Unable to switch back to vertx event loop thread pool after switching to executor thread pool

[ozzyozbourne]

@GET
@Path("/p1")
public Uni<Response> p1() {
    Log.info("Entry (event loop)");

    return Uni.createFrom()
            .item("input")
            .onItem().transform(input -> {
                Log.info("Light transform");
                return input.toUpperCase();
            })
            .emitOn(Infrastructure.getDefaultWorkerPool()) // ← Switch to worker
            .onItem().transform(data -> {
                Log.info("Heavy work (worker)");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException ignored) {
                }
                return "Processed: " + data;
            })
            .emitOn(Infrastructure.getDefaultExecutor()) // doesn't work
            .onItem().transform(result -> {
                Log.info("Back on event loop");
                return Response.ok(result).build();
            });
}

2025-08-11 04:13:58,404 INFO [com.exa.ExampleResource] (vert.x-eventloop-thread-1) Entry (event loop)
2025-08-11 04:13:58,405 INFO [com.exa.ExampleResource] (vert.x-eventloop-thread-1) Light transform
2025-08-11 04:13:58,405 INFO [com.exa.ExampleResource] (executor-thread-1) Heavy work (worker)
2025-08-11 04:13:59,407 INFO [com.exa.ExampleResource] (executor-thread-1) Back on event loop

I am unable to switch back to the event loop thread by using emitOn(Infrastructure.getDefaultExecutor()).
What am I doing wrong?
How can I go back to the event loop after doing CPU-intensive work inthe worker thread pool?

[cescoffier]

Infrastructure.getDefaultExecutor() does not go back on the event loop. In addition, you should NOT go back to the same event loop, but go back to the same context (slightly different).

So, to do this you either need to capture the context and switch back using:

.emitOn(runnable -> captured.runOnContext(x -> runnable.run())

Or use a a context aware executor as documented on https://quarkus.io/guides/vertx-reference#use-a-vert-x-context-aware-scheduler (recommended)