Support annotation `@Transactional` for panache

Description

Currently panache doesn’t support javax.transaction.Transactional. For example:

    @PUT
    @Path("{id}")
    @Transactional
    public Uni<Response> update(@RestPath Long id, Fruit fruit) {
        return Fruit.<Fruit> findById(id)
                    .onItem().ifNotNull().invoke(entity -> entity.name = fruit.name)
                .onItem().ifNotNull().transform(entity -> Response.ok(entity).build())
                .onItem().ifNull().continueWith(Response.ok().status(NOT_FOUND)::build);
    }

Currently we need something like this:

    @PUT
    @Path("{id}")
    public Uni<Response> update(@RestPath Long id, Fruit fruit) {
        return Panache
                .withTransaction(() -> Fruit.<Fruit> findById(id)
                    .onItem().ifNotNull().invoke(entity -> entity.name = fruit.name)
                )
                .onItem().ifNotNull().transform(entity -> Response.ok(entity).build())
                .onItem().ifNull().continueWith(Response.ok().status(NOT_FOUND)::build);
    }

[gavinking]

In my opinion, @Transactional doesn’t imply an XA transaction. It just means “some sort of container-demarcated transaction”. It can absolutely be a resource-local transaction, which it would be (at least initially) in the reactive case.

[DavideD]

I don’t think it matters how big the application is, if you mix the two paradigms you need to be aware where the reactive code is or it’s not going to work.

If you have reactive code that doesn’t return an Uni (or similar) somewhere, you will have problems anyway. In your original example, if putOne is reactive and nobody awaits for it’s conclusion, moveStuff will return before putOne has finished and the transaction will be closed prematurely. And because moveStuff returns void, we don’t know when we need to commit the transaction.