Make coroutine dispatcher(coroutine context) changeable in WebFlux

Motivation

There are cases where the coroutine dispatcher is changed when performing blocking processing.

@GetMapping("/sample")
suspend fun sample(): String {
    // do non-blocking call

    withContext(blockingDispatcher) {
        // do blocking call
    }

    // do non-blocking call

    ...
}

But now WebFlux uses Unconfined dispatcher. https://github.com/spring-projects/spring-framework/blob/5.3.x/spring-core/src/main/java/org/springframework/core/CoroutinesUtils.java#L74

So once we change the dispatcher, subsequent processing will also be executed by that dispatcher. For example, in the following example, it will be executed by blockingExecutor thread even after withContext.

@GetMapping("/test/hello1")
suspend fun hello1(): String {
    // reactor-http-nio-N thread
    log.info("thread={}", Thread.currentThread().name)

    withContext(blockingDispatcher) {
        // blockingExecutor-N thread
        log.info("thread={}", Thread.currentThread().name)

        // do blocking call
    }

    // blockingExecutor-N thread
    log.info("thread={}", Thread.currentThread().name)

    return "hello"
}

We can work around this issue by using the default dispatcher instead of the Unconfined dispatcher.

@GetMapping("/test/hello2")
suspend fun hello2(): String {
    return withContext(Dispatchers.Default) {
        // DefaultDispatcher-worker-N thread
        log.info("thread={}", Thread.currentThread().name)

        withContext(blockingDispatcher) {
            // blockingExecutor-N thread
            log.info("thread={}", Thread.currentThread().name)

            // do blocking call
        }

        // DefaultDispatcher-worker-N thread
        log.info("thread={}", Thread.currentThread().name)

        "hello"
    }
}

However, writing withContext(Dispatchers.Default) on all controller methods can be tedious. So I want to be able to change the coroutine dispatcher used by WebFlux.

How

How about making it possible to change by registering a class such as CoroutinesDispatchersProvider in the bean?

interface CoroutinesDispatchersProvider {
    fun provide(request: ServerHttpRequest): CoroutineDispatcher
}

For example, a framework called armeria allows we to specify a dispatcher https://armeria.dev/docs/server-annotated-service#coroutine-dispatcher

[zsiegel]

This would be very helpful in regards to MDC context as mentioned above.

I would be happy to put together a PR if any of the team members can point me in a direction that might be suitable to put this configuration?

[wplong11]

@2hyjun This feature is not implemented. That commit only added new invokeSuspendingFunction that accepts CoroutineContext. To implement this feature, InvocableHandlerMethod must be fixed to use new invokeSuspendingFunction and CoroutinesDispatchersProvider should be added.

https://github.com/spring-projects/spring-framework/blob/983c6e233fa21e2672484ee8f4f8e8c8bcd51f95/spring-webflux/src/main/java/org/springframework/web/reactive/result/method/InvocableHandlerMethod.java#L139-L145