I am curious to see where async would fit. I recognize distributed async is beyond scope for Rex. Given the treatment of content addressable storage (CAS), one could specify workflow states declaratively, poll artifacts for next, and fire tasks to a delegate and then move forward. It would be nice to have a subsystem do that. A question is where parallelism is useful in your scientific workflows, and where it blocks. Quantum chemistry use cases (QDX is behind Rex) are presently beyond me, so I have no idea.

I am thinking that Rex programs are inteneded as an output from a higher level worklow specification language. It would be interesting to see that.