@s-spindler what are you scheduling?

[simonbasle]

from a purely Reactor perspective:

@s-spindler it might not be as simple as you think, because there might not be a "default Scheduler"... If the source is executing on the main thread for instance, then there is no way in Java to go back to that thread once you've gotten off of it (Thread doesn't allow task submission).

There is an open issue about it (reactor/reactor-core#1177) with a recent PR that explores a potential solution provided there is some explicit Scheduler switch upstream of your publishOn (reactor/reactor-core#1644), but that won't be in before reactor-core 3.3.

[s-spindler]

@simonbasle, @spencergibb Thanks for the feedback!
Regarding what I'm scheduling: it's a DB query to store when a device last communicated with the server. I don't want this to block or fail the proxying of the actual request.

The request is handled in a filter implementing org.springframework.cloud.gateway.filter.GlobalFilter.
When debugging I see that I'm running on a thread called reactor-http-epoll (AFAIR followed by a digit).

Apart from my filter this project is pretty much vanilla Spring Cloud Gateway.
@spencergibb Can you shed some light on how SCG schedules incoming requests?

[spring-projects-issues]

Closing due to age of the question. If you would like us to look at this issue, please comment and we will look at re-opening the issue.