The worst problem is that eval
is opaque to Drools and so must be re-evaluated every time. The impact this can have on performance may range from negligible to huge, however.
Using eval
is first and foremost a code smell in Drools and an indication that the solution should be rethought. In the final analysis it is almost never truly necessary, and if it is necessary, then the whole Drools system may be a wrong tool for the job.