Spring WebFlux

Mapping a Flow of Datastar events onto a reactive response.

On the reactive side there is no servlet stream to write into. There is a Flow of events, and Spring knows how to turn ServerSentEvent into a response. The adapter’s whole job is the step between: turning Streamlord’s DatastarEvent into Spring’s ServerSentEvent.

@GetMapping("/counter", produces = [org.springframework.http.MediaType.TEXT_EVENT_STREAM_VALUE])
fun counter(): kotlinx.coroutines.flow.Flow<org.springframework.http.codec.ServerSentEvent<String>> =
    ticks
        .map { n -> PatchElements("""<span id="counter">$n</span>""") }
        .asServerSentEvents()

That is the entire API surface for WebFlux: asServerSentEvents() on a Flow<DatastarEvent>, and toServerSentEvent() on a single DatastarEvent for the times you are not holding a flow. Everything before them is your flow, and everything after is Spring’s.

The guard

asServerSentEvents() uses Streamlord.Default, which has the elements guard switched off. To run your HTML through the guard, pass the bean you declared:

@GetMapping("/counter", produces = [org.springframework.http.MediaType.TEXT_EVENT_STREAM_VALUE])
fun guarded(
    streamlord: Streamlord,
): kotlinx.coroutines.flow.Flow<org.springframework.http.codec.ServerSentEvent<String>> =
    ticks
        .map { n -> PatchElements("""<span id="counter">$n</span>""") }
        .asServerSentEvents(streamlord)

Reading signals

There is no HttpServletRequest here. Take the signals the way WebFlux gives them to you, either a @RequestParam named datastar on a GET or the request body on a POST, and hand the JSON to the codec yourself. The protocol rule for where the signals live is the same as everywhere; only the plumbing differs.

Cancellation

When the client goes away, WebFlux cancels the subscription, which cancels the coroutine behind the flow. A flow { } with a finally releases what it held, and nothing in Streamlord holds anything past the frame it is writing.

What this page does not cover

Content Security Policy is on Security, including the three lines that put a nonce on an exchange from a WebFilter of your own.

Backpressure strategy, schedulers and the rest of the reactive toolbox are Spring’s, and the adapter deliberately does not reach into them. If your flow is hot and fast, the usual buffer, conflate and sample operators apply before asServerSentEvents(), unchanged.