Reactive Quarkus app behaving differently when run as Java or native - quarkus

Question

I have a reactive quarkus app with hibernate-panache-reactive. The problem is it behaves differently when I run it as a Java app or a native app.
The app
loads a lot of data from a MySQL DB via hibernate-panache-reactive
builds a graph based on the data loaded
runs some time consuming algorithm on the graph
loads some more data from the DB based on the results returned from 3)
So initially the code looked something like this:
GraphProcessor graphProcessor = createInitialProcessor();
return Uni.createFrom().item(graphProcessor)
// 1) loading of initial data
.onItem().transformToUni(this::loadDataViaPanaceReactive1)
.onItem().transformToUni(this::loadDataViaPanaceReactive2)
.onItem().transformToUni(this::loadDataViaPanaceReactive3)
// 2) building of graph
.onItem().transform(graphProcessor::processLoadedData)
.onItem().invoke(graphProcessor::loadingComplete) //sync
// 3) running time consuming algorithm on graph
.onItem().transformToMulti(this::runTimeConsumingTask)
.onItem().invoke(this::prepareDBQueries)
// 4) load more data from DB
.onItem().transformToUniAndConcatenate(this::loadMoreData1)
.onItem().transformToUniAndConcatenate(this::loadMoreData2)
.onItem().transformToUniAndConcatenate(this::transformToPublicForm)
.onFailure().invoke(log::error);
That worked fine when run as a Java app but when I tried to run it as a native app it first complained that the computation in 2 and 3 were taking too long and this was blocking the calling thread.
I fixed that by using
.emitOn(Infrastructure.getDefaultWorkerPool())
Between 1 and 2
This time I got another error
java.lang.IllegalStateException: HR000069: Detected use of the
reactive Session from a different Thread than the one which was used
to open the reactive Session - this suggests an invalid integration;
original thread: 'vert.x-eventloop-thread-0' current Thread:
'vert.x-eventloop-thread-1'
I've fixed that by inserting
.emitOn(Infrastructure.getDefaultExecutor())
between 3 and 4.
GraphProcessor graphProcessor = createInitialProcessor();
return Uni.createFrom().item(graphProcessor)
// 1) loading of initial data
.onItem().transformToUni(this::loadDataViaPanaceReactive1)
.onItem().transformToUni(this::loadDataViaPanaceReactive2)
.onItem().transformToUni(this::loadDataViaPanaceReactive3)
// 2) building of graph
.emitOn(Infrastructure.getDefaultWorkerPool()) // Required for native mode
.onItem().transform(graphProcessor::processLoadedData)
.onItem().invoke(graphProcessor::loadingComplete)
// 3) running time consuming algorithm on graph
.onItem().transformToMulti(this::runTimeConsumingTask)
.onItem().invoke(this::prepareDBQueries)
.emitOn(Infrastructure.getDefaultExecutor()) // Required for native mode
// 4) load more data from DB
.onItem().transformToUniAndConcatenate(this::loadMoreData1)
.onItem().transformToUniAndConcatenate(this::loadMoreData2)
.onItem().transformToUniAndConcatenate(this::transformToPublicForm)
.onFailure().invoke(log::error);
That worked when run in native mode but now when I run it in Java I get the same exception (Detected use of the
reactive Session from a different Thread than the one which was used
to open the reactive Session)
The emitOn(Infrastructure.getDefaultExcecutor()) should have switched back to the original thread.
The odd thing is also that this exception is not thrown every time I hit the app.
So what am I doing wrong here? What is the best way to handle time consuming tasks and then having to do some more DB queries after?
You could use .runSubscriptionOn(Executor) but I would need to switch back to the original thread for part 4 again.
Thanks for you help.