A constraint is a rule or restriction about what constitutes valid instance data of the categories on the data model. The LDS notation does not accommodate constraints, and the process of controlled evolution discourages you from discussing most constraints. This is by design; we think premature discussion of constraints is a bad idea. This chapter describes why.
Postponing the consideration of constraints makes economic sense. (We are invoking the “Do what’s worth doing” principle articulated in the beginning of this book book.) You postpone discussing constraints for the following reasons: