Active database systems are a current focus of considerable research i
nterest, as a means of supporting a range of tasks including constrain
t enforcement, real-time applications and derived data management. How
ever, although many different proposals have been made-for active rule
systems, such proposals are normally described in an informal manner,
which makes it difficult to understand how different proposals differ
or how a set of rules will behave. This paper compares a range of for
mal specification methods, considering how suitable they are for descr
ibing active database functionality, and then shows how the model-base
d notation Object-Z, an object-oriented extension of Z, can be used to
specify the semantics of a representative active database system, nam
ely Starburst.