Upgrading Legacy Instances of Reactive Systems






A software product typically goes through 
many version changes over its lifetime. Reactive 
systems, such as email clients, software agents, 
proxies, traffic controllers, and telephone switches are 
no exception. Evolving such stateful systems is 
made difficult by the fact that new 
versions of the software must deal correctly 
with legacy instances; that is, users of 
earlier versions have invested significant resources in 
creating the state of the legacy instance, 
and usually require that this state be 
<I>upgraded</I> appropriately when the new system version 
is activated. However, validating the correctness of 
this upgrading behavior is particularly difficult, whether 
through testing or more formal techniques like 
model checking, because legacy states are typically 
unreachable to the new version of the 
software. This paper explores this problem, presents 
a simple conceptual and modeling/programming framework that 
allows upgrade behavior to be validated, and 
gives a technique for simplifying the validation 
problem.