A trace monitor observes the sequence of actions in a software system, and when it detects that this sequence matches a given pattern, it executes some extra code of its own. Trace monitors are often specified declaratively using patterns based on regular expressions, context free grammars or logical formulae, and then the trace monitor implementation is generated from the specification. Trace monitors are particularly useful for runtime verification, and many variations have been proposed. Despite this intense interest, there have been hardly any systems that implement the idea in its full generality, because it is hard to generate e#cient code from a purely declarative statement of the pattern. This paper identifies and addresses the challenges faced in generating e#cient trace monitors from declarative pattern-based specifications
