Process Engines vs State Engines

Process Engines, usually BPEL engines or Workflow engines, execute proces-steps based on a predefined process. This is a sound practice, however not for long-running processes.

Long-running processes have the nasty habit to change over time. This will affect all active and running processes. Why? Because the process engines usually store the process state and corresponding data in a persistence store. The effect of a change in the process requires you to migrate all those saved instances, unless you are able to leave them running the old version of the process.

A different approach is a State engine, where the next step in the process is not determined by the predefined steps in the process, but on the current state of the subject and the event that has been received. This leads to more decoupling between process and services. However, so far I haven't seen any Process engine that works like this.

In theory there's not much difference between the two approaches. You could state that a process engine uses implicit states as defined in the process, whereas a state engine uses explicit states. The end result could/should be exactly the same. A state engine would give some more flexibility, but also needs more functionality to be able to have an overview and control of the process.

Anyone knows of any tool that works like a state engine? I'd really like to know.