Typically, infopads are used for application monitoring. Infopads have a fixed set of rows and columns. Another variant of infopad -- the dynamic infopad -- has fixed number of columns but rows can be added or removed as required. Both these structures internally use an array. Adding and removing elements in an array structure is not efficient. Additionally, to access a particular value in an infopad you must traverse all the elements in it. These issues can be overcome by using another type of persistent structure, "Persistent Map". A persistent map provides you a dynamic structure, and quick and easy access to values in it.
The persistent framework generates two module files, one for rules and another one for the persistent data. The rule module contains rules whereas the data module contains persistent structures. This allows you to refresh the rule modules without affecting the persistent data. To distinguish between rule and data modules, "rule" and "data" keywords are used.
