A FORMAL FRAMEWORK FOR ONLINE SOFTWARE VERSION CHANGE

The usual way of installing a new version of a software system is to s hut down the running program and then install the new version. This ne cessitates a sometimes unacceptable delay during which service is deni ed to the users of the software. An on-line software replacement syste m replaces parts of the software while it is in execution, thus elimin ating the shutdown. While a number of implementations of on-line versi on change systems have been described in the literature, little invest igation has been done on its theoretical aspects. In this paper, we de scribe a formal framework for studying on-line software version change . We give a general definition of validity of an on-line change, show that it is in general undecidable and then develop sufficient conditio ns for ensuring validity for a procedural language.